figurec

11institutetext: Argelander-Institut für Astronomie (AIfA), Universität Bonn,
Auf dem Hügel 71, 53121 Bonn, Germany
11email<EMAIL_ADDRESS>22institutetext: Physics and Astronomy,
University of Kentucky, 505 Rose street, Lexington, KY 40506, USA
33institutetext: Center for Astrophysics — Harvard and Smithsonian, 60 Garden
Street, Cambridge, MA 02138, USA 44institutetext: Max-Planck-Institut für
extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
55institutetext: Universitaets-Sternwarte Muenchen, Ludwig-Maximilians-
Universität München, Scheinerstraße, München, Germany 66institutetext: INAF -
Osservatorio Astronomico di Trieste, via Tiepolo 11, I-34143 Trieste, Italy
77institutetext: IFPU - Institute for Fundamental Physics of the Universe, Via
Beirut 2, I-34014 Trieste, Italy 88institutetext: Australian Astronomical
Optics, Macquarie University, 105 Delhi Rd, North Ryde, NSW 2113, Australia
99institutetext: CSIRO Astronomy and Space Science, P.O. Box 76, Epping, NSW
1710, Australia 1010institutetext: Western Sydney University, Locked Bag 1797,
Penrith, NSW 2751, Australia 1111institutetext: CSIRO Astronomy and Space
Science, PO Box 1130, Bentley, WA 6102, Australia 1212institutetext: Institute
for Astroparticle Physics, Karlsruhe Institute of Technology, 76021 Karlsruhe,
Germany 1313institutetext: ICRAR, University of Western Australia, 35 Stirling
Hwy, Crawley, WA 6009, Australia

# The eROSITA view of the Abell 3391/95 field: The Northern Clump

The largest infalling structure in the longest known gas filament observed
with eROSITA, XMM-Newton, and Chandra

Angie Veronica 11 Yuanyuan Su 22 Veronica Biffi 55 6 6 7 7 Thomas H. Reiprich
11 Florian Pacaud 11 Paul E. J. Nulsen 33 13 13 Ralph P. Kraft 33 Jeremy S.
Sanders 44 Akos Bogdan 33 Melih Kara 1212 Klaus Dolag 55 Jürgen Kerp 11 Bärbel
S. Koribalski 99 10 10 Thomas Erben 11 Esra Bulbul 44 Efrain Gatuzz 44
Vittorio Ghirardini 44 Andrew M. Hopkins 88 Ang Liu 44 Konstantinos Migkas 11
Tessa Vernstrom 1111

(Received ; accepted)

###### Abstract

Context. Galaxy clusters grow through mergers and the accretion of
substructures along large-scale filaments. Many of the missing baryons in the
local Universe may reside in such filaments as the warm-hot intergalactic
medium (WHIM).

Aims. SRG/eROSITA performance verification (PV) observations revealed that the
binary cluster Abell 3391/3395 and the Northern Clump (the MCXC J0621.7-5242
galaxy cluster) are aligning along a cosmic filament in soft X-rays, similarly
to what has been seen in simulations before. We aim to understand the
dynamical state of the Northern Clump as it enters the atmosphere ($3\times
R_{200}$) of Abell 3391.

Methods. We analyzed joint eROSITA, XMM-Newton, and Chandra observations to
probe the morphological, thermal, and chemical properties of the Northern
Clump from its center out to a radius of 988 kpc ($R_{200}$). We utilized the
ASKAP/EMU radio data, the DECam optical image, and the Planck y-map to study
the influence of the wide-angle tail (WAT) radio source on the Northern
Clump’s central intracluster medium (ICM). Using eROSITA data, we also
analyzed the gas properties of the Northern Filament, the region between the
virial radii of the Northern Clump and the A3391 cluster. From the Magneticum
simulation, we identified an analog of the A3391/95 system along with an
infalling group resembling the Northern Clump.

Results. The Northern Clump is a weak cool-core cluster (WCC) centered on a
WAT radio galaxy. The gas temperature over $0.2-0.5R_{500}$ is
$k_{B}T_{500}=1.99\pm 0.04$ keV. We employed the mass-temperature ($M-T$)
scaling relation and obtained a mass estimate of $M_{500}=(7.68\pm 0.43)\times
10^{13}M_{\odot}$ and $R_{500}=(636\pm 12)$ kpc. Its X-ray atmosphere has a
boxy shape and deviates from spherical symmetry. We identify a southern
surface brightness edge, likely caused by subsonic motion relative to the
filament gas in the southern direction. At $\sim\\!R_{500}$, the southern
atmosphere (infalling head) appears to be 42% hotter than its northern
atmosphere. We detect a downstream tail pointing toward the north with a
projected length of $\sim\\!318\>\mathrm{kpc}$, plausibly the result of ram
pressure stripping. Through a two-temperature fit, we identify a cooler
component in the Northern Filament with
$k_{B}T=0.68_{-0.64}^{+0.38}\leavevmode\nobreak\ \mathrm{keV}$ and
$n_{e}=1.99_{-1.24}^{+0.88}\times 10^{-5}\leavevmode\nobreak\
\mathrm{cm}^{-3}$, which are consistent within the expected ranges of WHIM
properties. The analog group in the Magneticum simulation is experiencing
changes in its gas properties and a shift between the position of the halo
center and that of the bound gas, while approaching the main cluster pair.

Conclusions. The Northern Clump is a dynamically active system and far from
being relaxed. Its atmosphere is affected by an interaction with the WAT and
by gas sloshing or its infall toward Abell 3391 along the filament, consistent
with the analog group-size halo in the Magneticum simulation.

###### Key Words.:

Galaxies: clusters: individual: Northern Clump, MCXC J0621.7-5242, Abell 3391
– X-rays: galaxies: clusters – Galaxies: clusters: intracluster medium

##  1 Introduction

Figure 1: eROSITA particle-induced background subtracted, exposure corrected,
absorption corrected image in the energy band $0.5-2.0\leavevmode\nobreak\
\mathrm{keV}$, zoomed-in to A3391 and the Northern Clump region. The image has
been adaptively-smoothed with S/N set to 15. The point sources have been
removed and refilled with their surrounding background values. The green
circles depict some characteristic radii of A3391, while the white circle
depicts the $R_{200}$ of the Northern Clump. The yellow box marks the region
used for the spectral analysis of the Northern Filament.

Clusters of galaxies are formed by the gravitational infall and mergers of
smaller structures (Sarazin 2002; Springel et al. 2006; Markevitch & Vikhlinin
2007). They are the high density nodes of the so-called Cosmic Web, connected
by cosmic filaments (Bond et al. 1996). These cosmic filaments are of
importance, such that they are the passages for matter being accreted onto
galaxy clusters (West et al. 1995; Bond et al. 2010), as well as where the
warm-hot intergalactic medium (WHIM) can be found (Rost et al. 2021). The
large-scale cosmological simulations of a $\Lambda$-dominated cold dark matter
($\Lambda$CDM) model (Cen & Ostriker 1999; Davé et al. 2001) predict that
about $30-40\%$ of the total baryons in the Universe reside in the WHIM, and
thus it is believed that the WHIM might be a solution to the missing baryon
problem. However, with electron densities falling between $n_{e}\approx
10^{-6}-10^{-4}\leavevmode\nobreak\ \mathrm{cm}^{-3}$ and temperatures in the
range of $T=10^{5}-10^{7}$ K (Nicastro et al. 2017), detecting the WHIM is
rather difficult (Werner et al. 2008). Bond et al. (1996) predicted that the
presence of the filaments are expected to be the strongest between highly
clustered and aligned clusters of galaxies. Based on this prediction, it is
sensible to conduct the search for the WHIM between a pair of galaxy clusters
(e.g., Kull & Böhringer 1999; Werner et al. 2008; Fujita et al. 2008), and
also in the outskirts of galaxy clusters (e.g., Reiprich et al. 2013; Bulbul
et al. 2016; Nicastro et al. 2018). The binary galaxy cluster system Abell
3391 and Abell 3395 (A3391/95) is amongst the best candidates to conduct this
search.

The A3391/95 system has been observed by numerous instruments of different
wavelengths. Earlier studies of the system using ROSAT and ASCA show that the
gas from the region between the northern cluster (A3391) and the double-peaked
southern cluster (A3395S/N) is consistent with a filament (Tittley & Henriksen
2001). Meanwhile, more recent studies using XMM-Newton, Chandra, and Suzaku
show that the properties of the gas are more in agreement with tidally
stripped intracluster medium (ICM) gas that originated from A3391 and A3395,
indicating an early phase of the cluster merger (Sugawara et al. 2017; Alvarez
et al. 2018).

The A3391/95 system is also one of the performance verification (PV) targets
of the newly launched German X-ray telescope on board the Spectrum-Roentgen-
Gamma (SRG) mission, the extended ROentgen Survey with an Imaging Telescope
Array (eROSITA). With large field-of-view (FoV) of $\sim\\!1$ degree in
diameter, the scan-mode observation, and superior soft X-ray effective area
(the effective area of the combined seven eROSITA telescope modules (TMs) is
slightly higher than the one of XMM-Newton (MOS1+MOS2+pn) in the
$0.5-2.0\leavevmode\nobreak\ \mathrm{keV}$ energy band) (Merloni et al. 2012;
Predehl et al. 2021), eROSITA allows us to capture the A3391/95 system beyond
$R_{100}$ with a good spectral resolution.

In total, four eROSITA PV observations were performed on the A3391/95 system,
including three raster scans and one pointed observation. By combining all the
available observations, an area of $\sim\\!15$ deg2 is covered. This reveals a
breathtaking view of the A3391/95 system that has only been seen in
simulations before, such as the discovery of a long continuous filament
($\sim\\!15$ Mpc) from the projected north toward the south of the A3391/95
system. This indicates that the system is part of a large structure. eROSITA
also found that the bridge gas consists of not only the hot gas, which further
confirms the tidally stripped originated gas scenario, but also of warm gas
from a filament (Reiprich et al. 2021). A detailed eROSITA study focusing on
the bridge will be discussed in Ota et al. (in prep.) Additionally, some
clumps that seem to fall into the system have also been discovered, two of
which reside in the Northern and the Southern Filaments, namely the Northern
Clump111As the extended objects detected in the A3391/95 field are called
”clumps”, we refer this specific galaxy cluster found in the north of the main
system as the Northern Clump (MCXC J0621.7-5242 cluster, Piffaretti et al.
2011 and also referred to as MS 0620.6-5239 cluster, De Grandi et al. 1999;
Tittley & Henriksen 2001) and the Little Southern Clump, respectively
(Reiprich et al. 2021).

After eROSITA’s discovery, follow-up observations were performed by XMM-Newton
and Chandra on the Northern Clump. This biggest infalling clump in the
A3391/95 field is a galaxy cluster, located at redshift $z=0.0511$ (Tritton
1972; Piffaretti et al. 2011). It hosts a Fanaroff-Riley type I (FRI) radio
source, PKS 0620-52 (Trussoni et al. 1999; Venturi et al. 2000). This source
is associated with the cluster’s brightest cluster galaxy (BCG), 2MASX
J06214330–5241333 (Brüggen et al. 2021). This source is featured with a pair
of asymmetric wide-angle tail (WAT) radio lobes (Morganti et al. 1993;
Trussoni et al. 1999; Venturi et al. 2000), with the northeast lobe extending
up to $4^{\prime}$ (240 kpc), while the northwestern lobe up to $2.5^{\prime}$
(150 kpc) (Brüggen et al. 2021).

In this work, we investigate the morphology, thermal, and chemical properties
of the Northern Clump utilizing all available eROSITA, XMM-Newton, and Chandra
observations of the cluster. The cluster core is probed further with the help
of the ASKAP/EMU radio data (Norris et al. 2011), the Planck-y map, and the
DECam optical data. Furthermore, we identify an infalling galaxy group,
resembling the observed Northern Clump, in the analog A3391/95 system from the
Magneticum Simulation (Biffi et al. 2021) and discuss its properties in
comparison to observations to support their interpretation.

This paper is organized as follows: in Sect. 2, we describe the observations,
the data reduction steps, and the analysis strategy. In Sect. 3, we present
and discuss the analysis results. In Sect. 4, we provide some insights about
the analog A3391/95 system and the infalling group from the Magneticum
Simulation. In Sect. 5, we summarize our findings and conclude the results.

Unless stated otherwise, all uncertainties are at the 68.3% confidence
interval. The assumed cosmology in this work is a flat $\Lambda$CDM cosmology,
where $\Omega_{m}=0.3$, $\Omega_{\Lambda}=0.7$, and $H_{0}=70$
$\mathrm{k}\mathrm{m}\mathrm{/}\mathrm{s}\mathrm{/}\mathrm{M}\mathrm{p}\mathrm{c}$.
At the Northern Clump redshift $z=0.0511$, $1^{\prime\prime}$ corresponds to
0.998 kpc.

##  2 Data reduction and analysis

We list all eROSITA, XMM-Newton, and Chandra observations used for imaging and
spectral analysis in Table 1.

Table 1: All observations of the Northern Clump used in this work. Observing Date | ObsID | Exposure∗ [ks] | R.A. (J2000) | Dec. (J2000)
---|---|---|---|---
eROSITA
October 2019 | 300005 (scan) | 55 | 06h 26m 49.44s | -54d $04^{\prime}19.20^{\prime\prime}$
October 2019 | 300006 (scan) | 54 | 06h 26m 49.44s | -54d $04^{\prime}19.20^{\prime\prime}$
October 2019 | 300016 (scan) | 58 | 06h 26m 49.44s | -54d $04^{\prime}19.20^{\prime\prime}$
October 2019 | 300014 (pointed) | 35 | 06h 26m 49.44s | -54d $04^{\prime}19.20^{\prime\prime}$
XMM-Newton
March 2020 | 0852980601 | 73, 77, 65 | 06h 21m 42.55s | -52d $41^{\prime}47.4^{\prime\prime}$
Chandra
October 2010 | 11499 | 20 | 06h 21m 43.30s | -52d $41^{\prime}33.3^{\prime\prime}$
April 2020 | 22723 | 30 | 06h 21m 25.05s | -52d $43^{\prime}09.9^{\prime\prime}$
May 2020 | 22724 | 30 | 06h 21m 42.53s | -52d $45^{\prime}24.7^{\prime\prime}$
April 2020 | 22725 | 30 | 06h 21m 22.30s | -52d $42^{\prime}10.7^{\prime\prime}$
∗The exposure times listed for eROSITA are the average filtered exposure time
across the available TMs of each observation.
The listed XMM-Newton exposure times are the values after the SPF filtering
for MOS1, MOS2, and pn cameras, respectively.
The exposure times listed for Chandra are also the filtered exposure time.

###  2.1 eROSITA

The A3391/95 PV observations are listed in Table 1. In this work we used
eROSITA data processing of configuration c001. The data reduction of all 16
eROSITA data sets were realized using the extended Science Analysis Software
(eSASS, Brunner et al. 2021) version eSASSusers_201009. The eROSITA data
reduction and image correction steps are described in great detail in Sect.
2.1 of Reiprich et al. (2021). The image correction includes particle-induced
background (PIB) subtraction, exposure correction, and Galactic absorption
correction across the FoV.

####  2.1.1 Imaging analysis

The energy band for the imaging analysis was restricted to
$0.5-2\leavevmode\nobreak\ \mathrm{keV}$, with the lower energy limit used for
the telescope modules (TMs) with on-chip filter (TM1, 2, 3, 4, 6; the
combination of these TMs is referred to as TM8) set to 0.5 keV, while for the
TMs without on-chip filter (TM5 and 7; the combination of these TMs is
referred to as TM9) it was set to 0.8 keV due to the optical light leak
contamination (Predehl et al. 2021). The count rates of the final combined
image correspond to an effective area given by one TM with on-chip filter in
the energy band $0.5-2\leavevmode\nobreak\ \mathrm{keV}$. The final PIB
subtracted, exposure corrected, and Galactic absorption corrected image is
shown in Fig. 1. The image has been adaptively-smoothed to enhance low surface
brightness emission and large scale features, with signal-to-noise ratio (S/N)
was set to 15. We note that the Northern Clump appears boxy, which could be
the result of ram-pressure stripping as it interacts with the atmosphere of
the A3391 cluster or the filament, or both. In this work, we focus on studying
the properties of the Northern Clump out to its $R_{200}$, while a more
detailed study about the filaments observed by eROSITA around the A3391/95
system will be carried out by Veronica et al. (in prep).

Thanks to the large eROSITA FoV and the SRG scan mode, in which most of the
observations were taken, the cosmic X-ray background (CXB) could be modeled
from an annulus with an inner and outer radius of $25^{\prime}$ and
$35^{\prime}$, respectively. The annulus is centered at the X-ray emission
peak of the cluster determined by Chandra at
$(\alpha,\delta)=(6\\!:\\!21\\!:\\!43.344,\>-52\\!:\\!41\\!:\\!33.0)$. Since
the CXB region is at $1.5-2.1R_{200}$222All the cluster radii in this work are
calculated using the relations stated in Reiprich et al. (2013), e.g.,
$r_{500}\approx 0.65r_{200}$, $r_{100}\approx 1.36r_{200}$, and
$r_{2500}\approx 0.28r_{200}$., it ensures minimum cluster emission in the CXB
region. This region also stays beyond $R_{100}$ of A3391. Within the
$13.5^{\prime}$ radius, we utilized the XMM-Newton point source catalog and
beyond this radius we used eROSITA detected point source catalog, which was
generated following the same procedure as XMM-Newton (see Sect. 2.2). The
point sources are excluded both in imaging and spectral analyses.

####  2.1.2 Spectral analysis

All eROSITA spectra, the Ancillary Response Files (ARFs), and the Response
Matrix Files (RMFs) for the source and background region were extracted using
the eSASS task srctool. Unless stated otherwise, the spectral fitting for all
eROSITA spectra was performed with XSPEC (Arnaud 1996) version: 12.10.1.

We use the third eROSITA scan observation (ObsID: 300016), where all seven
eROSITA TMs are available, for the spectral analysis of the Northern Clump.
Due to the contamination by the nearby bright star, Canopus, some portions of
the data were rejected from the analysis (see Reiprich et al. 2021), which
further reduces the number of photons for the Northern Clump. To ensure enough
photon counts, the only eROSITA spectral analysis done for the Northern Clump
in this work is to constrain the $k_{B}T_{500}$ from an annulus with inner and
outer radii of $2.4^{\prime}$ and $6^{\prime}$ (corresponding to
$0.2-0.5R_{500}$, see Subsect. 2.2.2). The same definition of CXB region as in
the imaging analysis was used.

Spectral analysis was also performed for the filament region between the
$R_{200}$ of the Northern Clump and the A3391 cluster, the Northern Filament.
The filament region was chosen as a box centered at
$(\alpha,\delta)=(6\\!:\\!23\\!:\\!54.654,\>-53\\!:\\!05\\!:\\!26.637)$ with
height and width of $25^{\prime}$ and $50^{\prime}$, respectively. The
configuration of the Northern Filament box is shown in Fig. 1 (yellow box).
The CXB region for this analysis is a circular region with a radius of
$30^{\prime}$ centered at
$(\alpha,\delta)=(6\\!:\\!18\\!:\\!18.275,\>-54\\!:\\!33\\!:\\!37.880)$. The
average hydrogen column density at the box and its corresponding CXB region is
$N_{H}=0.048\times 10^{22}\leavevmode\nobreak\
\mathrm{atoms\leavevmode\nobreak\ cm}^{-2}$.

We followed the X-ray spectral fitting procedure described in Ghirardini et
al. (2021) and Liu et al. (2021) with some modifications for consistency with
the XMM-Newton and Chandra spectral fitting. The total eROSITA model includes
the absorbed thermal emission for the cluster, which is modeled using
phabs$\times$apec, two absorbed thermal models for the Milky Way Halo (MWH)
and the Local Group (LG), an unabsorbed thermal model for the Local Hot Bubble
(LHB), and an absorbed power-law for the unresolved AGN. The parameters of the
eROSITA CXB components are listed in Table 2. The instrumental background was
modeled based on the results of the eROSITA Filter Wheel Closed (FWC) data
analysis. The normalizations of the instrumental background components were
thawed during the fit. The fitting was restricted to the
$0.7-9.0\leavevmode\nobreak\ \mathrm{keV}$ band, with the lower energy limit
set to 0.8 keV for TM5 and TM7 and to 0.7 keV for the other TMs. Source and
CXB spectra for the individual TMs were fitted simultaneously. An example of
eROSITA spectrum and its fitted model is shown in Fig. 2.

The solar abundance table from Asplund et al. (2009) was adopted and the
C-statistic (Cash 1979) was implemented.

Figure 2: eROSITA spectrum of the Northern Clump from an annulus of $2.4-6^{\prime}$, fitted in the energy range $0.7-9.0$ keV. The spectra and the corresponding response files of TM3 and TM4 are merged for a better visualization. The black points are the Northern Clump spectral data, while the black, red, green, and blue lines represent the total model, the cluster model, the instrumental background model, and the CXB model, respectively. Table 2: Information of the parameters and their starting fitting values of the eROSITA CXB components for the Northern Clump $R_{500}$ analysis. Component | Parameter | Value | Comment
---|---|---|---
$phabs$ | $N_{H}^{\dagger}$ | $0.04$ | fix
$apec$ (LHB) |  $k_{B}T$ [keV] | 0.099 | fix
| $Z\leavevmode\nobreak\ [Z_{\odot}]$ | 1 | fix
| $z$ | 0 | fix
| $norm^{*}$ | $1.0\times 10^{-8}$ | vary
$apec$ (MWH) |  $k_{B}T$ [keV] | 0.225 | fix
| $Z\leavevmode\nobreak\ [Z_{\odot}]$ | 1 | fix
| $z$ | 0 | fix
| $norm^{*}$ | $1.0\times 10^{-8}$ | vary
$apec$ (LG) |  $k_{B}T$ [keV] | 0.5 | vary
| $Z\leavevmode\nobreak\ [Z_{\odot}]$ | 1 | fix
| $z$ | 0 | fix
| $norm^{*}$ | $1.0\times 10^{-8}$ | vary
$pow$ | $\Gamma$ | 1.4 | fix
(unresolved AGN) | $norm^{**}$ | $3.4\times 10^{-7}$ | vary
†[1022 atoms cm-2]
${}^{*}[\mathrm{cm}^{-5}/\mathrm{arcmin}^{2}]$
∗∗[photons/keV/cm2/s/arcmin2 at 1 keV]

###  2.2 XMM-Newton

The data reduction steps of the XMM-Newton data sets of the Northern Clump
observation were realized using HEASoft version 6.25 and the Science Analysis
Software (SAS) version 18.0.0 (xmmsas_20190531_1155). For all XMM-Newton
instruments, we filtered out the time intervals contaminated by soft proton
flares (SPF) (e.g., De Luca & Molendi 2004; Kuntz & Snowden 2008; Snowden et
al. 2008). The procedure started by constructing the light curve in the energy
band $0.3-10.0\leavevmode\nobreak\ \mathrm{keV}$ from the entire FoV. The mean
value of the counts was estimated by fitting a Poisson law to the counts
histogram. Then a $\pm 3\sigma$ clipping was applied, such that any time
intervals above and below the $3\sigma$ threshold were considered contaminated
by the SPF and rejected from further steps.

Light curves from each of XMM-Newton’s detectors are shown in Appendix A. To
check whether there was still any persisting soft proton background in our
data, we calculated the ratio of count rates within the FoV (IN) to those of
the unexposed region of the MOS detectors (OUT) (De Luca & Molendi 2004). The
obtained IN/OUT ratios for each detector are around unity, which implies that
the retained data are not contaminated by soft protons. After this SPF
filtering, we are left with $\sim\\!73$ ks, $\sim\\!77$ ks, and $\sim\\!65$ ks
for MOS1, MOS2, and pn, respectively.

We checked for “anomalous state” in the detectors’ chips. This is usually
indicated by a low hardness ratio and high total background rate (Kuntz &
Snowden 2008). Since, in such cases, there is not a robust method yet to model
the spectral distribution of the low energy background, any CCD affected by
this anomalous state was rejected in further analysis. This resulted in the
conservative exclusion of MOS1-CCD4 and MOS2-CCD5. In addition, MOS1-CCD6 and
MOS1-CCD3 were not operational during this observation, due to the previous
micro-meteorite impacts (ESA: XMM-Newton SOC 2020).

Utilizing the FWC data, the quiescent particle background (QPB) (Kuntz &
Snowden 2008) was modeled and then subtracted from the observation. We
followed the procedure described in Ramos-Ceja et al. (2019). First, the
cleaned observation data were matched with the suitable FWC data and then this
FWC template was re-projected to the sky position. Afterwards, rescaling
factors were calculated to renormalize the QPB template to a level comparable
with that of the observation. This renormalization was calculated separately
for each MOS CCD and pn quadrant based on their individual count rates in
unexposed regions. The renormalization of the central MOS CCD chips that do
not have any unexposed regions relies on the unexposed regions of the best
correlated surrounding CCD chips as determined by Kuntz & Snowden (2008). For
spectroscopic analysis, global renormalization factors were calculated in the
energy bands $2.5-5.0\leavevmode\nobreak\ \mathrm{keV}$ and
$8.0-9.0\leavevmode\nobreak\ \mathrm{keV}$ for MOS cameras, and
$2.5-5.0\leavevmode\nobreak\ \mathrm{keV}$ for pn camera. This energy
selection aims to include the bands where the instrumental background would
have the most impact on the spectral analysis, while avoiding the fluorescence
X-ray (FX) emission lines present for the EPIC cameras. For imaging analysis,
on the other hand, the template renormalization process relies on the exact
same band that was used for the science analysis.

Another contamination when studying the ICM gas might come from the point
sources that are distributed across the FoV. Therefore, it is required to
detect and excise the point sources, for example, in the calculation of
surface brightness profiles and spectral analysis. To detect the point sources
in the observation, we implemented the procedure described in Pacaud et al.
(2006) and Ramos-Ceja et al. (2019). Firstly, wavelet filtering was applied on
the combined detector image in the soft energy band
($0.5-2.0\leavevmode\nobreak\ \mathrm{keV}$). Afterwards, an automatic
detection of point sources and catalog generation were performed on the
wavelet-filtered image by the Source Extractor software (SExtractor, Bertin &
Arnouts 1996). Lastly, we carried out a manual inspection on the soft and hard
energy band images ($2.0-10.0\leavevmode\nobreak\ \mathrm{keV}$) to list any
point sources that were missed by the automatic detection. To generate a
surface brightness image, the areas where the point sources were removed were
refilled with random background photons, that is the ghosting procedure. The
ghosted surface brightness images were used only for visual inspection.

####  2.2.1 Imaging analysis

Figure 3: XMM-Newton count rate image of the Northern Clump in the energy band
$0.5-2.0$ keV, overlaid with the ASKAP/EMU radio contour (black). The
anomalous CCD chips are removed and the QPB is subtracted from the image. The
sectors are south (S), west (W), northwest (NW), north (N), northeast (NE),
and east (E). The white circle and cyan square depict the outermost annuli of
corresponding setups. The radius of the circle is $10^{\prime}$ and the half-
width of the square is $7.5^{\prime}$. Gaussian smoothing with kernel radius
of 7 pixels is applied to the image.

We constructed XMM-Newton’s X-ray surface brightness ($S_{X}$) profiles using
the combined photon image and exposure map from the XMM-Newton detectors
(MOS1, MOS2, and pn) in the energy band $0.5-2.0$ keV. Since the ghosting
procedure used for generating surface brightness images does not represent the
exact true counts from the ICM gas, we preferred to mask these point source
regions in the calculation of the $S_{X}$ profile. The first $S_{X}$ profile
was calculated from 60 concentric annuli placed around the Chandra X-ray
emission peak at
$(\alpha,\delta)=(6\\!:\\!21\\!:\\!43.344,\>-52\\!:\\!41\\!:\\!33.0)$. With
$10^{\prime\prime}$ increments between each annulus, the whole arrangement
covers radial distance of $10^{\prime}$ from the center. While the QPB was
subtracted from the data, the CXB was estimated from an annulus with an inner
radius of $12^{\prime}$ and an outer radius of $13.25^{\prime}$, respectively.
Then, the estimated $S_{X}$ from this background region was subtracted from
the source regions.

A second $S_{X}$ profile was also constructed. The setup consists of 45
concentric box-shaped “annuli” centered at the same peak coordinates. Between
each annulus there is a $20^{\prime\prime}$ width increment. This implies that
the width of each box-shaped annulus corresponds to the diameter of each
circular annulus. The half-width of the outermost box annulus is
$7.5^{\prime}$. The setup was angled at $173^{\circ}$, which was chosen
visually to follow the shape of the Northern Clump. The same X-ray background
region as in the circular annuli setup was used. To investigate, whether the
boxiness could be an artifact originating from the MOS detectors’ central
chips, a $S_{X}$ profile from pn-only data and using the box annuli setup was
calculated as well.

We fit each of the setups with a single $\beta$-model (Cavaliere & Fusco-
Femiano 1976), which is written as

$S_{X}(R)=S_{X}(0)\left(1+\frac{R^{2}}{r_{c}^{2}}\right)^{-3\beta+\frac{1}{2}},$
(1)

where $R$ is the projected distance from the center, $S_{X}(0)$ is the
normalization factor, $r_{c}$ is the estimated core radius, and $\beta$ is the
surface brightness slope.

To better address the features in any particular direction, the $S_{X}$
profiles in the projected south, west, northwest, north, northeast, and east
directions were constructed, as well. For these sectorized $S_{X}$ profiles,
all XMM-Newton detectors were employed in both circular and box annuli setups.
The configuration of the sectors is displayed in Fig. 3.

Figure 4: Left: Mosaic Chandra ACIS-I image of the northern clump. The image
is blanksky background subtracted, exposure corrected, and point-source
refilled. Right: The mosaic exposure map corresponding to the left image. Red,
black, and magenta pie regions represent the regions used to derive the
surface brightness profiles of the south, west, and northwest directions as
shown in Fig 8.

####  2.2.2 Spectral analysis

Unless stated otherwise, the spectral fitting for all XMM-Newton spectra was
performed with XSPEC (Arnaud 1996) version: 12.10.1 using the following model,

$\begin{split}\mathtt{Model_{XMM}=}&\quad\mathtt{constant\times(apec_{1}+phabs\times(apec_{2}+}\\\
&\quad\mathtt{powerlaw)+gauss_{1}+gauss_{2}+gauss_{3}+}\\\
&\quad\mathtt{gauss_{4}+gauss_{5})\,+phabs\times apec_{3}},\\\ \end{split}$
(2)

where the first term consists of the CXB and XMM-Newton’s known instrumental
lines, scaled to the areas of the source regions (constant, specified in units
of $\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{m}\mathrm{i}\mathrm{n}^{2}$). The
absorption parametrized by the hydrogen column density, $N_{H}$, along the
line of sight is represented by phabs. The $N_{H}$ values used in this work
are taken from the A3391/95 total $N_{H}$ map described in Sect. 2.5 of
Reiprich et al. (2021). The X-ray emission components to account for the LHB
and the MWH are represented by apec1 and apec2, respectively. The power-law
component, powerlaw, is to account for the unresolved AGN. The information
regarding the CXB components and their starting fitting parameter values are
listed in Table 3. Since the FX emission lines of the XMM-Newton cameras are
not convoluted in the ARF, we modeled the lines with the other background
components as the gaussi’s. The modeled FX lines include the Al-Kα at 1.48 keV
(for MOS and pn cameras), Si-Kα at 1.74 keV (MOS cameras), Ni-Kα at 7.47 keV
(pn camera), Cu-Kα at 8.04 keV (pn camera), and Zn-Kα at 8.63 keV (pn camera),
respectively. The second term, phabs$\times$apec3, represents the absorbed
cluster emission from the source region. The temperature $k_{B}T$, metallicity
$Z$, and normalization $norm$ of this cluster emission component were left to
vary and linked across the detectors.

The apec’s parameter $norm$ is defined as

$norm=\frac{10^{-14}}{4\pi[D_{A}(1+z)]^{2}}\int n_{e}n_{H}\mathrm{d}V,$ (3)

where $D_{A}$ is the angular diameter distance to the source in the unit of
cm, while $n_{e}$ and $n_{H}$ are the electron and hydrogen densities in the
unit of cm-3, respectively.

Table 3: Information of the parameters and their starting fitting values of the XMM-Newton CXB components. Component | Parameter | Value | Comment
---|---|---|---
$phabs$ | $N_{H}^{\dagger}$ | $0.046$ | fix
$apec_{1}$ (LHB) |  $k_{B}T$ [keV] | 0.099 | fix
| $Z\leavevmode\nobreak\ [Z_{\odot}]$ | 1 | fix
| $z$ | 0 | fix
| $norm^{*}$ | $1.7\times 10^{-6}$ | vary
$apec_{2}$ (MWH) |  $k_{B}T$ [keV] | 0.225 | fix
| $Z\leavevmode\nobreak\ [Z_{\odot}]$ | 1 | fix
| $z$ | 0 | fix
| $norm^{*}$ | $7.3\times 10^{-7}$ | vary
$pow$ | $\Gamma$ | 1.4 | fix
(unresolved AGN) | $norm^{**}$ | $5\times 10^{-7}$ | vary
†[1022 atoms cm-2]
${}^{*}[\mathrm{cm}^{-5}/\mathrm{arcmin}^{2}]$
∗∗[photons/keV/cm2/s/arcmin2 at 1 keV]

To ensure a cluster-emission-free background region, the ROSAT All-Sky Survey
(RASS) data were utilized to constrain all of the sky background components,
in particular the MWH as well as CXB normalization. The RASS data were
extracted using the X-ray background tool333HEASARC: X-Ray Background Tool.
The specified background region is an annulus centered at the Chandra center,
with an inner radius of $0.58^{\circ}$ and an outer radius of $0.9^{\circ}$.
To avoid having cluster emission from A3391, we manually removed some of the
RASS background region that is within $R_{100}$ of the A3391 cluster. This
reduces the number of detected pixels from 38 to 34.

The variety of overlap area between each annulus and each individual camera
and CCD combination can result in some very low S/N spectra being extracted,
for which a background model can not always be robustly constrained. For this
reason, we opted to combine the spectra of all CCDs for a given annulus, and
statistically subtract the rescaled FWC spectrum rather than modeling their
spectra. The co-addition of rescaled spectra for the different CCDs implies
that our background spectra no longer follow Poisson statistics and therefore
we used the $\chi^{2}$-statistic for parameter estimation, after ensuring that
each bin contains enough photons (¿25). Additionally, since we use the RASS
background, which is Gaussian data, to estimate the CXB normalizations, the
choice is believed to be sensible. However, we also performed several tests
using pgstat statistic from XSPEC, where the source data were treated as
Poisson data (C-statistic) and the background as Gaussian. The resulting
cluster parameters of these tests are always consistent within the $1\sigma$
error bars of the $\chi^{2}$-statistic fitting results. Hence, the choice of
the used statistics for XMM-Newton spectral fitting would not change any main
conclusions. We present the results of the pgstat tests in Appendix D, Fig.
21. To minimize the effect of the detector noise in the lower energy, photons
with $E<0.7$ keV were ignored. The solar abundance table from Asplund et al.
(2009) was adopted.

We determined the $k_{B}T_{500}$ in order to calculate the $M_{500}$,
consequently the $R_{500}$. We note that $k_{B}T_{500}$ was calculated through
an iterative procedure, such that we extracted and fit spectra from an annulus
centered at the Chandra center, varying the inner and outer radii until they
correspond roughly to the $0.2-0.5R_{500}$ cluster region. Based on this, we
obtained $k_{B}T_{500}=1.99\pm 0.04$ keV from an annulus with inner and outer
radii of $2.4^{\prime}$ and $6.0^{\prime}$, respectively. We input this value
into the mass-temperature ($M-T$) scaling relation by Lovisari et al. (2015),

$\log(M/C1)=a\cdot\log(T/C2)+b,$ (4)

where $a=1.65\pm 0.07$, $b=0.19\pm 0.02$, $C_{1}=5\times
10^{13}h_{70}^{-1}M_{\odot}$, and $C_{2}=2.0$ keV. Assuming spherical symmetry
and taking 500 times the critical density of the Universe at the Northern
Clump redshift as $\rho_{500}(z=0.0511)=4.82\times 10^{-27}$ g cm-3, we obtain
$M_{500}=(7.68\pm 0.43)\times 10^{13}M_{\odot}$ and $R_{500}=(10.62\pm
0.20)^{\prime}\approx(636.03\pm 12.03)$ kpc. These values are 19% and 7% lower
than the reported values in Piffaretti et al. (2011) ($M_{500}=9.47\times
10^{13}M_{\odot}$ and $R_{500}=682$ kpc), which are determined using the $L-M$
relation, instead.

We also analyzed the spectra in three different directions of the cluster,
such as the south, west, and north with position angle (P.A.) from
$250^{\circ}$ to $305^{\circ}$, $20^{\circ}$ to $305^{\circ}$, and
$70^{\circ}$ to $100^{\circ}$, respectively. At larger radii, i.e.,
$4-12.5^{\prime}$, due to the removed MOS1-CCD chips we used pn-only spectra
in the north, and MOS2+pn spectra in the south and west.

We always keep the first bin as a full annulus, instead of a sector in any
specific directions. We conducted a more detailed analysis to take into
account the emission originated from the AGN of the BCG, such that we
implemented three different fitting methods for this particular bin. The first
method is to mask the AGN with a circle of $15^{\prime\prime}$ radius. This is
the method that is implemented to the central bin during the simultaneous
fitting described above. The second method involves constraining the AGN
photon index and normalization from Chandra observation. Here, the masking was
lifted out. Since XMM-Newton and Chandra observations of the Northern Clump
were performed almost simultaneously, little to no temporal variation from the
AGN can be expected (e.g., Maughan & Reiprich 2019). The method and the
resulting AGN parameter values from the spectral fitting are discussed in
Subsect. 2.3.3. We froze Chandra’s parameter values during the XMM-Newton
spectral fitting of the first bin. Lastly, to account for the expected multi-
temperature structure in this region, we performed a two-temperature structure
fit in addition to the AGN component. Due to the statistics, the normalization
and the metallicity of the multi-temperature components were linked, while the
second temperature were set to be half of the first.

###  2.3 Chandra

We analyzed a total of 110 ks Chandra observations on the Northern Clump as
listed in Table 1, including three ACIS-I observations taken in 2020 and one
ACIS-S observation in 2010. CIAO 4.12 and CALDB 4.9.3 were used for the
Chandra data reduction. All the observations were reprocessed from level 1
events using the CIAO tool chandra_repro to guarantee the latest and
consistent calibrations. Readout artifacts from the central bright AGN were
removed from the reprocessed events using acisreadcorr. We filtered background
flares beyond $3\sigma$ using the light curve filtering script lc_clean; clean
exposure times are listed in Table 1. We combined all the observations to
resolve faint point sources. The Chandra PSF varies significantly across the
FoV. For each observation, we produced a PSF map for an energy of 2.3 keV and
an enclosed count fraction of 0.393. We obtain an exposure-time weighted
average PSF map from these four observations. Point sources were detected in a
$0.3-7.0$ keV mosaic image with wavdetect, supplied with this exposure-time
weighted PSF map. The detection threshold was set to $10^{-6}$. The scales
ranged from 1 to 8 pixels, increasing in steps of a factor of 2. We adopted
the blank sky background for the imaging and spectral analyses. A compatible
blank sky background file was customized to each of the event file using the
blanksky script. As shown in Subsect. 3.2, the surface brightness of our blank
sky background is consistent with the sum of the stowed background and the
astrophysical background.

####  2.3.1 Imaging analysis

We only include the three ACIS-I observations taken in 2020 for the imaging
analysis due to the different backgrounds, responses, FoVs, and calibrations
between these observations and the ACIS-S observation taken in 2010. All the
observations were reprojected to a common tangent point using reproject_obs.
Images in the $0.5-2.0$ keV energy band and their exposure maps were produced
using flux_obs. Point sources identified by wavdetect were replaced with the
surface brightness from the immediate surrounding regions using dmfilth.
Scaled blanksky images were produced using blanksky_image. A final background
subtracted and exposure corrected mosaic image of the Northern Clump is shown
in Figure 4.

####  2.3.2 Spectral analysis

We include all four existing Chandra observations for the spectral analysis.
Spectra were extracted using specextract from point-source excised event files
for regions of consecutive sectional annuli in the south and west directions,
ranging from $\delta r=0.5^{\prime}$ at the cluster center to $\delta
r=1.67^{\prime}$ at $r=6.68^{\prime}$. The corresponding response files and
matrices were produced for each spectrum using specextract. Blanksky
background normalized to the count rate in the $9.5-12.0$ keV energy band of
each observation was applied. All the spectra were grouped to have at least
one count per energy bin and restricted to the $0.7-7.0$ keV range. The
spectral fitting was performed with PyXspec 2.0.3. Our spectral model takes
the form of phabs$\times$apec, representing a foreground-absorbed thermal
component. The total $N_{H}$ value of $4\times 10^{20}$ cm-2 is taken from the
A3391/95 total $N_{H}$ map described in Reiprich et al. (2021). C-statistics
(Cash 1979) was used for parameter estimation. The solar abundance table of
Asplund et al. (2009) was adopted. Due to low photon counts, we found it
necessary to fix the metallicity at $0.3Z_{\odot}$.

####  2.3.3 Central AGN

We characterize the property of the central AGN detected at equatorial
coordinates of
$(\alpha,\delta)=(6\\!:\\!21\\!:\\!43.304,\>-52\\!:\\!41\\!:\\!33.153)$.
Spectra of the central $r=2^{\prime\prime}$ were extracted from clean events
files with point sources included. Spectra from the surrounding
$r=2-5^{\prime\prime}$ were used as the local background. We fit the spectrum
to a simple absorbed power-law model and fixed the column density
$N_{H}=4\times 10^{20}cm^{-2}$. We obtain a steep photon index of $2.5\pm
0.07$ and a best-fit luminosity of $5.7\pm 0.3\times 10^{41}$ erg/s in the
$2.0-10.0$ keV when using the observation taken in 2020, which is used for the
AGN modelling in the XMM-Newton analysis.

###  2.4 ASKAP/EMU, DECam, and Planck Data

We use the ASKAP/EMU radio image and DECam images, as well as the Planck y-map
for overlays generated as described in detail in Reiprich et al. (2021).

##  3 Results

###  3.1 Boxiness, tail, and X-ray cavity in images

Figure 5: XMM-Newton surface brightness images (left) and adaptively-smoothed
images (right) in the energy band $0.5-2$ keV. The S/N of the adaptively-
smoothed images is set to 15. Top: MOS1+2+pn combined. Bottom: pn-only. Figure
6: Left: Chandra unsharp masked image of the central region of the Northern
Clump. Right: XMM-Newton residual image. The northern yellow arrow shows the
$5.3^{\prime}$ projected length of the tail emission. The enhancement in the
south is indicated by the southern yellow arrow and the center of A3391
cluster is pointed by the dashed-line white arrow. Gaussian smoothing with
kernel radius of 3 pixels is applied to the image. Black and white contours in
both images represent the ASKAP/EMU radio continuum emission.

As observed in the clean images shown in Fig. 19, the main body of the
Northern Clump is well-embedded in the central chips of MOS1 (top left) and
MOS2 (top right) cameras. A question arises, whether the boxyness of the
system is intrinsic to the cluster itself, or rather an artifact caused by the
central MOS chips. The boxy shape of the cluster can still be recognized in
the pn image (bottom left), although not as prominent. To further investigate
this question, we generated exposure corrected images, as well as adaptively-
smoothed images using MOS1+2+pn data and pn-only data, where the latter should
not be affected by the CCD chips placement. The adaptive smoothing procedure
was realized using the SAS task, asmooth, smoothstyle=‘adaptive’. We set the
S/N of the adaptive smoothing to 15. Through adaptive smoothing we also aim to
enhance any soft emission, especially in the cluster outskirts. The resulting
images are displayed in Fig. 5. We observe similarity in the morphology of the
cluster in the two types of images, such that the boxy shape and indication of
tail-like emission projected northward.

We present a composite X-ray+radio+optical image in Fig. 12 (left), focusing
on the central region of the Northern Clump. We utilized the XMM-Newton count
rate image in soft band (blue color and white contour), the ASKAP/EMU radio
image (red color and yellow contour), and DECam Sloan g-band image (green).
The denser X-ray emission of the Northern Clump shows an asymmetrical
morphology and has a sharp edge pointing at the projected southwest direction
– towards the A3391 center. We notice that the opening angle of the WAT radio
source indicates movement towards the south. While looking at the bent
direction of both radio lobes towards the northeast, the central radio source
seems to move towards the southwest. We speculate that the radio lobes might
have ”escaped” from hot gas confinement and the change in direction is due to
motion through a lower density medium, as indicated by the contours. The
central radio source may be moving locally within the Northern Clump towards
the southwest as a result of, for example, sloshing, while globally, the
Northern Clump moves towards the southeast.

We used unsharp masks in the Chandra image to highlight substructures in the
ICM. The original image was smoothed using two different scales and one
smoothed image was subtracted from the other to make the edges more visually
appealing. This technique can be mathematically expressed as

$\hat{U}=I*g-I*h$ (5)

where $I$ stands for the original image and $g$ and $h$ are the two smoothing
scales, for which we chose $g=30$ and $h=100$ pixels. As shown in Fig. 6
(left), the unsharp masked image reveals regions with a possible deficient
surface brightness $r\approx 2.09^{\prime}\>(125\>\mathrm{kpc})$ to the east
and west of the cluster center. We further overlaid the ASKAP/EMU radio
continuum emission of the cluster center. A seeming picture of X-ray cavities
filled with radio lobes starts to emerge, as observed for other clusters
(e.g., Bîrzan et al. 2004; McNamara & Nulsen 2007).

From the presented XMM-Newton and Chandra images, and with the help from
ASKAP/EMU radio image, as well as the DECam optical image, we identify clear
indications of Northern Clump’s boxyness, tail-like emission in the north, and
hint of X-ray cavities that coincide with the radio lobes.

###  3.2 X-ray surface brightness ($S_{X}$) profiles

####  3.2.1 XMM-Newton

Figure 7: Left: Instrument independent, absorption corrected, and background
subtracted $S_{X}$ profiles of Chandra (green), XMM-Newton (blue), and eROSITA
(red) in the energy band $0.5-2.0$ keV. The blue, red, and black horizontal
dashed lines are the CXB levels of XMM-Newton, eROSITA, and eROSITA with XMM-
Newton defined CXB region, respectively. Black points are the CXB corrected
XMM-Newton $S_{X}$. The various radii are plotted in brown. Right: XMM-Newton
$S_{X}$ profiles in different directions of the Northern Clump using the box
annuli setup (top) and the relative differences with respect to the average
surface brightness (bottom). The grey shaded areas in both plots are the
$1\sigma$ confidence region of the average $S_{X}$. The various radii are
plotted in brown. The CXB and QPB level are plotted in orange and cyan dashed
lines.

The full azimuthal $S_{X}$ profiles of the three different setups can be found
in Appendix C. In all three profiles we observe a steep drop at around
$5^{\prime}<r<6^{\prime}$ ($299.4\mathrm{\>kpc}<r<359.28\mathrm{\>kpc}$).
Moreover, the overall shapes and residuals seen in the $S_{X}$ profiles of the
box annuli setups using all XMM-Newton detectors and pn-only data support the
case that the boxy appearance of the cluster is not an artifact caused by the
placement of the central MOS chip.

We present the sectorized $S_{X}$ profiles (using the box annuli setup) in
Fig. 7 (right, top plot). The bottom plot of the Figure shows the relative
differences with respect to the corresponding $S_{X}$ of the full azimuthal
setups, which is to examine any excess or deficit of the surface brightness.
The $1\sigma$ confidence region of the full azimuthal $S_{X}$ is plotted in
gray. Hereafter, the full azimuthal $S_{X}$ profile will be referred to as the
average $S_{X}$ profile. The sectorized $S_{X}$ profiles using the circular
annuli setup can be found in Appendix C.

Some noticeable features among the different directions are present, such as
the $S_{X}$ depression in the northeast, excess of the $S_{X}$ in the south,
and the enhancement towards the north. These features are seen in both setups,
but they are more pronounced in the box annuli setup.

The depression in the northeast is at $1^{\prime}<r<3^{\prime}$
($59.88\mathrm{\>kpc}<r<179.64\mathrm{\>kpc}$). The $S_{X}$ gets closer to the
average at around $4^{\prime}$ ($239.52\mathrm{\>kpc}$) and drops rapidly
beyond this point. The largest fractional decrement in this direction, of 36%,
occurs at $1.67^{\prime}$ ($100\mathrm{\>kpc}$).

In the southern sector, $S_{X}$ enhancement is observed at around
$2^{\prime}<r<6^{\prime}$ ($119.76\mathrm{\>kpc}<r<359.28\mathrm{\>kpc}$). The
peak is at $4.33^{\prime}$ ($259.20\mathrm{\>kpc}$) with a 66% enhancement. We
suggest that the origin of the surface brightness excess in this specific
direction is either from compressed gas caused by ram pressure or an accreted
clump of cold gas from the Northern Filament, where the cluster resides. In
both scenarios, the Northern Clump should be falling towards the A3391 cluster
along with the filament and within the region where its mean density
dominates, collapses from the filament will continue onto the system.

We observe significant enhancement at larger radii in the north direction
consistent with the observed tail feature in the image. A relative difference
of 170% occurs at $7.5^{\prime}$ ($449.1\mathrm{\>kpc}$).

The features seen in the sectorized $S_{X}$ profiles are present in the
residual image (Fig. 6, right). The residual image is the result of dividing
the surface brightness image by the spherically symmetric model. We overlaid
the residual image with the ASKAP/EMU radio contour (white contour). Surface
brightness depression at around $r\approx 2.05^{\prime}$ (122.75 kpc) to the
northeast and northwest of the center are observed. They coincide with the
lobes of the WAT radio source and also in agreement with what is found in
Chandra unsharp masked image (Fig. 6, left). In the south, a density
enhancement relative to other directions is revealed at predominantly around
$2^{\prime}<r<5^{\prime}$ ($119.76\mathrm{\>kpc}<r<229.40\mathrm{\>kpc}$).
Furthermore, we detect a clear tail of emission north of the Northern Clump,
$\sim\\!7^{\prime}$ ($419.16\mathrm{\>kpc}$) away from the center. This tail
has a projected length of $\sim\\!5.3^{\prime}$ ($317.4\mathrm{\>kpc}$), as
indicated by the northern yellow arrow in Fig. 6 (right).

####  3.2.2 Chandra

Figure 8: Left: Blanksky background subtracted surface brightness profiles of
the south (red), west (west), and northwest (magenta) directions in units of
counts/s/arcsec2 as measured with Chandra. The level of the tailored blanksky
background (green dashed) and that of the stowed background plus the
astrophysical background (purple dashed) for the south direction are
consistent. The blue dashed line represents the total emission from both the
source and background of the south direction. The vertical pink line marks the
position of a possible density jump in the south direction. Right: surface
brightness profile zoomed in around the surface brightness edge in the south
direction in units of photon/s/cm2/arcsec2. We fit the data with a broken
powerlaw density model (pink) and obtain a best-fit density jump of 1.45.

We derived the X-ray surface brightness profiles in south, west, and northwest
directions, ranging from the cluster center to radii of $11.70^{\prime}$ (700
kpc) as shown in Fig. 8. We note a region of enhanced brightness at around
$r=5.84^{\prime}$ (350 kpc) in the south direction relative to other
directions. We fit the profile to a broken power law profile with a density
jump projected along the line of sight

$S(r)=norm\int F(\omega)^{2}dl,\leavevmode\nobreak\ \leavevmode\nobreak\ {\rm
where}\leavevmode\nobreak\ \omega^{2}=r^{2}+l^{2}.$ (6)

$F(\omega)$ is the deprojected density profile:

$F(\omega)=\cases{\omega}^{-\alpha_{1}}\leavevmode\nobreak\
\leavevmode\nobreak\ {\rm if}\leavevmode\nobreak\ \omega<r_{\rm break}\\\
\frac{1}{jump}\omega^{-\alpha_{2}}\leavevmode\nobreak\ {\rm otherwise,}{}$ (7)

where $\alpha_{1}$, $\alpha_{2}$, $jump$, and $r_{\rm break}$ are free
parameters. We obtain a best-fit density jump of $1.45\pm 0.32$ at $r=5.74\pm
0.17^{\prime}$ ($344\pm 10$ kpc). As the ICM temperature appears to be uniform
across the edge (Fig. 9), we assume that the pressure discontinuity equals the
density jump and calculate the associated Mach number (Landau & Lifshitz
1959):

$\frac{p_{0}}{p_{1}}=\left(1+\frac{\gamma-1}{2}\mathcal{M}\right)^{\gamma/(\gamma-1)}$
(8)

for which we obtain $\mathcal{M}=0.48^{+0.30}_{-0.34}$. Given a sound speed of
802 km$/$s for a 2.5 keV ICM, the central region of the Northern Clump is
moving towards south at a speed of $386^{+239}_{-274}$ km$/$s relative to the
motion of the gas in the southern direction.

####  3.2.3 eROSITA, XMM-Newton, and Chandra

We present $S_{X}$ profiles of all the used X-ray instruments in the energy
band $0.5-2.0$ keV in Fig. 7 (left). To correct for the effective area of
different instruments and absorption, each of the three profiles had been
multiplied by a conversion factor. For eROSITA and XMM-Newton, this conversion
factor was calculated by taking the ratio of the unabsorbed energy flux to the
absorbed count rates in the $0.5-2$ keV band. We supplied the best-fit cluster
parameters in the $2.4-6^{\prime}$, the $(k_{B}T_{500})$ region and the on-
axis response files of an on-chip TM and MOS camera. To account for possible
temperature changes across the radial ranges, we computed the conversion
factor for 3 keV and 1.5 keV cluster emission, as well. The variation is found
to be less than $3\%$. For Chandra, whose exposure maps already include a
correction for the telescope effective area in addition to vignetting, the
conversion factor is simply the ratio of the unabsorbed energy flux to the
absorbed photon flux. The factor is $9.87\times 10^{-12}\leavevmode\nobreak\
\mathrm{ergs/cm^{2}/counts}$ for eROSITA, $7.55\times
10^{-12}\leavevmode\nobreak\ \mathrm{ergs/cm^{2}/counts}$ for XMM-Newton, and
$1.72\times 10^{-9}\leavevmode\nobreak\ \mathrm{ergs/photons}$ for Chandra.

The annulus used to estimate the XMM-Newton CXB level
($12.0-13.25^{\prime}\leavevmode\nobreak\ (1.13-1.24R_{500})$, shown as the
horizontal blue line) may still contain some cluster emission. In order to
correct for this, we calculated the surface brightness of this same background
region using eROSITA data (horizontal black dashed lines) and estimated the
excess cluster surface brightness as the difference between the two background
estimate (horizontal red dashed line). We added back this excess surface
brightness to obtain the corrected XMM-Newton data points (plotted in black).
The CXB correction results in better agreement between XMM-Newton and eROSITA,
especially in the $7.0-10.0^{\prime}$ radial range. The CXB subtracted surface
brightness profile is nearly flat beyond the $R_{500}$ and out to the
$R_{200}$. This may imply that at this regime, there is still some emission
either from the cluster or the filament in which the Northern Clump resides,
or both.

Through this exercise, we show the complementary nature of the three
instruments. For instance, Chandra (green points) with the best on-axis
spatial resolution can cover as far as $3^{\prime\prime}$ from the center,
XMM-Newton (blue and black points) covers the intermediate radial range with
good statistics, and eROSITA (red points) with its wide FoV and scan mode
delivers the cluster surface brightness out to $R_{200}$. The discrepancies in
the center can be well-explained by the PSF effect. Moreover, although masked,
the bright point source in the cluster center is likely to contribute some of
its emission to the central bins of eROSITA and XMM-Newton profiles. Other
causes, such as point source detection and different background treatment
could also play a role in the observed differences among the profiles.

###  3.3 Spectral analysis

Figure 9: Normalization (top), temperature (middle), and metallicity (bottom)
profiles of the Northern Clump derived in the energy band $0.7-7.0$ keV. Left:
Southern region profiles. The blue and red data points represent XMM-Newton
and Chandra, respectively. Right: Western region profiles. The cyan and
magenta data points represent XMM-Newton and Chandra, respectively. Due to
modest photon counts, Chandra metallicity could not be constrained in any
sectors. The central data points fitted with AGN component are shown in grey,
light green and dark green. These data points are shifted for visualization
purposes. The grey dashed lines indicate the $R_{2500}$ of the cluster. The
x-axis error bars are not the $1\sigma$ error range, but the full width of the
bins.

####  3.3.1 eROSITA cluster parameters

We list the eROSITA cluster parameters for the $k_{B}T_{500}$ region (an
annulus of $2.4-6.0^{\prime}$) in Table 4. The metallicity returned by eROSITA
is consistent with the one returned by XMM-Newton, both in the wide and the
small soft energy band, while the normalizations and temperatures are lower.
Fitting in the soft band does improve the consistency between eROSITA and XMM-
Newton temperatures while not fully solving the discrepancies. While this
could be an effect of instrumental calibration uncertainties, it could also be
caused by multi-temperature structure. As predicted by simulations, in the
case of multi-temperature plasma eROSITA that has superior soft response was
shown to return lower temperatures in comparison to other instruments
(Reiprich et al. 2013). Fitting in the soft band only should minimize such
effects as the relative effective areas are similar in this range for eROSITA
and XMM-Newton. This issue will be addressed further in the future, employing
also eROSITA mock observations based on hydrodynamical simulations, and
divising methodologies similar to the algorithm suggested by Vikhlinin (2006)
to predict the temperature in the case of multi-temperature plasma.

Table 4: eROSITA and XMM-Newton cluster parameters from the $k_{B}T_{500}$ region ($0.2-0.5R_{500}$). Instrument | $norm^{\dagger}$ | $k_{B}T$ | $Z$
---|---|---|---
|  $[$keV$]$ | $[Z_{\odot}]$
full energy band∗
eROSITA | $1.59_{-0.19}^{+0.17}$ | $1.67_{-0.11}^{+0.20}$ | $0.36_{-0.12}^{+0.11}$
XMM-Newton | $2.35_{-0.045}^{+0.046}$ | $1.99\pm 0.04$ | $0.31_{-0.026}^{+0.025}$
$0.8-2.0$ keV
eROSITA | $1.62_{-0.23}^{+0.26}$ | $1.70_{-0.11}^{+0.22}$ | $0.36_{-0.14}^{+0.19}$
XMM-Newton | $2.45_{-0.052}^{+0.053}$ | $1.99_{-0.054}^{+0.050}$ | $0.27_{-0.025}^{+0.027}$
† $[10^{-5}$ cm${}^{-5}/$arcmin${}^{2}]$
∗ $0.7/0.8-9.0$ keV for eROSITA TM8/TM9.
$0.7-7.0$ keV for XMM-Newton

####  3.3.2 Inner of south and west

We plot the results of the spectral analysis of XMM-Newton and Chandra in the
south and west directions of the Northern Clump in Fig. 9 left and right
panel, respectively. The top, middle, and bottom panels are the normalization
per area, temperature, and metallicity profiles. Due to modest photon counts,
the metallicities in Chandra could only be constrained using bins of full
azimuthal direction. The profile can be found in Appendix D, Fig. 22. The
information regarding the source regions and their best-fit parameter values
are listed in Table 8.

In both directions, we observe a flattening towards the central region of the
cluster. The temperature profile towards the south indicates an elevated
temperature around $2^{\prime}<r<3^{\prime}$
($119.76\mathrm{\>kpc}<r<179.64\mathrm{\>kpc}$). The metalicity also appears
enhanced in this region. Beyond the fourth bin, the temperature profile
flattens at an average temperature of 1.94 keV.

####  3.3.3 Outskirts of north, south, and west

Figure 10: XMM-Newton normalization (top), temperature (middle), and
metallicity (bottom) profiles of the outer regions of the Northern Clump in
the different directions in the energy band $0.7-7.0$ keV. The northern,
southern, and western regions are represented by orange, blue, and red data
points, respectively. The grey dashed (dotted) lines indicate the $R_{2500}$
($R_{500}$) of the cluster. The x-axis error bars are not the $1\sigma$ error
range, but the full width of the bins. The metallicity in the last two bins of
the south and west has been frozen to $0.3Z_{\odot}$.

We compare spectral properties of the north, south, and west directions over
$R_{2500}$ to $R_{500}$ ($r=4-12.5^{\prime}=239.52-733.53\mathrm{\>kpc}$) as
shown in Fig. 10. We compare only XMM-Newton values since, unfortunately, the
outer north of the Northern Clump is not covered by the Chandra observations.
We found it necessary to fix the metallicity in the last two bins of the south
and west to $0.3Z_{\odot}$. The best-fit temperatures and normalizations vary
within $1\sigma$ when we fixed the metallicity of south and west to the best-
fit value of the north or fixed the metallicity of the north to
$0.3Z_{\odot}$. In general, our treatment of metallicity does not change the
following results.

We note a higher normalization per area in the last two bins of the northern
region. In comparison to the south (west) third and fourth data points, the
normalization per area in the north are 56 (75)% and 83 (97)% higher,
respectively. From XMM-Newton residual image (Fig. 6, right), we estimated
that the excess emission in the north associated with the tail feature is
located at $\sim\\!7^{\prime}$ ($419.16\mathrm{\>kpc}$) from the center and
stretched out to $12.5^{\prime}$ ($733.53\mathrm{\>kpc}$). Therefore, we see
an agreement between the imaging and spectral analyses results. This tail
feature is often observed in other merging systems as a result of ram pressure
stripping. For example, the tail from M86 and M49 as they fall into the Virgo
Cluster (Randall et al. 2008; Su et al. 2019. Su et al. 2014, 2017a, 2017b,
Also see), the northeast tail feature observed in A2142 cluster (Eckert et al.
2014. Liu et al. 2018, Further confirmed through dynamical study of the member
galaxies in), and a more extreme case of complete stripping is observed in the
galaxy cluster Zwicky 8338 (Schellenberger & Reiprich 2015).

In the south, higher temperature in comparison to the other two directions is
apparent. At $11.25^{\prime}$ ($673.65\mathrm{\>kpc}$), the temperature in the
south is higher by 42 (20)% than in the north (west). This elevated
temperature may be associated with shock or compression heated gas as the
Northern Clump falls towards the south, into the A3391 cluster. At
$5.0^{\prime}$ ($299.4\mathrm{\>kpc}$), the normalization per area is higher
than the other two directions, which is in agreement with the residual image
(Fig. 6, right) and the southern $S_{X}$ profile (Fig. 7, right).

The properties of the outskirts of the Northern Clump are similar to those
systems listed above, particularly the case of the M49 group residing beyond
the virial radius of the Virgo Cluster (Su et al. 2019). In addition to the
stripped tail, a temperature enhancement is noted in front of the cold front
in M49, resembling the temperature elevation and surface brightness edge (see
Subsect. 3.2.2) observed at $\sim\\!11.25^{\prime}$ ($673.65\mathrm{\>kpc}$)
and $5^{\prime}$ ($299.4\mathrm{\>kpc}$), respectively in the south of the
Northern Clump. Our findings indicate that the Northern Clump may be
undergoing the same infalling process with M49.

Figure 11: Left: Profiles of apec normalization per area measured with XMM-
Newton (blue solid) and Chandra (red solid). Red dashed lines indicate the
best-fit 3D density profiles for an empirical density profile of Equation 9
through a joint-fit of XMM-Newton and Chandra normalization profile. The
corresponding normalization per area profiles derived with the best-fit 3D
density profiles are shown in the cyan line, displaying good agreement with
the measurements. Black dashed line is the deprojected density profile derived
from Chandra surface brightness profile and the Chandra spectral analysis of
the innermost bin. Right : Projected temperature profile measured with XMM-
Newton (blue solid line). The best-fit deprojected temperature profile is
shown in cyan. The corresponding projected temperature profile derived from
Equation 16 is shown in yellow, in good consistent with the measurement.

####  3.3.4 Central AGN

We notice a metallicity drop in the core of the Northern Clump. While it is
commonly believed that this phenomena, which is often observed in galaxy
groups and clusters with X-ray cavities, is the result of the AGN mechanical
feedback and/or depletion of Fe into cold dust grains (Panagoulia et al. 2015;
Lakhchaura et al. 2019; Liu et al. 2019), it could also be an artifact that
surfaces as the central AGN is not accounted properly in the spectral fitting
(Mernier et al. 2017). To investigate this, we therefore implemented the
second and third fitting methods (Subsect. 2.2.2). We present the best-fit
parameter values of the three methods in Table 7 and the spectra with their
best-fit model are displayed in Fig. 23.

Since the on-axis encircled energy fraction (EEF) for MOS1 camera is 68% at a
$15^{\prime\prime}$ radius444XMM-Newton Science Analysis System: Encircled
energy function, this indicates that there might be some AGN emission left in
the source region while performing the fit using the first method. While 92%
of the total energy are encircled within a $60^{\prime\prime}$ radius, having
a mask of this size in the core region is undesirable. The second and third
methods are a better solution to this problem. However, as noticed from the
metallicity profiles, the low metallicity persists even when the AGN component
is added.

The apparent low core metallicity is better explained by the above-mentioned
mechanical processes, for instance by the AGN feedback, through which a
fraction of the core metal-rich ICM gas is distributed outwards (Sanders et
al. 2016; Mernier et al. 2017) and/or depletion of core Fe into dust grains
(Panagoulia et al. 2015; Lakhchaura et al. 2019; Liu et al. 2019).

Additionally, we also observe lower Sunyaev-Zel’dovich (SZ) signal at the
center of the cluster, as shown in Fig. 12. The decrease of the SZ signal at
this region is likely due to the core of the radio AGN, and similarly seen at
the center of the Perseus Cluster (Erler et al. 2019, Figure 5 in. Lin et al.
2009; Gupta et al. 2017, For more detailed review on AGN contamination of SZ
clusters see). Unless special effort is done, such clusters may be missed in
SZ surveys.

(a)  Figure 12: Left: Planck y-map overlaid with X-ray (from XMM-Newton, in
magenta) and radio (ASKAP/EMU, in black) contours. Black line depicts the
$3\times R_{200}$ of A3391 and black dashed-line points toward the center of
A3391. Right: Radio+X-ray+optical composite image of the central region of the
Northern Clump. The image is generated utilizing ASKAP/EMU image (red color
and yellow contour), XMM-Newton image (blue color and white contour), and
DECam Sloan g-band image (green color).

####  3.3.5 Deprojection analysis and central entropy

We obtain the best-fit apec normalization profiles for Chandra spectra
extracted from concentric annuli ranging from $r=20$ kpc to $r=200$ kpc and
for XMM-Newton ranging from $r=20$ kpc to $r=400$ kpc as shown in Fig. 11
(left). We assume the deprojected density profile takes the form of

$n(r)=\sqrt{{n_{01}}^{2}\left(1+\frac{r^{2}}{{r_{c1}}^{2}}\right)^{-3\beta_{1}}+{n_{02}}^{2}\left(1+\frac{r^{2}}{{r_{c2}}^{2}}\right)^{-3\beta_{2}}}\,.$
(9)

A normalization profile can be derived by substituting the 3D density profile
(Equation 9) into Eq. 3. We can obtain the best-fit parameters for the
deprojected density profiles by fitting the normalization profiles measured
with Chandra and XMM-Newton to Equation 3, as shown in Fig. 11 (left). We use
the $R^{2}$-score to evaluate the goodness of the fit, defined as

$R^{2}(y,\hat{y})=1-\frac{\Sigma_{i=1}^{n}(y_{i}-\hat{y_{i}})^{2}}{\Sigma_{i=1}^{n}(y_{i}-\bar{y_{i}})^{2}}$
(10)

where $\hat{y_{i}}$ is the best-fit value of the $i$-th data point and
${y_{i}}$ is the measured value for total $n$ data points, and
$\bar{y}=\frac{1}{n}\Sigma_{i=1}^{n}y_{i}$. We obtain a $R^{2}$-score of 0.95
for the deprojected density analysis.

We also derived the deprojected density profiles using the Chandra surface
brightness profile following Hudson et al. (2010). We fit the surface
brightness profile of the west direction to a double-$\beta$ model, which
takes the form of

$S(r)=S_{01}\left(1+\frac{r^{2}}{{r_{c1}}^{2}}\right)^{(-3\beta_{1}+0.5)}+S_{02}\left(1+\frac{r^{2}}{{r_{c2}}^{2}}\right)^{(-3\beta_{2}+0.5)},$
(11)

corresponding to a density profile of Eq. 9. The central density
$n_{0}=\sqrt{{n_{01}}^{2}+{n_{01}}^{2}}$ can be estimated from

$n_{0}=\left[\frac{10^{14}4\pi(\Sigma_{12}\rm LI_{2}+LI_{1}){\it
D_{A}D_{L}}\zeta N}{\Sigma_{12}\rm
LI_{2}EI_{1}+LI_{1}EI_{2}}\right]^{\frac{1}{2}},$ (12)

where $\Sigma_{12}=S_{01}/S_{02}$ is the ratio of the central surface
brightness as in Eq. 11 and N is the normalization of the apec model for the
innermost bin of Chandra projected spectral analysis. Additionally, $D_{A}$
and $D_{L}$ are the angular diameter distance and the luminosity distance,
respectively, and $\zeta=1.2$ is the ratio of electrons to protons. ${\rm
EI}_{i}$ is the emission integral divided by the central density for component
$i$:

${\rm
EI}_{i}\equiv\int\left(\frac{n}{n_{0}}\right)^{2}dV=2\pi\int_{-\infty}^{\infty}\int_{0}^{R}x\left(1+\frac{x^{2}+l^{2}}{r_{ci}^{2}}\right)^{-3\beta_{i}}dxdl,$
(13)

where $R$ is the outer radius of the spectrum extraction region. The line
emission measure for component $i$, ${\rm LI}_{i}$, is defined as

${\rm
LI}_{i}\equiv\int_{-\infty}^{\infty}\left(1+\frac{l^{2}}{{r_{ci}}^{2}}\right)^{-3\beta_{i}}dl,$
(14)

and $n_{01}$ and $n_{02}$ can be further related by,

$\frac{{n_{01}}^{2}}{{n_{02}}^{2}}=\rm\frac{\Sigma_{12}LI_{2}}{LI_{1}}.$ (15)

We can solve these equations using the surface brightness parameters to obtain
a density profile of Eq. 9 as shown in Fig. 11 (left).

We adopted an empirical formula to model the deprojected temperature profile
(Andrade-Santos et al. 2017; Su et al. 2019)

$T_{\rm
3D}(r)=\frac{T_{0}}{[1+(r/r_{t})^{2}]}\times\frac{[r/(0.075r_{t})]^{1.9}+T_{m}/T_{0}}{[r/(0.075r_{t})]^{1.9}+1}.$
(16)

The corresponding projected temperature can be obtained by projecting the gas
density weighted $T_{\rm 3D}$ along the line of sight. We fit the azimuthally
averaged projected temperature profile measured with XMM-Newton to the 2D
formula,

$T_{\rm 2D}=\frac{\int{{\rho}_{g}}^{2}{T_{\rm
3D}^{1/4}dz}}{\int{{\rho}_{g}}^{2}{T_{\rm 3D}^{-3/4}dz}},$ (17)

for which the density profile derived with XMM-Newton is applied. The best-fit
2D and 3D temperature profiles are plotted in Fig. 11 (right). The 2D
temperature model and the measured projected temperature profile give a
$R^{2}$-score of 0.88. The discrepancies between XMM-Newton and Chandra
normalizations may be caused by, such as, different bin size, scattering,
point source detection, as well as instrumental calibration uncertainties.

The Northern Clump has an extrapolated central electron density of
$\lesssim\\!7\times 10^{-3}$ cm-3, falling short of $\sim\\!1.5\times 10^{-2}$
cm-3, a conventional threshold for cool-core clusters (Andrade-Santos et al.
2017; Su et al. 2020). Based on the deprojected density and temperature
profiles, we extrapolate an entropy ($K\equiv k_{B}T_{\rm gas}/n_{e}^{2/3}$)
of 30 keV cm2 for the central $r<0.004R_{500}$. According to Hudson et al.
(2010), the Northern Clump can be best described as a weak cool-core (WCC)
cluster.

####  3.3.6 Northern Filament

We consider the region between the virial radii of the Northern Clump and the
A3391 cluster as an inter-cluster filament as shown in Fig. 1. We used the
measured apec normalization to derive the electron density, $n_{e}$, of the
filament. We assume a simple geometry, such that the filament is a cylinder
with its axis in the plane of the sky and located at the mean redshift of both
clusters, $z=0.0533$, such that Eq. 3 can be rewritten as,

$\begin{split}n_{e}=&\left[1.52\times 10^{-10}\leavevmode\nobreak\
\mathrm{cm}^{-1}\times norm\times(1+z)^{2}\right.\\\
&\left.\times\left(\frac{D_{A}}{\mathrm{Mpc}}\right)^{2}\times\left(\frac{r}{\mathrm{Mpc}}\right)^{-2}\times\left(\frac{h}{\mathrm{Mpc}}\right)^{-1}\right]^{\frac{1}{2}},\end{split}$
(18)

where $D_{A}(z=0.0533)=214.09\leavevmode\nobreak\ \mathrm{Mpc}$, radius of the
cylinder $r=1.56\leavevmode\nobreak\ \mathrm{Mpc}$, and its height
$h=1.81\leavevmode\nobreak\ \mathrm{Mpc}$. The Hydrogen density is taken to be
$n_{H}\approx n_{e}/1.17$.

We present the results of the eROSITA spectral analysis for the Northern
Filament in Table 5. Comparing our results to the WHIM properties predicted by
the cosmological hydrodynamic simulations (Davé et al. 2001, e.g.,. Nicastro
et al. 2018; Kovács et al. 2019, For observational constraints of WHIM
properties see, e.g.,), we find that the best-fit projected temperature
obtained from the one-temperature (1T) component fit is beyond the expected
temperature range of $T=10^{5}-10^{7}\leavevmode\nobreak\ \mathrm{K}$, while
the electron density is within the expected range of $n_{e}\approx
10^{-6}-10^{-4}\leavevmode\nobreak\ \mathrm{cm}^{-3}$.

We further performed a two-temperature (2T) component fit, for which we fixed
the abundances of the filament to 0.1 of Asplund et al. (2009) due to the
statistics. We identify a cooler component whose temperature and electron
density are consistent with the properties of WHIM. The hotter component is
likely to be cluster emission from either or both the Northern Clump and the
A3391 cluster. Further study, for instance, constraining the properties of the
ICM in the outskirts of A3391, as well as calculating the surface brightness
profile along the filament, is needed to draw more conclusions (Veronica et
al. in prep.).

Table 5: eROSITA properties of Northern Filament. Fitting | $norm$ | $k_{B}T$ | $Z$ | $n_{e}$
---|---|---|---|---
$[10^{-3}$ cm${}^{-5}]$ |  $[$keV$]$ | $[Z_{\odot}]$ |  $[10^{-5}$ cm${}^{-3}]$
1T | $1.34_{-0.30}^{+0.64}$ | $1.52_{-0.54}^{+0.57}$ | $0.06_{-0.06}^{+0.19}$ | $4.85_{-0.58}^{+1.05}$
2T | $1.06_{-0.29}^{+0.25}$ | $1.72_{-0.29}^{+1.02}$ | $0.1$ | $4.32_{-0.65}^{+0.48}$
$0.22_{-0.19}^{+0.24}$ | $0.68_{-0.64}^{+0.38}$ | $1.99_{-1.24}^{+0.88}$

##  4 Insights from the Magneticum Simulation

In the $20\>$cMpc$/h$-scale region extracted from the Magneticum Simulation
volume, that comprises an analog of the A3391/95 system (see Reiprich et al.
2021) at $z=0.07$, we also identify various structures with the size of galaxy
groups (Biffi et al. 2021). Among these additional structures, we consider in
particular a small halo (hereafter, “group B”), in order to better interpret
the history of the observed Northern Clump. Group B is a group-size object
that has entered the atmosphere of the upper pair member (marked as “GC1”;
Biffi et al. 2021) at about $z=0.16$, and has reached its outskirts (at
$d_{\rm 3D}\sim 1.5\times R_{200}^{\rm GC1}$) by $z=0.07$. The configuration
at redshift $z=0.07$ of the simulated pair and the group B is shown in Figure
13. The Figure shows the gas density map projected along the $z$ axis of the
simulation box, and comprises a volume of $10\>$cMpc$/h$ per side, centered on
the pair center of mass. There we mark the positions of the two pair members,
GC1 and GC2, and of group B. Furthermore, we mark cluster radii that are of
interest for the present discussion.

Figure 13: Gas density map of the A3391/95 analog pair from the Magneticum
Simulation, at $z=0.07$. The region shown comprises a cubic volume of
$10\>$cMpc$/h$ per side centered on the pair center of mass. The two main
clusters GC1 and GC2 are marked, with their $R_{200}$ and $R_{500}$ (defined
w.r.t. the critical density of the Universe), as well as the position of group
B.

Group B has a mass of $M_{500}\sim 3.6\times 10^{13}\>{\rm M}_{\odot}$ at
redshift $z\sim 0.16$, just before the merging starts. At lower redshifts,
($0.07<z\lesssim 0.16$) the group is identified as a gravitationally-bound
substructure of the GC1 cluster. By tracking the history of its most-massive
progenitor, we investigate its properties along the trajectory towards the
node of the Cosmic Web where the cluster pair is finally settling by $z=0.07$.

In the simulation analog, we find that the group movement towards the GC1
cluster is well defined below $z\lesssim 1$, according to several indicators.
In Fig. 14 (upper inset) we show the redshift evolution of the alignment
between the subhalo velocity and the infall direction, quantified by the
cosine of the angle defined by those two directions. Different colors refer to
the direction towards the final positions at $z=0.07$ of the pair center of
mass (c.m.; blue) and the center of GC1 (red), as in the legend. We note that
values of the cosine below $z\sim 1$ are always comprised between $-0.8$ and
$-0.95$, indicating a preferential alignement towards the GC1 final position:
this confirms that group B is actually moving towards the GC1 cluster. This
result holds for both the total bound dark matter (DM; solid line) and gas
(dashed line) components and for all the gas (either bound or not) located
within $R_{500}$ (dotted lines). While the total bound gas follows more
closely the DM trend, the gas within $R_{500}$ is more subject to oscillations
due to the fact that the selection is purely geometrical and can therefore
comprise matter not yet gravitationally bound to the main halo potential. At
redshift $z\lesssim 0.2$ the cosine absolute value decreases and marks a
weaker alignment, and is also accompanied by a decrease of the total mass of
the DM and gas that are gravitationally bound to group B (lower inset). This
indicates a stripping process occurring while the group enters the cluster
virial radius: the bound gas mass, in particular, decreases by a factor of
$\sim 3$ between $z\sim 0.16$ and $z=0.07$.

The infall of group B towards GC1 is furthermore characterized by an
increasing difference between the radial components of gas and subhalo bulk
velocities below redshift $z\sim 0.25$. This is shown in Fig. 15, where we
report the radial components of bulk (solid) and bound gas (dashed) velocity,
as a function of the distance (comoving units) between the position of group B
at each redshift and the final position of GC1 (upper inset) — here larger
radial distances correspond to earlier times.

From the Figure, one can note the slowing down of the gas component with
respect to the DM (traced by the halo bulk velocity) at the smaller distances,
corresponding to the redshifts $0.07<z<0.25$. This is better quantified in the
lower inset of Fig. 15, where we report the relative difference between gas
and bulk radial velocities at each point. We note a monotonic increase of such
difference, while group B is getting closer than $\sim 2.5\>$cMpc$/h$ from the
final position of GC1, which corresponds to $z\lesssim 0.25$. At $z=0.25$, the
actual distance between the center of group B and the GC1 progenitor
corresponds to $d_{\rm 3D}\sim 3.5\>$cMpc$/h$, namely $d_{\rm 3D}\sim
1.7\times(R_{100}^{\rm B}+R_{100}^{\rm GC1})\sim 4\times R_{200}^{\rm GC1}$.
In the reference projection (Reiprich et al. 2021, $xy$ plane; see and Biffi
et al. 2021, for more details), this corresponds to a distance of $\sim
3\times R_{200}^{\rm GC1}$ from GC1. The projected configuration and distance
relative to the GC1 cluster found at $z=0.25$ are similar to the observed
Northern Clump case, supporting the possibility for the bigger accreting
cluster (A3391 in the observed system) and the infall movement to leave an
observable signature on the Northern Clump properties (such as the emission
tail — see Subect. 3.1). At such distances, we indeed expect to observe the
effects of the closeness to the GC1 cluster. In fact, this is also the typical
distance from clusters at which simulations predict to observe changes in the
gas properties of main filaments connected to them (namely at three to four
times the cluster virial radius; e.g., Dolag et al. 2006). A zoom-in onto the
group is shown in Fig. 17 by the simulated map of the projected gas density
within a $4\>$cMpc$/h$-side FoV centered on group B at redshift $z=0.25$
(left) and $z=0.07$ (right). In the maps, the movement of the group occurs
roughly in the top-right to bottom-left direction and the dot-dashed circle
indicates the radius corresponding to $\sim 3\times R_{200}^{\rm GC1}$ from
the GC1 progenitor at $z=0.25$. The black circles approximate the $R_{200}$
radius of group B at $z=0.25$ (solid line), and the $R_{200}$ radius of GC1 at
$z=0.07$ (dot-dashed line), when group B is no more identified as a separate
halo by our substructure-finding algorithm (SUBFIND, Springel et al. 2001;
Dolag et al. 2009).

We also find indications of halo sloshing throughout its evolution. This can
be inferred from the oscillations described in the radial velocity cosine
(upper inset in Fig. 14), when all the gas within $R_{500}$ is considered
(dotted curve) in comparison to the corresponding DM trend. This effect is
further confirmed by Fig. 16, where we report the redshift evolution of the
center shift for group B between $z\sim 1$ and $z=0.07$. The center shift is
computed as the three-dimensional difference between the halo center555The
halo center is identified with the position corresponding to the minimum of
the potential well. and the bound gas center of mass, in units of the group
$R_{500}$. In the Figure, we also report the separate components along the
major axes of the simulation volume, from which we note an oscillating
feature, especially between the $x$ and $y$ components — i.e. in the plane
perpendicular to the chosen l.o.s. This effect is likely connected to the
merging and accretion of smaller galaxy-size substructures (such as between
$z\sim 1$ and $z\sim 0.8$), which perturb the gas component in the group and
also produces elongated density contours.

Figure 14:  Redshift evolution of the cosine of the angle between the radial
direction towards the pair (center of mass, blue; GC1, red) and the velocity
of the subhalo (bulk, solid; bound gas, dashed; all gas within $R_{500}$,
dotted). The gas velocity alignment is reported only for the infall direction
towards GC1. Data points for all the gas within $R_{500}$ are only defined for
the redshifts when the clump is an independent group from GC1 ($z\gtrsim
0.16$). We also report the redshift evolution of the subhalo DM (solid lines)
and gas (dashed lines) bound mass in the lower inset of the figure.  Figure
15:  Top: radial component of the bulk (solid lines) and bound-gas velocity
(dashed lines) as a function of the radial distance from the final position of
GC1. Bottom: relative difference between bound-gas and bulk radial velocity,
as a function of the radial distance.  Figure 16:  Redshift evolution of the
center shift between the position of the halo center (coinciding with the
position of the minimum of the halo potential well) and the bound-gas center
of mass, in units of $R_{500}$. The three-dimensional value of the shift and
its three separate components along the major Cartesian axes of the
simulations are reported, as in the legend.

Figure 17:  Simulated projected maps of gas surface density, centered on the
infalling group B (black diamond marking its center), at $z=0.25$ (left) and
$z=0.07$ (right). The gas surface density map encloses a comoving volume of
$(4\,$cMpc$/h)^{3}$. In both panels, we mark the circles corresponding to
three times the GC1 cluster virial radius ($3\times R_{200}^{\rm GC1}$) and
$R_{200}$ of the group at $z=0.25$ (solid) and of the GC1 cluster at $z=0.07$
(dot-dashed; when group B is a substructure of GC1).

##  5 Discussion and conclusions

We presented results of the X-ray imaging and spectral analyses of the MCXC
J0621.7-5242 galaxy cluster, also known as the Northern Clump of the A3391/95
system, utilizing eROSITA, XMM-Newton, and Chandra observations. We used the
ASKAP/EMU radio data, Planck y-map, and DECam data to study the influence of
the wide-angle tail (WAT) radio source to the central intracluster medium
(ICM) gas. We compared the observations with an analog of the A3391/95 system
found in the Magneticum simulation. Our findings include:

  * •

The Northern Clump is not a relaxed system and its morphology deviates from
the common spherical or elliptical shapes. In fact, it has an interesting
pronounced boxy shape.

  * •

We identified a southern density enhancement, which indicates motion of the
filament gas relative to the Northern Clump resulting in compression, e.g., by
filament gas being accreted towards the Northern Clump while the whole system,
filament and the Northern Clump is being accreted towards A3391. The
temperature and density enhancement at around the same distance support this.

  * •

We detected a tail of emission north of the Northern Clump that supports the
relative motion as well. The tail starts at $\sim\\!7^{\prime}$
($419.16\mathrm{\>kpc}$) from the center with a projected length of
$\sim\\!5.3^{\prime}$ ($317.4\mathrm{\>kpc}$). We speculate that the gas is
either stripped from the Northern Clump and lagging behind, or infalling gas
from a possible continuation of the filament north of the Northern Clump.
While a more detailed analysis is required, the first picture is nevertheless
consistent with findings from the analog system identified in the Magneticum
simulations, where the gas in the clump start to lag behind with its infall
velocity reduced compared to dark matter, while getting closer than $3\times
R_{200}$ from the accreting cluster.

  * •

A bright WAT radio galaxy occupies the center of the Northern Clump. A weak
hint of X-ray surface brightness depressions coincident with the radio lobes
is found, suggesting interaction of the radio plasma with the Northern Clump
ICM.

  * •

We note that the morphology of the denser X-ray gas is asymmetric with a sharp
edge that points towards the A3391 center, in the southeast direction. The
lower density gas at around $\sim\\!5^{\prime}$ appears boxy, which may be the
result of ram pressure stripping as it enters the atmosphere of A3391 and/or
sloshing. However, the northeast bend of both lobes is not quite consistent
with them being “swept back” by that global motion. We speculate that the
lobes might have “escaped” some hot-gas confinement and the change of
direction is due to motion through a lower-density ambient medium. The central
radio source may be moving locally within the cluster toward the southwest as
a result of sloshing, while the Northern Clump is moving toward the southeast.

  * •

The electron density and entropy derived for the central region of the
Northern Clump indicates that the Northern Clump is a weak cool-core (WCC)
cluster. The temperature flattening towards the center further supports this
indication. The WCC nature could indicate some disturbance of the core either
through effects caused by entering the sphere of influence of A3391, through
infalling filament gas, or through the central radio AGN.

  * •

The low core metallicity derived for the core using three different methods
signifies AGN feedback blowing the metal-rich ICM gas outwards the central
region. This is consistent with the strong radio emission of the central
galaxy and with the fact that the cooling flow in this cluster is suppressed,
which as a result only forming a weak cool-core.

  * •

We performed spectral analyses in the region between the $R_{200}$ of the
Northern Clump and the A3391 cluster, the Northern Filament, where we assumed
a simple cylindrical geometry on the plane of the sky. Through a 2T fit, we
identified a cooler component whose temperature
($k_{B}T=0.68_{-0.64}^{+0.38}\leavevmode\nobreak\ \mathrm{keV}$) and electron
density ($n_{e}=1.99_{-1.24}^{+0.88}\times 10^{-5}\leavevmode\nobreak\
\mathrm{cm}^{-3}$) are consistent with the expected ranges of WHIM properties.

  * •

Analysis of a similar system found in the Magneticum simulation reveals that
an infalling clump (equivalent to the observed Northern Clump) starts to
“feel” the influence of the main accreting system (equivalent to the observed
A3391) at around $3\times R_{200}$, possibly related to accretion shocks
expected to be present at such distance. For instance, the infall velocity of
the gas component starts to decrease compared to the radial velocity of the
dark matter. Once the group approaches the cluster virial radius, it undergoes
further stripping processes with a decrease by a factor of $\sim\\!3$ in the
gravitationally-bound gas mass between $z=0.16$ and $z=0.07$. Throughout its
redshift evolution, we find evidences for halo sloshing marked by oscillations
of the infall velocity and the center shift of the gas component compared to
dark matter. In general, the findings in the Magneticum simulation are
consistent with the properties of the observed system.

Thanks to the wide eROSITA FoV and its superior soft response, significant
emission was observed in the A3391/95 system beyond the virial radii,
$R_{100}$, with good resolution. This has allowed us to discover the soft
X-ray emission from filaments connected to the system, as well as some
infalling clumps residing in these filaments, one of which is the Northern
Clump. This infalling galaxy cluster appears to have just entered the
atmosphere, or sphere of influence, of the A3391 cluster.

In this work, we demonstrated how eROSITA is complementary not only to other
X-ray instruments, e.g., XMM-Newton and Chandra, but also instruments from
other wavelengths, e.g., the ASKAP radio telescope, the DECam optical survey,
and the Planck microwave satellite. We also found that the eROSITA image of
the A3391/95 system and the results of our Northern Clump analysis are
consistent with each other, as well as with the analog simulated system from
the Magneticum simulations.

###### Acknowledgements.

We would like to thank the anonymous referee for their valuable feedback that
helped improve the manuscript. We thank Kaustuv moni Basu for discussion about
the Planck y-map. Funded by the Deutsche Forschungsgemeinschaft (DFG, German
Research Foundation) – 450861021. This research was supported by the
Excellence Cluster ORIGINS which is funded by the Deutsche
Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s
Excellence Strategy – EXC-2094 – 390783311. VB acknowledges funding by the
Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) — 415510302.
YS acknowledges support from Chandra grants AR8-19020A and GO1-22126X. AV is a
member of the Max-Planck International School for Astronomy and Astrophysics
(IMPRS) and of the Bonn-Cologne Graduate School for Physics and Astronomy
(BCGS), and thanks for their support. This work is based on data from eROSITA,
the soft X-ray instrument aboard SRG, a joint Russian-German science mission
supported by the Russian Space Agency (Roskosmos), in the interests of the
Russian Academy of Sciences represented by its Space Research Institute (IKI),
and the Deutsches Zentrum für Luft- und Raumfahrt (DLR). The SRG spacecraft
was built by Lavochkin Association (NPOL) and its subcontractors, and is
operated by NPOL with support from the Max Planck Institute for
Extraterrestrial Physics (MPE). The development and construction of the
eROSITA X-ray instrument was led by MPE, with contributions from the Dr. Karl
Remeis Observatory Bamberg and ECAP (FAU Erlangen-Nuernberg), the University
of Hamburg Observatory, the Leibniz Institute for Astrophysics Potsdam (AIP),
and the Institute for Astronomy and Astrophysics of the University of
Tübingen, with the support of DLR and the Max Planck Society. The Argelander
Institute for Astronomy of the University of Bonn and the Ludwig Maximilians
Universität Munich also participated in the science preparation for eROSITA.
The eROSITA data shown here were processed using the eSASS/NRTA software
system developed by the German eROSITA consortium. Partly based on
observations obtained with XMM-Newton, an ESA science mission with instruments
and contributions directly funded by ESA Member States and NASA. We
acknowledge the Wajarri Yamatji people as the traditional owners of the
Murchison Radio-astronomy Observatory, where ASKAP is located.

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##  Appendix A XMM-Newton – Lightcurves

Figure 18: Top panels show the lightcurves of MOS1 (top left), MOS2 (top
right), and pn (bottom) of the Northern Clump observation (ObsID: 0852980601).
The red line depicts the accepted portion of the observation, while the grey
is the rejected portion due to SPF. The dashed lines mark the $3\sigma$. The
bottom panels show the histograms of the associated lightcurves, fitted with
Gaussian (red curve). The red vertical lines mark the $3\sigma$.

##  Appendix B XMM-Newton – Images

Figure 19: XMM-Newton cleaned photon images, i.e., SPF filtered, QPB
subtracted, anomalous CCDs removed, in the energy band $0.5-2$ keV. Top left:
MOS1 image. Top right: MOS2 image. Bottom left: pn image. Bottom right:
MOS1+2+pn image.

##  Appendix C XMM-Newton – Surface brightness profile

The $S_{X}$ profiles of the three setups are compared in Fig. 20 (left). The
blue, green, and red solid lines are the $S_{X}$ profiles for circular annuli,
box annuli, and pn-only box annuli setup. The dotted lines of the respective
colors are their $\beta$-model fits. The $\beta$-model best-fit parameters can
be found in Table 6. In the bottom panel, their residuals are plotted in the
unit of $\sigma$.

Table 6: The $\beta$-model best-fit parameters. Setups | $S_{X}(0)$ | $\beta$ | $r_{c}$
---|---|---|---
$[$cts/s/arcmin${}^{2}]$ | $[^{\prime}]$
circular | $0.05\pm 0.004$ | $0.368\pm 0.007$ | $0.310\pm 0.04$
box | $0.0218\pm 0.001$ | $0.396\pm 0.010$ | $0.660\pm 0.053$
pn box | $0.0715\pm 0.002$ | $0.404\pm 0.011$ | $0.718\pm 0.053$

Figure 20: XMM-Newton $S_{X}$ profiles calculated from XMM-Newton data in the
energy band $0.5-2.0$ keV. Top: The full azimuthal $S_{X}$ profiles. The top
panel shows the $S_{X}$ profiles (solid lines) and the corresponding
$\beta$-model fits (dotted lines) of all XMM-Newton detectors with circular
annuli setup (blue) and box annuli setup (green), as well as the pn-only box
annuli setup (red). The bottom panel shows the residuals in unit of $\sigma$.
Bottom: XMM-Newton $S_{X}$ profiles of different sectors top and their
relative differences bottom from circular annuli setup. The grey shaded areas
are the $1\sigma$ confidence regions of the full azimuthal $S_{X}$. The brown
vertical lines indicate the various radii.

##  Appendix D Spectral analysis

We performed several tests using pgstat statistic provided in XSPEC, where the
XMM-Newton source data were treated as Poisson data (C-statistic) and the
background as Gaussian. Initially, we fit spectra that are grouped to have a
minimum of 5 counts (grouping5) and 25 counts (grouping25) per bin. Then, we
grouped the source spectra based on the background spectra, as such we would
have 25 counts per bin for the background spectra (bgdGrouping25). This
ensures the background spectra to be Gaussian. These tests were carried out
for all the Northern Clump spectral analysis regions. We show the results from
the full azimuthal region in Fig. 21 as a representative. In the plots we
compare the fitting results using $\chi^{2}-$statistic (blue points,
stat/dof=0.97), pgstat grouping5 (red, stat/dof=1.06), pgstat grouping25
(green, stat/dof=1.09), and pgstat bgdGrouping25 (orange, stat/dof=1.12). The
resulting cluster parameters of these tests in various regions are always
consistent within the $1\sigma$ error bars of the $\chi^{2}$-statistic fitting
results.

Figure 21: Normalization (top), temperature (middle), and metallicity (bottom)
profiles of the Northern Clump derived in the energy band $0.7-7.0$ keV. Blue
points are estimated using $\chi^{2}-$statistics, while the red, green, and
orange points are estimated using pgstat statistic with source spectrum
grouping 5, 25, and background spectrum grouping 25. Figure 22: Metallicity
profiles of various regions in the Northern Clump cluster in the energy band
$0.7-7.0$ keV. The blue and red shaded areas represent XMM-Newton and Chandra
metallicity in full azimuthal direction. Due to modest photon counts, Chandra
cannot constrain the metallicity. The dark blue, cyan, and magenta data points
represent XMM-Newton metallicity profiles in the south, west, and north
directions, respectively. The grey dashed lines indicate the $R_{2500}$ of the
Northern Clump. The x-axis error bars are not the $1\sigma$ error range, but
the full width of the bins.

Figure 23: XMM-Newton and RASS background spectra (black) of the central bin fitted with the three different central AGN models in the energy range of $0.7-7.0$ keV. Top: Method 1 – with $15^{\prime\prime}$ mask to mask the central AGN. Middle: Method 2 – AGN component is modeled from Chandra best fit values. Bottom: Method 3 – AGN is modeled and additional thermal component is added. Table 7: XMM-Newton best-fit values of central bin with different methods in energy $0.7-7.0$ keV. AGN | $norm^{\dagger}$ | $k_{B}T$ | $Z$
---|---|---|---
method |  |  $[$keV$]$ | $[Z_{\odot}]$
1 | $2.535_{-0.095}^{+0.096}$ | $2.532\pm 0.105$ | $0.217_{-0.056}^{+0.064}$
2 | $2.651_{-0.118}^{+0.12}$ | $2.411_{-0.116}^{+0.118}$ | $0.16_{-0.057}^{+0.065}$
3 | $1.682_{-0.277}^{+0.268}$ | $3.055_{-0.221}^{+0.208}$ | $0.155_{-0.05}^{+0.06}$
| $0.969_{-0.308}^{+0.344}$ | $1.528$ | –
† the presented values have been normalized
by their respective source areas
$[10^{-4}$ cm${}^{-5}/$arcmin${}^{2}]$
Table 8: XMM-Newton and Chandra best-fit spectral fitting values in the south, west, and north in the energy band $0.7-7.0$ keV. XMM-Newton | Chandra
---|---
annulus | $norm^{\dagger}$ | $k_{B}T$ | $Z$ | $norm^{\dagger}$ | $k_{B}T$
$[^{\prime}]$ |  $[10^{-4}$ cm${}^{-5}/$arcmin${}^{2}]$ |  $[$keV$]$ | $[Z_{\odot}]$ |  $[10^{-4}$ cm${}^{-5}/$arcmin${}^{2}]$ |  $[$keV$]$
South
$0.0-0.55$ | $2.539_{-0.094}^{+0.096}$ | $2.524_{-0.106}^{+0.107}$ | $0.214_{-0.056}^{+0.065}$ | $2.787_{-0.22}^{+0.22}$ | $2.74_{-0.36}^{+0.5}$
$0.55-1.65$ | $1.212_{-0.052}^{+0.051}$ | $2.593_{-0.123}^{+0.125}$ | $0.25_{-0.067}^{+0.082}$ | $1.137_{-0.046}^{+0.046}$ | $3.35_{-0.31}^{+0.43}$
$1.65-2.2$ | $0.66_{-0.043}^{+0.046}$ | $2.791_{-0.232}^{+0.25}$ | $0.528_{-0.157}^{+0.179}$ | $0.911_{-0.073}^{+0.073}$ | $1.87_{-0.19}^{+0.32}$
$2.2-2.75$ | $0.536_{-0.034}^{+0.034}$ | $2.732_{-0.178}^{+0.256}$ | $0.371_{-0.121}^{+0.152}$ | $0.636_{-0.041}^{+0.041}$ | $2.71_{-0.34}^{+0.52}$
$2.75-3.3$ | $0.482_{-0.033}^{+0.033}$ | $1.93_{-0.134}^{+0.131}$ | $0.227_{-0.067}^{+0.088}$ | $0.467_{-0.027}^{+0.033}$ | $1.86_{-0.22}^{+0.23}$
$3.3-3.85$ | $0.385_{-0.031}^{+0.029}$ | $1.756_{-0.098}^{+0.148}$ | $0.28_{-0.075}^{+0.1}$ | $0.395_{-0.028}^{+0.023}$ | $2.24_{-0.24}^{+0.4}$
$3.85-4.4$ | $0.295_{-0.026}^{+0.026}$ | $1.845_{-0.147}^{+0.131}$ | $0.477_{-0.113}^{+0.146}$ | $0.323_{-0.024}^{+0.024}$ | $1.89_{-0.2}^{+0.25}$
$4.4-4.95$ | $0.252_{-0.022}^{+0.024}$ | $1.9_{-0.155}^{+0.134}$ | $0.479_{-0.125}^{+0.156}$ | $0.324_{-0.022}^{+0.022}$ | $2.15_{-0.24}^{+0.44}$
$4.95-6.05$ | $0.157_{-0.013}^{+0.014}$ | $1.631_{-0.083}^{+0.067}$ | $0.29_{-0.075}^{+0.086}$ | $0.185_{-0.013}^{+0.013}$ | $2.17_{-0.24}^{+0.41}$
$6.05-8.45$ | $0.059_{-0.007}^{+0.008}$ | $1.732_{-0.069}^{+0.279}$ | $0.279_{-0.108}^{+0.171}$ | $0.047_{-0.004}^{+0.004}$ | $2.17_{-0.26}^{+0.58}$
$4.0-6.0$ | $0.211_{-0.011}^{+0.011}$ | $1.703_{-0.042}^{+0.103}$ | $0.344_{-0.054}^{+0.041}$ | * | *
$6.0-8.0$ | $0.068_{-0.009}^{+0.009}$ | $1.756_{-0.098}^{+0.261}$ | $0.3_{-0.109}^{+0.181}$ | * | *
$8.0-10.0$ | $0.024_{-0.004}^{+0.003}$ | $1.72_{-0.144}^{+0.326}$ | 0.3 | * | *
$10.0-12.5$ | $0.011_{-0.002}^{+0.003}$ | $1.709_{-0.368}^{+0.663}$ | 0.3 | * | *
West
$0.0-0.55$ | $2.536_{-0.095}^{+0.096}$ | $2.531_{-0.105}^{+0.105}$ | $0.217_{-0.056}^{+0.064}$ | $1.99_{-0.161}^{+0.108}$ | $3.55_{-0.55}^{+0.73}$
$0.55-1.65$ | $1.015_{-0.041}^{+0.041}$ | $2.811_{-0.144}^{+0.151}$ | $0.503_{-0.091}^{+0.104}$ | $1.129_{-0.034}^{+0.04}$ | $3.43_{-0.27}^{+0.3}$
$1.65-2.75$ | $0.532_{-0.023}^{+0.023}$ | $2.629_{-0.133}^{+0.153}$ | $0.299_{-0.075}^{+0.088}$ | $0.518_{-0.03}^{+0.034}$ | $3.5_{-0.69}^{+1.02}$
$2.75-3.85$ | $0.266_{-0.015}^{+0.016}$ | $2.548_{-0.168}^{+0.159}$ | $0.497_{-0.115}^{+0.138}$ | $0.291_{-0.013}^{+0.013}$ | $2.71_{-0.28}^{+0.33}$
$3.85-8.45$ | $0.081_{-0.006}^{+0.006}$ | $1.861_{-0.166}^{+0.142}$ | $0.35_{-0.089}^{+0.105}$ | $0.064_{-0.005}^{+0.005}$ | $2.79_{-0.47}^{+0.67}$
$4.0-6.0$ | $0.133_{-0.006}^{+0.006}$ | $1.958_{-0.098}^{+0.09}$ | $0.396_{-0.053}^{+0.082}$ | * | *
$6.0-8.0$ | $0.058_{-0.005}^{+0.005}$ | $1.666_{-0.166}^{+0.196}$ | $0.241_{-0.077}^{+0.08}$ | * | *
$8.0-10.0$ | $0.014_{-0.003}^{+0.002}$ | $1.363_{-0.128}^{+0.152}$ | 0.3 | * | *
$10.0-12.5$ | $0.002_{-}^{+0.002}$ | $1.365_{-0.499}^{+0.607}$ | 0.3 | * | *
North
$0.0-0.55$ | $2.537_{-0.095}^{+0.096}$ | $2.53_{-0.1}^{+0.11}$ | $0.22_{-0.06}^{+0.06}$ | * | *
$0.55-2.0$ | $0.989_{-0.056}^{+0.056}$ | $2.95_{-0.21}^{+0.21}$ | $0.6_{-0.14}^{+0.17}$ | * | *
$2.0-4.0$ | $0.363_{-0.024}^{+0.023}$ | $2.22_{-0.12}^{+0.19}$ | $0.42_{-0.1}^{+0.13}$ | * | *
$4.0-6.0$ | $0.159_{-0.016}^{+0.018}$ | $2.01_{-0.28}^{+0.29}$ | $0.18_{-0.1}^{+0.14}$ | * | *
$6.0-8.0$ | $0.069_{-0.016}^{+0.019}$ | $1.53_{-0.25}^{+0.38}$ | $0.12_{-0.1}^{+0.21}$ | * | *
$8.0-10.0$ | $0.056_{-0.015}^{+0.015}$ | $1.87_{-0.36}^{+0.71}$ | $0.22_{-0.19}^{+0.53}$ | * | *
$10.0-12.5$ | $0.063_{-0.022}^{+0.025}$ | $0.99_{-0.11}^{+0.08}$ | $0.11_{-0.07}^{+0.12}$ | * | *
† the presented values have been normalized by their respective source areas
* Chandra observations do not cover these regions