# The NuSTAR and Chandra view of CL 0217+70 and Its Tell-Tale Radio Halo

Ayşegül Tümer Department of Physics & Astronomy, The University of Utah, 115
South 1400 East, Salt Lake City, UT 84112, USA  Daniel R. Wik Department of
Physics & Astronomy, The University of Utah, 115 South 1400 East, Salt Lake
City, UT 84112, USA  Xiaoyuan Zhang Leiden Observatory, Leiden University, PO
Box 9513, 2300 RA Leiden, The Netherlands  SRON Netherlands Institute for
Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands  Duy N. Hoang
Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg,
Germany  Massimo Gaspari Department of Astrophysical Sciences, Princeton
University, 4 Ivy Lane, Princeton, NJ 08544-1001, USA  Reinout J. van Weeren
Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The
Netherlands  Lawrence Rudnick Minnesota Institute for Astrophysics, University
of Minnesota, 116 Church St. S.E., Minneapolis, MN 55455, USA  Chiara Stuardi
Dipartimento di Fisica e Astronomia, Università di Bologna, via Gobetti 93/2,
I-40129 Bologna, Italy  INAF - Istituto di Radioastronomia di Bologna, Via
Gobetti 101, I-40129 Bologna, Italy  François Mernier European Space Agency
(ESA), European Space Research and Technology Centre (ESTEC), Ke- plerlaan 1,
2201 AZ Noordwijk, The Netherlands  Aurora Simionescu SRON Netherlands
Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, The
Netherlands  Netherlands Institute for Radio Astronomy (ASTRON), P.O. Box 2,
7990 AA Dwingeloo, The Netherlands  Randall A. Rojas Bolivar Department of
Physics & Astronomy, The University of Utah, 115 South 1400 East, Salt Lake
City, UT 84112, USA  Ralph Kraft Harvard-Smithsonian Center for Astrophysics,
60 Garden Street, Cambridge, MA 02138, USA  Hiroki Akamatsu SRON Netherlands
Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
Jelle de Plaa SRON Netherlands Institute for Space Research, Sorbonnelaan 2,
3584 CA Utrecht, The Netherlands

###### Abstract

Mergers of galaxy clusters are the most energetic events in the universe,
driving shock and cold fronts, generating turbulence, and accelerating
particles that create radio halos and relics. The galaxy cluster CL 0217+70 is
a remarkable late stage merger, with a double peripheral radio relic and a
giant radio halo. A Chandra study detects surface brightness edges that
correspond to radio features within the halo. In this work, we present a study
of this cluster with NuSTAR and Chandra data using spectro-imaging methods.
The global temperature is found to be kT = 9.1 keV. We set an upper limit for
the IC flux of $\sim$2.7$\times$10-12 erg s-1 cm-2, and a lower limit to the
magnetic field of 0.08 $\mu$G. Our local IC search revealed a possibility that
IC emission may be dominant at the outskirts of a radio halo emission and
on/near shock regions within $\sim$0.6 r500 of clusters. We detected a “hot
spot” feature in our temperature map coincident a surface brightness edge, but
our investigation on its origin is inconclusive. If the “hot spot” is the
downstream of a shock, we set a lower limit of kT $>$ 21 keV to the plasma,
that corresponds to $\mathcal{M}$ $\sim$ 2\. We found three shock fronts
within 0.5 r500. Multiple weak shocks within the cluster center hint at an
ongoing merger activity and continued feeding of the giant radio halo. CL
0217+70 is the only example hosting these secondary shocks in multiple form.

X-rays: galaxies: clusters — galaxies: clusters: individual (CL 0217+70),
intracluster medium — methods: data analysis — radiation mechanisms: non-
thermal, thermal — shock waves

##  1 Introduction

Galaxy clusters are the largest gravitationally bound structures in the
universe, which contain hundreds of galaxies within a radius of 1–2 Mpc
(Abell, 1958). The intracluster medium (ICM) is an optically thin hot plasma
($\sim 10^{7}-10^{8}$ K) in X-rays, that fills in between the galaxies inside
galaxy clusters (e.g., Tümer et al. 2019, 2022).

Mergers between galaxy clusters are the most energetic (1063-1064 ergs) events
in the universe since the Big Bang. These mergers drive shocks that heat and
mix the thermal gas while also (re)accelerating electrons and cosmic rays
through Fermi-like acceleration processes across shocks and turbulent eddies
(Brunetti & Jones, 2014). Turbulence is a common phenomenon in the ICM,
recurrently driven by mergers at large scales (e.g., Gaspari & Churazov 2013;
Eckert et al. 2017), sustained by AGN feedback (e.g., Gaspari 2015; Hofmann et
al. 2016; Yang et al. 2019; Wittor & Gaspari 2020) and ram-pressure stripping
processes at small scales (e.g., De Grandi et al. 2016; Clavico et al. 2019).

The ICM carries the X-ray traces of mergers such as shock fronts that are
detected as spatial variations in the temperature and density (see, e.g.
Markevitch et al., 1999, and references therein). While surface brightness
(SB) edges hint at the existence of a shock or cold front, temperature
measurements at both sides of the SB edges can distinguish these two
scenarios. Shocks caused by mergers have relatively low Mach numbers,
$\mathcal{M}$ $\lesssim$ 3 (see, e.g. Gabici & Blasi, 2003; Ryu et al., 2003).

Mergers also produce relativistic particles emitting via non-thermal
synchrotron process that can be detected by radio observations. The diffuse
radio emission can be in the form of radio relics and radio halos. Relics are
defined as extended radio emission features that are not associated with an
active cluster radio galaxy or central region of a galaxy cluster (Giovannini
& Feretti, 2004). They are located at the peripheries of clusters and are
characterized by polarized radio emission with steep spectra and elongated
shapes (Ensslin et al., 1998). So far, $\sim$45 clusters are known to host
these structures and almost as many candidates (van Weeren et al., 2019).
Double radio relic systems, where relics appear on opposing sides of the
cluster, are even rarer. 17 such systems are known to date and only 7 of them
host giant radio halos (van Weeren et al., 2019).

Radio halos are Mpc size structures that are found at the center of
dynamically disturbed clusters with large scale particle acceleration
processes (Cassano et al., 2010; Feretti et al., 2012), yet they are not
ubiquitous (Schellenberger et al., 2019). While evidence suggests that radio
relics trace shock fronts from the initial collisions of clusters, the origin
of radio halos is less clear (Brunetti et al., 2012, see, e.g.). The
prevailing view is that the relativistic electrons producing radio halos are
accelerated by turbulence in the ICM.

Unlike relics, most radio halos have no known shock front-radio emission edge
connection except for a few cases (e.g., Abell 520 (Hoang et al., 2019), the
Toothbrush (van Weeren et al., 2016), the Coma Cluster (Brown & Rudnick, 2011;
Planck Collaboration et al., 2013), the Bullet Cluster (Shimwell et al.,
2014), and Abell 2146 (Hlavacek-Larrondo et al., 2017)). This small number of
cluster mergers with known shock front-radio edge connection suggests that
turbulence formed directly behind shocks is what accelerates the electrons in
radio halos, whether from the initial collision or a subsequent settling of
the ICM. The Bullet cluster may be an example of the former case (initial
collision), since its radio halo extends right up to the bow shock—driven by
the subcluster’s first core passage (Shimwell et al., 2014). In contrast, the
Coma cluster seems to be the only example of the latter scenario (subsequent
settling of the ICM) so far. Recent eROSITA images, combined with SZ and radio
data, convincingly illustrate the formation and connection to a radio halo
edge of a secondary shock (Zhang et al., 2020a). This secondary shock is also
detected in X-rays by Simionescu et al. (2013).

Table 1: Observation log.  | Observation ID | Start Date | Equatorial coordinates | Effective
---|---|---|---|---
|  | (YYYY-mm-dd) | (J2000) | exposure (ks)
NuSTAR | 70701001002 | 2021-07-04 | 02:16:58.2 +70:36:48 |  $\sim$230
| 70701001004 | 2021-07-08 | 02:17:06.9 +70:37:05 |  $\sim$106
Chandra | 16293 | 2014-12-01 | 02:16:49.00 +70:35:52.00 |  $\sim$25

In addition, the same relativistic electrons producing the synchrotron-powered
radio halo also upscatter cosmic microwave background (CMB) photons to X-ray
energies. The ratio of fluxes between the synchrotron and this inverse Compton
(IC) emission is simply the ratio of the energy densities of their respective
radiation fields: the magnetic field strength $B$ and the CMB, respectively
(Wik et al., 2014). Since the latter is very well known, an IC detection or
upper limit directly leads to an estimate or lower limit on the volume-
averaged value of $B$, a quantity that is poorly constrained in galaxy
clusters generally.

Past NuSTAR searches for IC emission (see, e.g. Wik et al., 2011, 2014; Cova
et al., 2019; Rojas Bolivar et al., 2021) constrain $B\gtrsim 0.1-0.2~{}\mu$G
comparable to the strength estimated from IC detections made with RXTE and
BeppoSAX. Those detections remain controversial, but if IC emission could be
definitively measured in a cluster, implying $B\sim 0.1~{}\mu$G, the dynamical
role of magnetic fields in clusters would be confirmed to be negligible, as is
currently assumed in simulations and mass scaling relations used in cluster
cosmology (see, e.g. Vikhlinin et al., 2009)).

One of these rare double radio relic system at z = 0.18 (Zhang et al., 2020b)
is CL 0217+70. This cluster hosts the most extended (3.5 Mpc) radio relic
known accompanied by a giant radio halo with the largest projected distance of
1.5 - 1.8 Mpc (Brown et al., 2011; Hoang et al., 2021). Its two radio relics
are mostly separated from its radio halo and lie at a large projected
distance, indicating the first core passage was not recent. Intriguingly, two
X-ray SB discontinuities or edges have recently been reported from a 25 ks
Chandra observation (Zhang et al., 2020b), which are roughly coincident with
spectral edges in the radio halo, i.e., locations where the spectral index
suddenly changes from flat to steep, suggesting a transition to a zone of
particle acceleration (Hoang et al., 2021). However, the nature of these X-ray
SB edges are unknown. A confirmation that at least one of these edges is a
shock front would add CL 0217+70 to a rare class of objects with the potential
to unravel how particles in radio halos are accelerated.

In addition, the Chandra image of CL 0217+70 exhibits a 1′-wide SB depression,
or “Channel” (Zhang et al., 2020b). Such structures can only exist if they are
supported by some form of non-thermal pressure: a locally enhanced value of
either $P_{B}$ or of the density of non-thermal particles. The former case
would suppress all X-ray emission, while the latter case would lead to
enhanced IC emission, more easily detected at hard X-ray energies.

In this work, we study CL 0217+70 using recent NuSTAR and archival Chandra
data to study the nature of these SB edges, as well as searching for global
and local IC emission.

The paper is organized as follows: observations, data reduction process and
the background assessment of the NuSTAR data are presented in Section 2. In
Section 3, methods used for the various analyses of the cluster and their
respective results are presented. We discuss our findings in Section 4, and
present our conclusions in Section 5.

Throughout this paper, we assume the $\Lambda$CDM cosmology with H0 = 70 km
s-1 Mpc-1, $\Omega_{M}$ = 0.3, $\Omega_{\Lambda}$ = 0.7. According to these
assumptions, at the cluster redshift (z = 0.18), a projected intracluster
distance of 100 kpc corresponds to an angular separation of $\sim$32$\arcsec$.
All uncertainties are quoted at the 68% confidence levels unless otherwise
stated.

##  2 Observations and data reduction

###  2.1 NuSTAR

In this work, we use NuSTAR (Nuclear Spectroscopic Telescope Array; Harrison
et al., 2013) observations of CL 0217+70 with Observation IDs 70701001002 and
70701001004 using both focal plane modules, namely, FPMA and FPMB. The
specifications of the data used in our analysis are summarized in Table 1.

Figure 1: Background subtracted, exposure corrected photon images of CL
0217+70 in the 3.0 - 10.0 keV NuSTAR (left panel), 10.0 - 20.0 keV NuSTAR
(middle panel), and 1.0 - 3.0 keV Chandra (right panel) bands. LOFAR contour
levels (white) correspond to $\sigma_{rms}\times$ [3, 6, 12, 24, 48]
($\sigma_{rms}$ = 330 $\mu$Jy/beam [46$\arcsec\times$45$\arcsec$]) (Hoang et
al., 2021). Red and blues arcs correspond to the location of northern and
southern SB edges proposed by Zhang et al. (2020b), respectively. The channel
region is indicated with dashed magenta rectangle.  Figure 2: LOFAR
26.6$\arcsec$ $\times$ 22.5$\arcsec$ resolution image (Hoang et al., 2021)
with 3.0-10.0 keV NuSTAR contours overlaid.

In order to filter the data, standard pipeline processing using HEASoft (v.
6.28) and NuSTARDAS (v. 2.0.0.) tools are used. To clean the event files, the
stage 1 and 2 of the NuSTARDAS pipeline processing script nupipeline are used.
Regarding the cleaning of the event files for the passages through the South
Atlantic Anomaly (SAA) and a “tentacle”-like region of higher activity near
part of the SAA, instead of using SAAMODE=STRICT and TENTACLE=yes calls, we
have created light curves and applied different filters to create good time
intervals (GTIs) manually without fully discarding the passage intervals.

The new set of GTIs are reprocessed with nupipeline stages 1 and 2, and images
are generated at different energy bands with XSELECT. To create exposure maps,
nuexpomap are used. To produce the corresponding spectra for the regions of
interest as well as the corresponding Response Matrix Files (RMFs) and
Ancillary Response Files (ARFs), stage 3 of nuproducts pipeline are used.

Main components of the NuSTAR background are instrument Compton scattered
continuum emission, instrument activation and emission lines, cosmic X-ray
background from the sky leaking past the aperture stops, reflected solar
X-rays, and focused and ghost-ray cosmic X-ray background. Modeling the
background where there is a lack of cluster emission regions is not
straightforward since the ICM emission becomes an additional component in the
background fitting procedure. To apply these models for the cluster background
assessment, a set of IDL routines called nuskybgd (Wik et al., 2014), which
defines the background spatially and spectrally, is utilized.

The procedure starts with selecting regions in the FOV, where the cluster
emission is inherently present yet not the most dominant. To account for the
ICM emission, an apec model is included in the full set of models, and jointly
fitted with the background. This global background model is then used to
create background images, which are then subtracted from the images and are
corrected by the corresponding exposure maps. Background subtracted, exposure
corrected images at different energy bands are presented in the left and
middle panels of Fig. 1. Region selection and the background fits are
presented in the Appendix section of this work.

Once the background is defined for any region in the FOV both spatially and
spectrally, the next step is to select regions of interest, to extract spectra
and the corresponding files and generating the specific background model,
followed by spectral fitting to evaluate the physical properties of the ICM.

###  2.2 Chandra

We use the observation and blank-sky background event files obtained from the
25 ks Chandra archival data (Observation ID: 16293) produced by Zhang et al.
(2020b) for the data analysis. We use Chandra Interactive Analysis of
Observations (CIAO) v4.12 package with CALDB 4.9.0 for extracting the spectra.
Background subtracted, exposure corrected image in the 1.0 - 3.0 keV band are
presented in the right panel of Fig. 1.

In addition to NuSTAR and Chandra data, we also represent the LOFAR data with
courtesy of Hoang et al. (2021, Fig. 4). The LOFAR image of the cluster
showing the relics and the central region is presented in Fig. 2.

##  3 Data analysis and results

###  3.1 Global properties

We began our analysis with the assessment of the global properties of the
cluster. We selected a circular region centered at the X-ray emission peak
with r = 5.2′ ($\sim$1 Mpc), following the extent of the radio halo emission.
We extracted a spectrum from NuSTAR which then was fit by a single temperature
apec model (Smith et al., 2001), i.e. constant $\times$ apec, i.e. 1T model,
with XSPEC (v. 12.11.1).

The redshift value of the cluster is freed to vary due to the gain issue of
NuSTAR (Rojas Bolivar et al., submitted). Once the best-fit values are found
for the redshift, we then freeze these parameters during error calculations.
We apply the maximum likelihood-based statistic (hereafter, C-stat)
appropriate for Poisson data as proposed by Cash (1979). Photon counts used in
spectral analysis are grouped to have at least 3 counts in each bin.

We then applied this method to the joint analysis of NuSTAR and Chandra data.
For all of the joint analyses, we convolve the model with TBabs since Chandra
is susceptible to NH, and we let this parameter free. CL 0217+70 lies within
9° of the Galactic plane and consequently suffers from a high foreground
column density ($N_{H}\sim 8\times 10^{21}$ cm-2), which absorbs most X-ray
photons with energies below a few keV. Near the plane, it is also expected
that the column density will be variable on arcminute scales, which cannot be
reliably predicted from HI estimates. Due to NuSTAR’s lack of bandpass
sensitivity below 3 keV, we are able to the ignore foreground absorption at
this level (Rojas Bolivar et al., 2021) for NuSTAR analysis for simplicity.
However, we report $\sim$0.2 keV difference with/without the inclusion of an
absorption model with $N_{H}\sim 8\times 10^{21}$ cm-2 for the global fit.
However, when the NH parameter is freed to vary, the face value becomes 1017
cm-2 where the lower limit hits the hard limit of zero with the higher limit
reaching on to 1019 cm-2.

Table 2: Spectral parameters and 1$\sigma$ uncertainty ranges of NuSTAR global spectrum fits in 3.0 - 20.0 band for 1T: constant $\times$ apec, 2T: constant $\times$ (apec \+ apec) and 1T + IC: constant $\times$ (apec \+ powerlaw). For the 2T model, abundance and redshift values of two apec components are tied to each other within instruments. apec normalization (n) is given in $\frac{10^{-14}}{4\pi\left[D_{A}(1+z)\right]^{2}}\int n_{e}n_{H}dV$ where powerlaw normalization ($\kappa$) is photons $\textit{keV}^{-1}~{}cm^{-2}~{}s^{-1}$ at 1 keV.  | 1T | 2T | 1T + IC
---|---|---|---
kT1 (keV) | 9.13 $\pm{0.12}$ | 10.60${}^{+0.79}_{-0.44}$ | 7.76${}^{+0.26}_{-0.25}$
Z1 (Z⊙) | 0.198$\pm{0.026}$ | 0.282${}^{+0.036}_{-0.034}$ | 0.399${}^{+0.069}_{-0.057}$
z1 | 0.222 | 0.222 | 0.222
norm1 (10-2) | 1.369 $\pm{0.018}$ | 1.139${}^{+0.060}_{-0.073}$ | 1.045${}^{+0.049}_{-0.046}$
kT2 |  $\cdots$ | 2.16${}^{+1.14}_{-0.52}$ |  $\cdots$
norm2 (10-2) |  $\cdots$ | 0.728${}^{+0.312}_{-0.156}$ |  $\cdots$
$\Gamma$ |  $\cdots$ |  $\cdots$ | 2.0 (fixed)
$\kappa$ (10-2) |  $\cdots$ |  $\cdots$ | 0.121${}^{+0.016}_{-0.018}$
C / $\nu$ | 1679.29/1686 | 1638.04/1684 | 1640.99/1685
Table 3: Same as Table 2 but for joint NuSTAR (3.0 - 20.0 keV) and Chandra (0.8 - 7.0 keV) joint global spectral fit. All models are convolved with TBabs. During the error calculations, redshifts (z) are fixed to the best-fit values. Subscripts “N” refer to NuSTAR, and “C” refer to Chandra parameters.  | 1T | 2T | 1T + IC
---|---|---|---
NH (1021 cm-2) | 9.17 $\pm{0.25}$ | 9.80$\pm{0.27}$ | 10.06$\pm{0.33}$
kT1 (keV) | 9.20${}^{+0.11}_{-0.12}$ | 21.56${}^{+2.71}_{-2.90}$ | 7.99${}^{+0.25}_{-0.24}$
Z1N (Z⊙) | 0.202 $\pm{0.026}$ | 0.214 $\pm{0.024}$ | 0.273${}^{+0.034}_{-0.033}$
Z1C (Z⊙) | 0.550${}^{+0.110}_{-0.105}$ | 0.572${}^{+0.102}_{-0.097}$ | 0.750${}^{+0.135}_{-0.128}$
z1N | 0.228 | 0.219 | 0.221
z1C | 0.188 | 0.185 | 0.186
norm1 (10-2) | 1.376$\pm{0.017}$ | 0.364${}^{+0.095}_{-0.062}$ | 1.081${}^{+0.048}_{-0.047}$
kT2 |  $\cdots$ | 6.36${}^{+0.40}_{-0.38}$ |  $\cdots$
norm2 (10-2) |  $\cdots$ | 1.133${}^{+0.061}_{-0.096}$ |  $\cdots$
$\Gamma$ |  $\cdots$ |  $\cdots$ | 2.0 (fixed)
$\kappa$ (10-2) |  $\cdots$ |  $\cdots$ | 0.107${}^{+0.018}_{-0.016}$
C / $\nu$ | 2104.29/2107 | 2067.33/2105 | 2071.99/2106

The spectral fit results of the parameters are presented in Table 2, and Table
3, for NuSTAR, and for joint NuSTAR and Chandra, respectively. To study the
non-isothermal gas expected from merging clusters of galaxies, as well as
possible non-thermal emission due to inverse Compton (IC) scattering, we
applied two more models to the spectra following the work of Rojas Bolivar et
al. (2021) and Wik et al. (2014).

We first added another apec component to the 1T model to describe a secondary
temperature structure, and fitted the spectra with constant $\times$ (apec \+
apec), namely 2T model. We then used another model 1T+IC : constant $\times$
(apec \+ powerlaw), accounting for the thermal emission and a possible IC
emission, where the IC component is represented with a power-law distribution.
We fixed the photon index to $\Gamma$ = 2 of the powerlaw obtained from the
spectral index found by Hoang et al. (2021).

The addition of a second apec and a powerlaw to the original single
temperature model for NuSTAR spectrum, resulted in an improvement of
$\Delta$C/$\Delta\nu$ = 41.25/2 and $\Delta$C/$\Delta\nu$ = 38.30/1 for 2T and
1T + IC model, respectively, with respect to single temperature fit. For the
joint NuSTAR and Chandra analysis, the fit also improved by an additional apec
or a powerlaw component, where we found $\Delta$C/$\Delta\nu$ = 36.96/2 and
$\Delta$C/$\Delta\nu$ = 32.30/1 for 2T and 1T + IC, respectively. Both NuSTAR,
and joint NuSTAR and Chandra analysis show that the central r = 5.2′emission
is best described by the 2T model.

We also calculated the luminosity of the cluster from the NuSTAR spectra
within this region (r = 312$\arcsec$ $\simeq$ 0.6 R500 adopting R500 =
500$\arcsec$ (Reiprich et al., 2013)). We find the X-ray bolometric (0.01-100
keV) luminosity to be LX,bol = 1.390 $\times$ 1045 erg s-1, and within 0.5-2.0
keV we find LX,0.5-2.0keV = 2.716 $\times$ 1043 erg s-1 using ${\tt XSPEC}$
convolution model ${\tt clumin}$, for the 1T model. Using E(z)=1.09 the
cosmological evolution factor Giles et al. (2016) at the cluster redshift, the
luminosity values become LX,bol = 1.275 $\times$ 1045 erg s-1 and
LX,0.5-2.0keV = 2.492 $\times$ 1043 erg s-1.

The resulting spectra are shown in the Appendix section of this work.

###  3.2 Surface brightness discontinuities

To study the correspondences of northern and southern Chandra SB edges (Fig.
1) with NuSTAR, we used Gaussian Gradient Magnitude (GGM)
filter111https://github.com/jeremysanders/ggm (Sanders et al., 2016). Assuming
Gaussian derivatives with a width of $\sigma$, the GGM filter calculates the
gradient of an image. To create the GGM filtered image, the image itself is
convolved by the gradient of a 1D Gaussian function for two axes, then these
two resulting images are combined for the 2D gradient image. The width,
$\sigma$, is varied to capture gradients on different scales, i.e., small
$\sigma$ values are used at the central regions where there are many counts,
and high values are better capturing the gradient at cluster outskirts with
low counts.

Figure 3: NuSTAR GGM filtered background subtracted, exposure corrected images
in the 3.0 - 30.0 keV (top panels), 3.0 - 10.0 keV (middle horizontal panels),
and 10.0 - 20.0 keV (lower panels) energy bands. White cross correspond to the
emission peak, red (blue) arc corresponds to the northern (southern) SB edge,
and dashed turquoise region represents the channel. The white arrow in all
images correspond to 1 Mpc.  Figure 4: $\sigma$ = 8, 3.0 - 10.0 keV NuSTAR GMM
(upper left ), NuSTAR 3.0 - 10.0 keV photon (upper right), Chandra 1.0 - 3.0
keV photon (lower left), and LOFAR 26.6$\arcsec$ $\times$ 22.5$\arcsec$
resolution (lower right) images. Dashed blue box corresponds to the same
regions in all images

Since NuSTAR PSF is much larger than the Chandra PSF for which the GGM filter
was intended for, the filters with low levels of sigma ($\sigma$ = 1, 2) were
mainly dominated by the noise. Therefore, we present the GGM filtered to the
background subtracted, exposure corrected images with using $\sigma$ = 4, 8,
and 16 detector pixels, as shown in Fig. 3.

GGM point to a sharp gradient best discerned in the panels presenting $\sigma$
= 8, 3.0 - 10.0 and 3.0 - 30.0 keV band, and $\sigma$ = 16, 3.0 - 10.0 keV
band, suggesting a SB edge. This edge can also be seen in NuSTAR, Chandra, and
LOFAR images. For a better visualization of this edge, we present the NuSTAR
GMM, NuSTAR photon, Chandra photon, and LOFAR images together in Fig. 4.

###  3.3 Temperature map

To study the temperature structure of the NuSTAR FOV, we created a temperature
map. We extracted count images from a 12$\arcmin$ $\times$ 12$\arcmin$ central
box region of the cluster in the 3.0 - 5.0, 6.0 - 10.0, and 10.0 - 20.0 keV
bands where the point sources detected by Chandra were excluded. We avoided
the 5.0 - 6.0 keV band due to the existence of FeK complex. The background
images and exposure maps were extracted for the same energy bands with
nuskybgd and nuproducts, respectively.

Figure 5: NuSTAR temperature map with radio contours from LOFAR (white, at
$\sigma_{rms}\times$ [3, 6, 12, 24, 48], where $\sigma_{rms}$ = 330
$\mu$Jy/beam [46$\arcsec\times$45$\arcsec$]) are overlaid. In the zoomed in
panel (black box, upper right corner), the “hot spot” is indicated by
turquoise circle, and the radio source is shown with blue circle. In the lower
right corner, the radio galaxy spectral map between 141 MHz (LOFAR) and 1.5
GHz (VLA) at 16$\arcsec$ resolution is shown Hoang et al. (2021, Fig. 6).

The background subtracted, exposure corrected images were then used for the
spectral extraction and predefined pixels were fitted with a single
temperature apec model, during which the abundance and redshift were fixed to
the values obtained from the global fit. The technique is described in detail
by Markevitch et al. (2000). The resulting temperature map is presented in
Fig. 5.

In the temperature map, we encountered a “hot spot”222“Hot spot” refers to the
feature of unknown origin seen in the temperature map, while without
quotations, we refer to a thermal plasma emission. that has a higher
temperature than the cluster core. In the vicinity ($\sim$ 0.5$\arcmin$) of
the “hot spot” (Fig. 5, turquoise circle), there is a radio source GB3
0212+704333https://vizier.cds.unistra.fr/viz-
bin/VizieR-5?-ref=VIZ620b3ce430f570&-out.add=.&-source=VIII/53/gb1&recno=3648
(Maslowski, 1972) (Fig. 5, blue circle) and LOFAR high resolution study shows
a tail morphology. The spectral index map between LOFAR 145 MHz and VLA 1.5
GHz also shows the steepening of the spectrum behind the tail. The tail radio
galaxy has flux density of S (141 MHz) = 12.1$\pm$1.2 mJy, S (1.4 GHz)= 2.9
+/- 0.2 mJy, S (1.5 GHz)= 2.7 +/- 0.2 mJy. The average spectral index is
$\alpha$= -0.62 $\pm$ 0.01.

Table 4: Spectral parameters and 1$\sigma$ uncertainty ranges of NuSTAR spectrum fits in 3.0 - 20.0 band of the “hot spot” scenarios.  | Hot spot | Obscured AGN | IC
---|---|---|---
NH (1022 cm-2) |  $\cdots$ | 5.11 ${}^{+3.63}_{-3.52}$ |  $\cdots$
kT (keV) | 12.10${}^{+2.01}_{-1.29}$ |  $\cdots$ |  $\cdots$
Z (Z⊙) | 0.612 ${}^{+0.388}_{-0.312}$ |  $\cdots$ |  $\cdots$
z | 0.249 ${}^{+0.046}_{-0.033}$ |  $\cdots$ |  $\cdots$
norm (10-4) | 1.497 ${}^{+0.187}_{-0.158}$ |  $\cdots$ |  $\cdots$
$\Gamma$ |  $\cdots$ | 2.34 ${}^{+0.18}_{-0.17}$ | 2.13 ${}^{+0.09}_{-0.08}$
$\kappa$ (10-4) |  $\cdots$ | 1.100${}^{+0.542}_{-0.356}$ | 0.657${}^{+0.120}_{-0.102}$
C / $\nu$ | 381.91/364 | 382.37/365 | 384.50/366

To study this feature in detail with NuSTAR, we extracted a circular region
with r = 0.6$\arcmin$ ($\sim$112 kpc) enclosing the most dominant region of
this “hot spot”, centered at 2:17:04.08 (RA) and +70:39:06 (Dec). The
temperature map indicates that, if of thermal origin, the temperature of the
plasma is $\geq$ 12 keV. This feature may be a part of a more extended
emission, yet it is difficult define this extension.

To explain its nature, we put forward three scenarios. Scenario 1 suggests
that the feature is the downstream shock region evidenced by its location on
the northern Chandra SB edge. Scenario 2 is an emission from a heavily
obscured AGN (since Chandra doesn not detect it) that may be due to a strong
chaotic cold accretion (CCA) rain that feeds the central supermassive black
hole (Gaspari et al., 2013, 2020). Scenario 3 is a localized IC emission where
the electrons in the nearby radio source is upscattered to X-ray regime by a
possible shock front.

We first fit the NuSTAR spectra with a single temperature apec model (Scenario
1). The temperature obtained from this fit was kT = 12.1${}^{+2.0}_{-1.3}$ keV
with C/$\nu$ = 381.91/364. To test Scenario 2, we used a powerlaw model
convolved with a TBabs model to account for a heavily obscured AGN. The
resulting hydrogen column density value is NH = 5.11$\times$1022 cm-2, where
the photon index for the hypothesized AGN emission is $\Gamma$ =
2.34${}^{+0.18}_{-0.17}$. The statistics of this fit is C/$\nu$ = 382.37/365.
And for Scenario 3, we used an unabsorbed powerlaw model to describe possible
IC emission. The resulting powerlaw slope is $\Gamma$ =
2.13${}^{+0.09}_{-0.08}$. The statistics of this fit is C/$\nu$ = 384.50/366.
The corresponding values and spectra of the fits from these scenarios are
presented in Table 4 and in the Appendix section, respectively. Statistics
show that these three scenarios are equally likely.

Lastly, to rule out a possible multi-temperature structure, we also fitted a
two temperature model to the spectrum. The high temperature component rose up
to kT $\sim$ 28.8 keV, where the low component was kT $\sim$ 7.41. In this
fit, both temperature parameters were unconstrained. In this localized spot, a
multi-temperature structure with such wide range temperatures is already
rather unphysical.

Figure 6: Regions selected for the cross-talk assessment.

###  3.4 Crosstalk analysis

Even single bright sources in the NuSTAR FOV causes the scatter photons due to
the large ($\sim$1$\arcmin$ Half Power Diameter, $\sim$18$\arcsec$ Full Width
at Half Maximum), slightly energy-dependent point spread function (PSF). This
results in a cross-contamination, namely crosstalk, of multiple emission
sources in the regions of interest, although the emission may not originate
from the selected region. nuproducts produces ARFs for point or diffuse
sources inside the user defined extraction regions. However, it does not
account for the ARFs for other sources whose emission originates outside these
extraction regions that contaminate the spectra of those regions. These ARFs
will be referred to as cross-ARFs.

Software composed of a set of routines created to account for this
contamination is nucrossarf444https://github.com/danielrwik/nucrossarf.
nucrossarf mainly calculates the cross-ARFs of a number of user defined
extracted regions, and disentangles the source distribution from the
contamination from other regions. For N extracted regions, nucrossarf
generates N $\times$ N ARFs to account for the wings of the PSF of each source
present in other regions. In this section we apply this new software,
nucrossarf.

In order to study the temperature jumps and prominent structures in the FOV
more accurately, we selected regions of interest in the NuSTAR data and used
nucrossarf to account for the crosstalk. The selected regions are overlaid on
the NuSTAR image on the left panel, and the corresponding region values are
presented in the right panel of Fig. 6.

Table 5: Crosstalk analysis spectral fit results where Region 6 is treated as a hot spot, obscured AGN and inverse Compton emission. In the obscured AGN scenario, the value NH is kept fixed at 5.11 $\times$ 1022 cm-2. ${\tt apec}$ normalization (norm) is given in $\frac{10^{-14}}{4\pi\left[D_{A}(1+z)\right]^{2}}\int n_{e}n_{H}dV$ (10-3 cm-5) where ${\tt powerlaw}$ normalization ($\kappa$) is photons keV-1 cm-2 s-1 at 1 keV (10-3).  |  $kt$ (keV) or $\Gamma$ |  norm or $\kappa$ |
---|---|---|---
Region | nuproducts | nucrossarf | nuproducts | nucrossarf | Notes
Number |  | Hot spot | AGN | IC |  | Hot spot | AGN | IC |
1 | 10.18${}^{+0.68}_{-0.33}$ | 9.90${}^{+1.90}_{-1.38}$ | 9.89${}^{+1.90}_{-1.38}$ | 9.90${}^{+1.90}_{-1.38}$ | 0.356${}^{+0.023}_{-0.021}$ | 0.459${}^{+0.056}_{-0.051}$ | 0.460${}^{+0.060}_{-0.051}$ | 0.459${}^{+0.056}_{-0.051}$ |
2 | 9.47${}^{+0.62}_{-0.73}$ | 7.47${}^{+2.44}_{-1.88}$ | 7.48${}^{+2.46}_{-1.87}$ | 7.43${}^{+2.45}_{-1.85}$ | 0.235${}^{+0.018}_{-0.019}$ | 0.217${}^{+0.061}_{-0.042}$ | 0.218${}^{+0.060}_{-0.042}$ | 0.219${}^{+0.061}_{-0.042}$ |
3 | 11.18${}^{+0.43}_{-0.20}$ | 12.53${}^{+0.94}_{-0.83}$ | 12.55${}^{+0.95}_{-0.82}$ | 12.51${}^{+0.94}_{-0.83}$ | 0.996${}^{+0.038}_{-0.034}$ | 1.406${}^{+0.060}_{-0.048}$ | 1.409${}^{+0.060}_{-0.048}$ | 1.407${}^{+0.061}_{-0.049}$ | Cin
4 | 10.61${}^{+0.49}_{-0.46}$ | 10.29${}^{+0.93}_{-0.71}$ | 10.25${}^{+0.91}_{-0.71}$ | 10.31${}^{+0.92}_{-0.71}$ | 0.573${}^{+0.030}_{-0.028}$ | 0.800${}^{+0.041}_{-0.042}$ | 0.801$\pm{0.041}$ | 0.799${}^{+0.041}_{-0.042}$ | Cout
5 | 9.92${}^{+0.16}_{-0.22}$ | 9.37${}^{+0.42}_{-0.44}$ | 9.35${}^{+0.42}_{-0.45}$ | 9.40${}^{+0.42}_{-0.44}$ | 3.452${}^{+0.089}_{-0.063}$ | 3.626${}^{+0.134}_{-0.123}$ | 3.635${}^{+0.139}_{-0.124}$ | 3.617${}^{+0.134}_{-0.121}$ |
6 | 12.02${}^{+1.76}_{-1.31}$ | 21.02 aaLower limit. |  $\cdots$ |  $\cdots$ | 0.149${}^{+0.017}_{-0.016}$ | 0.108$\pm{0.012}$ |  $\cdots$ |  $\cdots$ | Hot spot
6 | 2.34${}^{+0.18}_{-0.17}$ |  $\cdots$ | 2.34bbPhoton indices are fixed to the nuproducts values. |  $\cdots$ | 0.110${}^{+0.054}_{-0.036}$ |  $\cdots$ | 0.054$\pm{0.007}$ |  $\cdots$ | AGN
6 | 2.13${}^{+0.09}_{-0.08}$ |  $\cdots$ |  $\cdots$ | 1.38$\pm{0.26}$ | 0.066${}^{+0.012}_{-0.010}$ |  $\cdots$ |  $\cdots$ | 0.014${}^{+0.010}_{-0.006}$ | IC
7 | 9.29${}^{+0.27}_{-0.16}$ | 8.31${}^{+0.43}_{-0.41}$ | 8.57${}^{+0.45}_{-0.41}$ | 8.12${}^{+0.44}_{-0.40}$ | 1.862${}^{+0.051}_{-0.027}$ | 2.028${}^{+0.088}_{-0.082}$ | 2.014${}^{+0.084}_{-0.081}$ | 2.067${}^{+0.094}_{-0.089}$ | Nin
8 | 7.51${}^{+0.30}_{-0.53}$ | 6.70${}^{+0.52}_{-0.47}$ | 6.98${}^{+0.54}_{-0.49}$ | 6.56${}^{+0.51}_{-0.47}$ | 1.449${}^{+0.055}_{-0.114}$ | 1.342${}^{+0.098}_{-0.090}$ | 1.319${}^{+0.093}_{-0.086}$ | 1.374${}^{+0.105}_{-0.095}$ | Nout
9 | 8.98${}^{+0.39}_{-0.31}$ | 8.19${}^{+0.45}_{-0.44}$ | 8.20${}^{+0.45}_{-0.43}$ | 8.18${}^{+0.45}_{-0.43}$ | 1.527${}^{+0.061}_{-0.053}$ | 1.575${}^{+0.072}_{-0.064}$ | 1.578${}^{+0.072}_{-0.064}$ | 1.577${}^{+0.072}_{-0.065}$ | Sin
10 | 8.05${}^{+0.72}_{-0.59}$ | 6.02${}^{+0.97}_{-0.72}$ | 6.02${}^{+0.97}_{-0.72}$ | 6.02${}^{+0.98}_{-0.72}$ | 0.819${}^{+0.055}_{-0.054}$ | 0.645${}^{+0.090}_{-0.082}$ | 0.646${}^{+0.090}_{-0.082}$ | 0.645${}^{+0.090}_{-0.082}$ | Sout
11 | 7.13${}^{+0.54}_{-0.42}$ | 4.46${}^{+0.57}_{-0.62}$ | 4.44${}^{+0.56}_{-0.63}$ | 4.46${}^{+0.57}_{-0.61}$ | 1.195${}^{+0.075}_{-0.071}$ | 0.956${}^{+0.177}_{-0.112}$ | 0.960${}^{+0.181}_{-0.120}$ | 0.953${}^{+0.175}_{-0.119}$ |
12 | 4.72${}^{+0.29}_{-0.37}$ | 3.16${}^{+0.22}_{-0.20}$ | 3.15${}^{+0.22}_{-0.20}$ | 3.18${}^{+0.23}_{-0.21}$ | 2.921${}^{+0.226}_{-0.206}$ | 3.765${}^{+0.427}_{-0.380}$ | 3.803${}^{+0.426}_{-0.385}$ | 3.740${}^{+0.427}_{-0.379}$ |
13 | 9.97${}^{+0.82}_{-0.52}$ | 7.40${}^{+0.64}_{-0.60}$ | 7.37${}^{+0.63}_{-0.61}$ | 7.42${}^{+0.65}_{-0.60}$ | 1.819${}^{+0.104}_{-0.077}$ | 1.667${}^{+0.112}_{-0.107}$ | 1.672${}^{+0.123}_{-0.107}$ | 1.664${}^{+0.112}_{-0.107}$ |
14 | 8.25${}^{+2.26}_{-1.94}$ | 2.17${}^{+.....}_{-1.29}$ccUpper temperature limit hits 9 keV. | 2.20${}^{+.....}_{-1.31}$ccUpper temperature limit hits 9 keV. | 2.17${}^{+.....}_{-1.29}$ccUpper temperature limit hits 9 keV. | 0.052${}^{+0.016}_{-0.013}$ | 0.140${}^{+......}_{-0.130}$ddUpper norm limit hits 1 $\times$ 10-3 cm-5. | 0.138${}^{+......}_{-0.128}$ddUpper norm limit hits 1 $\times$ 10-3 cm-5. | 0.141${}^{+......}_{-0.131}$ddUpper norm limit hits 1 $\times$ 10-3 cm-5. | Channel
15 | 2.17$\pm{0.13}$ | 2.17bbPhoton indices are fixed to the nuproducts values. | 2.17bbPhoton indices are fixed to the nuproducts values. | 2.17bbPhoton indices are fixed to the nuproducts values. | 0.076${}^{+0.021}_{-0.017}$ | 0.040$\pm{0.006}$ | 0.040$\pm{0.006}$ | 0.040$\pm{0.006}$ | PS
16 | 1.76${}^{+0.14}_{-0.13}$ | 1.76bbPhoton indices are fixed to the nuproducts values. | 1.76bbPhoton indices are fixed to the nuproducts values. | 1.76bbPhoton indices are fixed to the nuproducts values. | 0.041${}^{+0.012}_{-0.010}$ | 0.019$\pm{0.003}$ | 0.019$\pm{0.003}$ | 0.019$\pm{0.003}$ | PS
17 | 2.79$\pm{0.17}$ | 2.79bbPhoton indices are fixed to the nuproducts values. | 2.79bbPhoton indices are fixed to the nuproducts values. | 2.79bbPhoton indices are fixed to the nuproducts values. | 0.183${}^{+0.062}_{-0.046}$ | 0.118${}^{+0.017}_{-0.016}$ | 0.118$\pm{0.016}$ | 0.118$\pm{0.016}$ | PS
Table 6: Mach numbers of shock fronts calculated from the temperature jumps obtained from the crosstalk results..  Regions | Mach Number
---|---
Post-shock | Pre-shock | Hot spot | Obscured AGN | IC
3 | 4 | 1.22 | 1.22 | 1.21
6 | 8 | 2.01 |  $\cdots$ |  $\cdots$
7 | 8 | 1.24 | 1.23 | 1.24
9 | 10 | 1.37 | 1.37 | 1.37

The motivation behind selecting regions are based on the recent Chandra study
proposing surface brightness edges, the results of our NuSTAR GGM and
temperature map results, and LOFAR images. Region 1 corresponds to the radio
SB depression as seen in the LOFAR image (Fig. 2, right panel). Region 2 is a
wedge, again evidenced by LOFAR image. We selected these regions to
investigate any correspondence of these radio features with the X-ray data.
Regions 3 (Cin) and 4 (Cout) correspond to the inner and outer side of the
central SB edge, respectively, as highlighted in Fig. 4. Region 5 represents
the central region of the cluster that excludes the emission from Regions 1,
2, 3, and 4. Region 6 is the “hot spot”, which we investigate with the
aforementioned three scenarios in Section 3.3. Regions 7 (Nin) and 8 (Nout)
are the inner and outer part of the northern SB edge, respectively, and
similarly, Regions 9 (Sin) and 10 (Sout) are the inner and outer sides of the
southern SB edge, respectively. Regions 11, 12 and 13 indicate the outer
cluster emission at different directions that is most likely isothermal.
Channel is represented by Region 14. Finally, Regions 15, 16, and 17 are point
sources as seen by Chandra and NuSTAR.

We extracted spectra from these 17 regions in total using nuproducts, and
found the best fit model for these regions. In total, we have 13 regions to
account for the ICM emission modeled by an apec, and 3 point sources (circular
regions in Fig. 6) modeled by a powerlaw. And for Region 6, we use three
different models as explained in detail in Section 3.3.

During the nucrossarf fit, we fitted 17 spectra, and their individual 17 ARFs
simultaneously, where we fixed the redshift and abundance values to the
individual nuproducts spectral fits. For Region 6, we studied the three
scenarios. We ran the nucrossarf code thrice, first treating Region 6 as a hot
spot, then obscured AGN and finally, localized IC emission. The results from
the nuproducts and nucrossarf spectral analysis are given in Table 5.

We note that Region 14, where Chandra detects the possible SB depression
(Channel), the upper limits for kT and apec normalization were quite large,
therefore we constrained the upper limits to 9 keV and 1 $\times$ 10-3 cm-5
for the temperature and apec normalization, respectively.

For all of the AGN powerlaw models, we fixed the photon index to the
nuproducts value to prevent an artificial gauge of any excess in the hard band
regime by the powerlaw model. Since these regions are treated as point sources
and since the regions attributed to them is comparable to the NuSTAR PSF,
their emission should be localized, meaning the shape of the slope would be
conserved.

To better visualise the effect of the inclusion of crosstalk analysis, we
present temperature maps created by filling the regions of interest with the
obtained temperature values from the fits in Fig. 7. This figure showing the
temperature results from all scenarios are also a testament to the stability
of the crosstalk analysis, as they visualize the almost identical temperature
values when we assume difference emission models for Region 6.

Figure 7: Temperature maps created with the results of nuproducts and
nucrossarf analysis. We use color black to fill in the regions where a power-
law model is used to describe the data, that is regions 15, 16 and 17 for all
maps, and Region 6 for obscured AGN and IC scenarios.  Figure 8: Chandra X-ray
surface brightness profiles (blue) and the best-fit model (red) of northern,
southern and central SB edges.

We then studied the Chandra SB profiles of the SB edges evidenced by NuSTAR,
Chandra, LOFAR images, as well as our GGM analysis and temperature maps. These
regions are 3 - 4 (Cin \- Cout), 7 - 8 (Nin \- Nout), and 9 - 10 (Sin \-
Sout), where the first region in these region couples are presumed post-shock
and the second one represents the pre-shock.

Although Zhang et al. (2020b) study the SB profiles of northern and southern
SB edges, we extracted new Chandra SB profiles from the exact regions used in
this work for precision. The profiles and the best fit model are shown in Fig.
8. Mach numbers obtained from the density jumps are, $\mathcal{M}$ = 1.35,
1.58, and 1.50 for the northern, southern and central edges, respectively. The
Mach numbers calculated by the temperature jumps from our crosstalk analysis
range between 1.22 - 1.37. In addition, we calculated the Mach number for the
temperature jump between regions 6-8, accounting for the possibility that “hot
spot” is truly a hot plasma where the post-shock is localized at Region 6,
i.e., a bullet-like feature, and Region 8 is the pre-shock region. This
approach results in a $\mathcal{M}$ $\sim$ 2\. These Mach numbers are
presented in Table 6.

The details of the nucrossarf analysis are presented in the Appendix section.

###  3.5 Local IC search

We did not find strong evidence for extended IC emission from our global
spectra fit results, but we continued to search for IC locally within the
radio halo, with the motivation that IC may start to dominate the emission in
narrower regions moving away from the center of the cluster where ICM
dominates. For this analysis, we used annular regions, where Hoang et al.
(2021) studies radio spectral indices and we extracted NuSTAR spectra from 5
annuli as shown in Fig. 9. Although 6 regions are selected by Hoang et al.
(2021, Fig. 7) including a central circular region, we exclude that central
region from our analysis expecting for the ICM to dominate the total emission,
hence we studied 5 annular regions. We applied the same analysis done for the
NuSTAR global spectra to these annular spectra. We also allowed for metal
abundances to be free. Model parameter results are shown in Table 7.

Figure 9: Selected regions (white annuli) for IC search overlaid on NuSTAR 3.0
- 10.0 keV image. VLA L-band D configuration (magenta, at
3$\sigma_{rms}\times$ [1, 2, 4, 8] levels, where ($\sigma_{rms}$ = 70
$\mu$Jy/beam), and LOFAR (blue, at $\sigma_{rms}\times$ [3, 6, 12, 24, 48].
($\sigma_{rms}$ = 330 $\mu$Jy/beam [46$\arcsec\times$45$\arcsec$]) radio
contours are also shown.

Annulus 1 spectrum is best described by a single temperature model, whereas
for annuli 2 - 3 - 4, 2T model describes the spectra the best-fit models. The
outermost annulus, annulus 5, is best described by 1T+IC model, but almost
equally well as the 2T model does. This region roughly passes through the
Regions 8 and 10 of our crosstalk analysis.

Table 7: IC search sprectral fit results from selected annuli shown in Fig. 9. Model norms are described in Table 2. IC flux is given for the 20.0 - 80.0 keV band in 10-13 erg s-1 cm-2.  Annulus |  | $kt$ | Z1 | norm |  $kt$ or $\Gamma$ |  norm or $\kappa$ | IC |
---|---|---|---|---|---|---|---|---
Number | Model | (keV) | (Z⊙) | (10-3) | keV or $\cdots$ | (10-3 cm-5) or (10-4) | Flux | C / $\nu$
1 | 1T | 10.28${}^{+0.21}_{-0.20}$ | 0.31$\pm{0.06}$ | 2.808${}^{+0.062}_{-0.061}$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 1218.21 / 1318
| 2T | 10.34${}^{+0.23}_{-0.21}$ | 0.33${}^{+0.07}_{-0.06}$ | 2.786${}^{+0.065}_{-0.067}$ | 0.16${}^{+0.27}_{-0.13}$ | 5.390E7${}^{+1.572E10}_{-5.401E7}$ |  $\cdots$ | 1216.92 / 1316
| 1T + IC | 10.28${}^{+0.25}_{-0.28}$ | 0.31$\pm{0.07}$ | 2.373${}^{+0.102}_{-0.231}$ | 2.10 (fixed) | 0.300${}^{+0.880}_{-0.300}$ | 0.46 | 1218.21 / 1317
2 | 1T | 9.42${}^{+0.21}_{-0.20}$ | 0.31$\pm{0.06}$ | 2.846${}^{+0.069}_{-0.068}$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 1385.03 / 1365
| 2T | 10.34${}^{+0.91}_{-0.54}$ | 0.36$\pm{0.07}$ | 2.527${}^{+0.178}_{-0.371}$ | 2.33${}^{+2.13}_{-1.02}$ | 0.915${}^{+1.235}_{-0.423}$ |  $\cdots$ | 1380.11 / 1363
| 1T + IC | 8.90${}^{+0.37}_{-0.43}$ | 0.38${}^{+0.08}_{-0.07}$ | 2.384${}^{+0.239}_{-0.240}$ | 2.10 (fixed) | 1.853${}^{+0.937}_{-0.934}$ | 2.85 | 1381.48 / 1364
3 | 1T | 8.80${}^{+0.26}_{-0.27}$ | 0.13$\pm{0.06}$ | 2.165${}^{+0.066}_{-0.065}$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 1321.22 / 1351
| 2T | 12.91${}^{+2.71}_{-2.78}$ | 0.22${}^{+0.11}_{-0.08}$ | 1.371${}^{+0.329}_{-0.294}$ | 2.87${}^{+1.30}_{-1.64}$ | 1.689${}^{+1.809}_{-0.334}$ |  $\cdots$ | 1303.74 / 1349
| 1T + IC | 6.84${}^{+0.68}_{-0.60}$ | 0.24${}^{+0.09}_{-0.08}$ | 1.464${}^{+0.163}_{-0.162}$ | 2.05 (fixed) | 2.842${}^{+0.611}_{-0.610}$ | 5.25 | 1306.49 / 1350
4 | 1T | 7.34${}^{+0.28}_{-0.27}$ | 0.05${}^{+0.06}_{-0.05}$ | 1.897${}^{+0.079}_{-0.078}$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 1329.64 / 1361
| 2T | 7.70${}^{+0.39}_{-0.33}$ | 0.09${}^{+0.07}_{-0.06}$ | 1.778${}^{+0.094}_{-0.102}$ | 0.39${}^{+0.25}_{-0.16}$ | 1.271E4${}^{+2.378E8}_{-1.319E4}$ |  $\cdots$ | 1324.24 / 1359
| 1T + IC | 7.23${}^{+0.37}_{-0.59}$ | 0.05${}^{+0.06}_{-0.05}$ | 1.837${}^{+0.89}_{-0.215}$ | 2.10 (fixed) | 0.231${}^{+0.841}_{-0.231}$ | 0.36 | 1329.53 / 1360
5 | 1T | 7.38$\pm{0.39}$ | 0.12$\pm{0.08}$ | 1.491${}^{+0.087}_{-0.079}$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 1308.92 / 1435
| 2T | 24.40${}^{-25.08}_{-15.63}$ | 0.17${}^{+0.12}_{-0.08}$ | 0.348${}^{+0.722}_{-0.193}$ | 4.62${}^{+1.35}_{-2.57}$ | 1.457${}^{+0.208}_{-0.546}$ |  $\cdots$ | 1304.14 / 1433
| 1T + IC | 5.55${}^{+1.01}_{-1.14}$ | 0.26${}^{+0.19}_{-0.13}$ | 0.975${}^{+0.228}_{-0.198}$ | 2.15 (fixed) | 2.300${}^{+0.798}_{-0.943}$ | 2.94 | 1304.25 / 1434

##  4 Summary and Discussion

CL 0217+70 hosts a giant radio halo and double radio relics pointing to a late
stage merger. The location of radio relics points to a merger plane
perpendicular to the line of sight. Motivated by the SB edges detected by
Chandra (Zhang et al., 2020b) at the central region of the cluster, we
observed the cluster with NuSTAR with the aim of studying the nature of these
SB edges. We applied spectro-imaging methods using NuSTAR and Chandra data,
and relying on the recent LOFAR study by Hoang et al. (2021).

###  4.1 Global view

We first studied the photon images of NuSTAR, Chandra and LOFAR data. To
capture any strong SB gradients, we used GGM filter on the NuSTAR photon
images and we saw that an interesting SB edge feature consistently appears at
the central region of the cluster with NuSTAR, Chandra and LOFAR images as
well as NuSTAR GGM filter.

Background extracted, exposure corrected images (Fig. 1) of NuSTAR and Chandra
data point to a localized, enhanced emission at the central region, where we
assigned Region 3 to, for our crosstalk analysis (Fig. 6). Furthermore, GGM
and LOFAR images suggest a sharp edge between Regions 3 and 4, and the
correspondence of this X-ray edge with the radio band, respectively, as seen
in Fig. 4.

In addition, NuSTAR image at hard (10.0 - 20.0 keV) energy band hints at high
temperature or the existence of non-thermal emission enclosed by the SB edges
suggested by Zhang et al. (2020b) using Chandra data. We found that the this
hard excess is due to the hot ICM, given that we did not find strong IC
component. We do not directly detect a clear SB depression at the proposed
(Zhang et al., 2020b) “Channel” region from NuSTAR images.

The offset between the diffuse radio and X-ray central halo peak seen in the
photon images (Fig. 1), as well as the NuSTAR SB and radio contours overlaid
on the temperature map (Fig. 5), is also observed by Brown et al. (2011) for
this cluster, as well as in Coma cluster Deiss et al. (1997). This is thought
to be due the non-equilibrium state of the merger system (Brown et al., 2011).

The global analysis of the cluster constitutes the selection of a circular
region (5.2$\arcmin$) containing the radio halo (roughly enclosing Regions
1-9), extraction of spectra of this region from both NuSTAR and Chandra data,
and fitting the spectra with single temperature, two temperature and single
temperature plus non-thermal emission models. The purpose of this analysis is
to assess the global properties of the cluster and to search for possible
extended IC emission in the cluster. Assuming an isothermal plasma, the global
temperature found by NuSTAR is kT = 9.1$\pm{0.1}$ keV, and for joint NuSTAR
and Chandra fit, we find kT = 9.2$\pm{0.3}$ keV. The results of NuSTAR, and
joint NuSTAR and Chandra data analyses show that the statistics improve with
an additional apec or powerlaw component, with respect to a single temperature
apec model. The powerlaw component was more dominant in NuSTAR fit than joint
NuSTAR and Chandra fit, due to the band pass of Chandra being narrower than
that of NuSTAR. Therefore, joint fit captures more of the thermal emission.
Two temperature model for the NuSTAR fit has a high temperature component of
TH = 10.6${}^{+0.8}_{-0.4}$ keV and a lower temperature component with TL =
2.2${}^{+1.1}_{-0.5}$ keV. However, our further detailed analyses on the the
temperature structure, i.e., temperature map and crosstalk analysis, do not
show signs of dominant emission around this low temperature value. Apparently,
the multi-temperature structure of the cluster would theoretically be better
described with a multi-temperature model, yet the temperature values of
various regions within the radio halo being high and close to each other makes
it statistically difficult to disentangle these components.

In the joint NuSTAR and Chandra fit, however, we witnessed a subtle high
temperature component of TH = 21.56 ${}^{+2.71}_{-2.90}$ keV and a dominant
low component of TL = 6.4$\pm{0.4}$ keV. The lower limit found for Region 6
for the hot spot scenario in the crosstalk analysis results, is actually
within 1$\sigma$ of this global high temperature component. The lower
component, however, is too low with respect to what we see in the central
region throughout this work, and also with respect to the findings of Zhang et
al. (2020b, Table 1). Overall, the statistics for both NuSTAR, and joint
NuSTAR and Chandra fit improved by adding a second apec model with
$\Delta$C/$\Delta\nu$ = 41.25/2 and $\Delta$C/$\Delta\nu$ = 36.96/2,
respectively.

The addition of a powerlaw component to the single temperature model improved
the statistics by $\Delta$C/$\Delta\nu$ = 38.3/1 and $\Delta$C/$\Delta\nu$ =
32.3/1 for NuSTAR, and joint NuSTAR and Chandra analysis, respectively.
Statistically a 2T fits the data better than a 1T+IC description (Table 2 and
Table 3). The temperature component of the 1T+IC model is $\sim$8 keV. NuSTAR,
and joint NuSTAR and Chandra analysis results agree within 1$\sigma$.

Although the data is better described by the 2T model, we provide IC flux only
to report an upper limit. The powerlaw component was more dominant in the
NuSTAR fit than that of the joint fit. Therefore, we use NuSTAR fit results
for our flux calculations. The upper limit to the IC flux in the 20.0 - 80.0
keV band was calculated using the cflux model applied to the powerlaw of the
NuSTAR fit. The resulting flux is 2.695 $\times$ 10-12 erg s-1 cm-2. Using the
LOFAR data (Hoang et al., 2021), this refers to a lower limit of 0.082 $\mu$G
for the average magnetic field.

###  4.2 The “hot spot”

Our temperature map in Fig. 5 shows a hot spot ($\sim$12 keV) at the location
of the northern SB edge. We extracted NuSTAR spectra from this region to study
the nature of the emission. We propose three scenarios to explain the emission
from this source, which are (1) a hot spot where the shock heated gas is
concentrated at that region (bullet-like), (2) a highly obscured AGN, and (3)
a localized IC emission connected to the radio source in the vicinity.

Shocks driven by a piston-like object, like a cool core, do not produce shocks
with uniform Mach number across it, but will have higher Mach numbers near the
piston and lower farther away. If the “hot spot” is actually of thermal
origin, this would be a similar case as in the bullet cluster. If the shock
surface is considered as a 2D sheet, the middle will have the highest Mach
number (in this case $\mathcal{M}$ = 2), and it will be lower everywhere else.
At the hot spot-Region 8 interface the temperature jump will be highest and
the particle acceleration will be most efficient.

Supported by theoretical and high-resolution hydrodynamical simulations
(Gaspari et al. 2017, 2019), AGN in massive hot halos often become heavily
obscured via the CCA triggered by the top-down multiphase condensation of warm
and cold clouds, which rain onto the central supermassive black hole. Such
obscuring CCA rain has been also probed in lower energy bands, e.g., with ALMA
and MUSE absorption/emission-line features (Rose et al. 2019; Olivares et al.
2022; Temi et al. 2022).

However, at the “hot spot” location, there are no known AGNs. We also carried
out a simple mid-IR selection criteria following (Somboonpanyakul et al.,
2022) using AllWISE2020 Catalog
555https://irsa.ipac.caltech.edu/Missions/wise.html. Within r=1$\arcmin$ of
the “hot spot” position, we found an IR object with ID J021703.24+703909.0 and
found W1-W2=-0.598. This value is much lower than what is expected from an AGN
emission (Somboonpanyakul et al., 2022). However we note that, mid-IR
selection for AGN mostly select extremely bright AGN (high accretion rate).
This implies that the object may not be a strong AGN, but could still be a
fainter/smaller AGN.

Furthermore, the vicinity of the radio galaxy may indeed be causing the IC
emission due to the reacceleration of relativistic particles injected from the
site, or by adiabatic compression, the merger shock may be “re-energizing” the
radio plasma. The radio galaxy spectral index found by LOFAR is within
1$\sigma$ of the IC emission photon index we found.

Our NuSTAR analysis using nuproducts shows that all three scenarios are almost
equally likely as represented in Table 4. However, 2T model fit to the joint
NuSTAR and Chandra data revealing a similar high temperature (Table 3) for the
global assessment with the crosstalk results for Region 6 of the hot spot
(Table 5) may not be a coincidence. Although we would expect NuSTAR to capture
the higher temperature component model better with respect to Chandra, the
addition of the Chandra data may have helped better constraining the lower
temperature model. Since Chandra data is quite shallow and foreground
absorption is moderately high, it is difficult to make a conclusion.

###  4.3 Crosstalk analysis and Shock fronts

To account for the scattering due to NuSTAR PSF, we applied nucrossarf to
disentangle the emission originating from a specific region, from the other
regions in the field of view. Our crosstalk analysis (Table 5) reveal several
temperature jumps between the regions selected with the guidance of NuSTAR,
Chandra and LOFAR images, as well as temperature and GGM maps. These weak
jumps occur at Region 7-8 (northern) and 9-10 (southern), and Region 3-4
(central) interface. We calculated Mach numbers for these jumps, and found
$\mathcal{M}$ = 1.22 - 1.37 (Table 6), hinting at multiple weak shocks within
the radio halo. Our Chandra SB profiles show that these temperature jumps have
corresponding density jumps with $\mathcal{M}$ = 1.35 - 1.58. The physical
distances of the temperature (density) jumps are $\sim$500 ($\sim$440) kpc,
$\sim$650 ($\sim$780) kpc, and $\sim$100 ($\sim$72) kpc from the emission peak
center for the northern, southern and central fronts, respectively.

On the technical side, we present a new code in this work, nucrossarf, aimed
at accounting for the X-ray scattering in and out of spectral extraction
regions within the NuSTAR FOV. We ran the code for three different scenarios
for 17 sources, where the temperature and model normalization values of all
sources are almost identical for the scenarios that points to the stability of
the code. Furthermore, nucrossarf and nuproducts temperature values are in
agreement within 1$\sigma$ for Regions 1-2-4-5-8-9 and 2$\sigma$ for Regions
3-7-10. Temperature values for Regions 11-12-13, that are at the outskirts of
the cluster drop by $\sim$2 keV, pointing to a contamination of hard excess
from the points sources enclosed by them, which is supported by the fact that
the normalizations of these point source emissions are reduced by a factor of
2 after the crosstalk treatment. Regions 6- the “hot spot”, and Region 14 -the
Channel temperatures from nuproducts are quite different from nucrossarf
results.

For the “Channel” region, we see that the temperature and normalization
parameters were poorly constrained. The scattered emission from the
neighbouring regions may be responsible for this instability, hinting at a
very faint emission at the Channel region.

The total fit C statistics for the crosstalk analyses are C/$\nu$ =
14669.14/15218, C/$\nu$= 14694.08/15219, C/$\nu$ = 14666.06/15218 for hot
spot, obscured AGN, and IC scenarios that were assumed to explain the Region 6
emission, respectively. Although the best fit is obtained for the IC scenario,
at this depth, all scenarios are almost equally likely.

###  4.4 IC search

We also carried out a local search for IC for regions where radio spectral
indices are given by Hoang et al. (2021) inside the radio halo. The outermost
annulus fit results show that 1T+IC model describes the spectra the best, but
almost equally well with 2T model. For this region, we find an upper limit of
2.64 $\times$ 10-13 erg s-1 cm-2 in the 20.0 - 80.0 keV band for the IC flux,
if were detected. This region where 1T+IC model and 2T models have quite
similar statistics, passes through Region 8 and 10 that is the outer region to
the northern and southern shock fronts. Whereas annulus 3 has the highest IC
flux, again has similar statistics with 2T model, and coincides with the
northern and southern SB edges, that is expected due to shock enhancement
(Sarazin, 2002, see, e.g.).

##  5 Conclusion

Based on the NuSTAR and Chandra data, complemented by a previous LOFAR study
(Hoang et al., 2019), we studied the late stage galaxy cluster merger CL
0217+70 . We present our conclusions of this work in this section.

All three scenarios, namely; hot spot, obscured AGN and localized IC emission,
proposed for the “hot spot” captured by our temperature map seem to be equally
likely. If the “hot spot” is indeed a thermal plasma of $\sim$12 keV, it
indicates a shock front $\sim$500 kpc away from the cluster emission peak. For
the obscured AGN or IC emission scenarios deep optical and deep low frequency
radio observations are required to study the galaxy distribution at the site.
In essence, what is witnessed here may be even a combination of a hot spot +
IC.

We confirm two shock fronts at Region 7-8 (northern) and 9-10 (southern)
suggested by Zhang et al. (2020b), and detect another shock front at Region
3-4 interface (central), within 0.5 r500 of the cluster. The physical
distances of the temperature (density) jumps are $\sim$500 ($\sim$440) kpc,
$\sim$650 ($\sim$780) kpc, and $\sim$100 ($\sim$72) kpc from the emission peak
center for the northern, southern and central fronts, respectively. These weak
shocks may be partially responsible for sustaining the giant radio halo
emission by the (re)acceleration of the relativistic particles at the central
region of the cluster.

The axis connecting these northern and southern secondary shock fronts are
almost perpendicular to the axis connecting the radio relics, suggesting that
these secondary shocks are unrelated to the first core passage. The
coincidence of the northern and southern shock fronts with the radio halo
emission edge adds this cluster a short list of clusters that suggests the
radio halos formation may be boosted by the turbulence formed directly behind
shocks. Deeper high angular resolution X-ray observations should reveal $\sim$
100 kpc scale turbulence eddies, taking advantage of the merger plane lying
perpendicular to the line of sight.

To the best of our knowledge, this cluster is the second known case where a
secondary shock coinciding with the radio halo emission in the form of
subsequent settling of the ICM is observed within 1 Mpc, the first being Coma
(Simionescu et al., 2013).

Unlike Coma, CL 0217+70 shows multiple shock fronts, which makes this cluster
the only known case where multiple secondary shock fronts are formed within
0.5 r500. We may have captured this cluster before the settling of the ICM,
hence, further studies beyond local systems are needed to be the carried out
to investigate the ubiquity of these secondary shocks to understand the
evolution of galaxy cluster mergers and radio halo formation mechanisms.
Galaxy clusters hosting giant radio halos can be the first place to look for
similar cases.

AT & DRW acknowledges support from NASA NuSTAR GO grant 80NSSC22K0066 and from
NASA ADAP award 80NSSC19K1443. MG acknowledges partial support by NASA Chandra
GO9-20114X and HST GO-15890.020/023-A, and the BlackHoleWeather program. RJvW
acknowledges support from the ERC Starting Grant ClusterWeb 804208. CS
acknowledges support from the MIUR grant FARE “SMS”. AT thanks Ross Silver and
Taweewat Somboonpanyakul for valuable discussions. This research has made use
of data from the NuSTAR mission, a project led by the California Institute of
Technology, managed by the Jet Propulsion Laboratory (JPL), and funded by by
the National Aeronautics and Space Administration (NASA). In this work, we
used the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the
ASI Science Data Center (ASDC, Italy) and the California Institute of
Technology (USA). The data for this research have been obtained from the High
Energy Astrophysics Science Archive Research Center (HEASARC), provided by
NASA’s Goddard Space Flight Center. This research has also made use of data
obtained from the Chandra Data Archive and the Chandra Source Catalog, and
software provided by the Chandra X-ray Center (CXC) in the application package
CIAO.

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##  Appendix A NuSTAR Background assessment

In this section, we present the spectra from the background model fits using
both FPMs and both Obs. ID in Fig. 10. As shown in the left panel of Fig. 10,
we selected 4 regions in the FOV from each detector chip, excluding the ICM
emission as much as possible but also including as much data as available.
Region selection is somewhat an experienced guess, and we tested the fits for
smaller and larger regions to optimize the stability of the fit. The method is
described on Wik et al. (2014).

Figure 10: Selected regions for background analysis (left panel) laid over
cleaned NuSTAR 3-10 keV photon images. White squares indicate regions where
the background spectra is extracted. Joint-fit of background and cluster
emission of NuSTAR FPMA (middle panel) and FPMB (right panel) for Obs. ID
70701001002 (upper panel) and Obs. ID 70701001004 (lower panel). Each color
represents a region selected for the background fit. For plotting purposes,
adjacent bins are grouped until they have a significant detection at least as
large as 15$\sigma$, with maximum 15 bins.  Figure 11: Global fits of NuSTAR
(upper panel), and joint NuSTAR and Chandra data (lower panel) with 1T (left
panel), 2T (middle panel), and 1T + IC (right panel) of the spectra extracted
from the central 5$\arcmin$.2 circular region. Black indicates FPMA and red
indicates FPMB for ObsID 70701001002, whereas for ObsID 70701001004, FPMA and
FPMB are indicated by blue and orange, respectively. Chandra ACIS-I is
indicated by turquoise. The data and models are shown as the higher curve and
lower lines correspond to background spectrum. The dashed curves correspond to
the model components to visualize their contribution to the composed model.
The “ratio” panel shows data to model ratios describing the goodness of the
fit. For plotting purposes, adjacent bins are grouped until they have a
significant detection at least as large as 8$\sigma$, with maximum 12 bins.

##  Appendix B NuSTAR and Chandra global fit spectra

We present the results of the global spectral fit in this section for only
NuSTAR and joint NuSTAR and Chandra data using 1T, 2T, and 1T + IC models.
Since the statistics are quite similar, the differences in ratios are not
discernible, yet the model contribution curves provide an insight to the
contribution of different models.

##  Appendix C “hot spot” scenarios

The resulting spectra from the three scenarios proposed for the explanation of
the Region 6 emission is presented in this section. The statistics are again
similar, which makes it difficult to conclude from the figures which model
best describes the data. However, in the spectra, we encountered a strange dip
around 4.8 keV. We extracted a spectra from a larger (1$\arcmin$) circular
region at the location at the “hot spot”, as well as different data and
plotting bins. The dip feature still persists for those spectra as well,
pointing to a reason different from a mere coincidence of statistical
fluctuations. We could not found an explanation for this feature, and we
refrain from any speculations.

Figure 12: Spectra of the NuSTAR “hot spot” region fits with constant $\times$
apec (left panel), constant $\times$ TBabs $\times$ powerlaw (middle panel),
and constant $\times$ powerlaw (right panel). Black indicates FPMA and red
indicates FPMB for ObsID 70701001002, whereas for ObsID 70701001004, FPMA and
FPMB are indicated by green and blue, respectively. For plotting purposes,
adjacent bins are grouped until they have a significant detection at least as
large as 4$\sigma$, with maximum 8 bins.

##  Appendix D nucrossarf

With this detailed study, we also happen to test the new nucrossarf code. In
this section, we elaborate the inner workings of the code, and what to expect
from the code for future users.

The final fitted spectra for the obscured AGN scenario is presented in the
left panel of Fig. 13 as an example. This figure shows the spectra from each
source, the total ARF curve and the individual contributions from all regions,
for two observations and for both FPMA and FPMB: (1+1+17) $\times$ 2 $\times$
2 curves in total. In order to present a much less complicated version, we
plotted the spectrum from Region 1, the total ARF curve and the individual
contributions from all regions to Region 1 emission, for Obs ID 70701001002
and for FPMA as seen in the right panel of Fig. 13

As seen in the nucrossarf spectral fit of Region 1 spectra for ObsID
70701001002 FPMA, the shape of the weakest model seen in the right panel in
Fig. 13 has an artifact- a dip within 5.0 - 10.0 keV. This model corresponds
to the scattering from Region 17 to Region 1. We further investigated the
spectra for similar artifacts and saw that point source crosstalk
contributions to the regions that are far away from these sources show a
similar behaviour. These curves are 15-10, 15-4, 16-2, 16-9, 16-14, 16-17,
17-1, 17-2, 17-12, 17-16, where the first numbers in the couples point to the
origin of the scattered light, and the seconds correspond to where the
scattered light reaches. These crossarfs with artifacts for Obs ID 70701001002
are plotted in the lower panel of Fig. 14, where the upper panel shows the
entirety of the ARFs.

Figure 13: Total cross-talk spectra for the AGN scenario final fit, (left
panel) and the spectra for the AGN scenario, for Region 1, ObsID 70701001002
FPMA (right panel). In the right panel, black indicates the data, orange
presents the total model, blue curve is the local contribution model from
Region 1 and gray curves represent the scattering from other regions.

Since for the production of ARFs, the PSF images at various bands are used,
these artifacts are bound to happen for faint point sources that are close to
the edge of the FOV where the calibration for PSF is less than perfect.
nucrossarf samples the PSF images for extended sources therefore averaging out
very small fluctuations. Therefore we do not see any similar artifacts from
Region 14, Channel, which is also close to the edge of the FOV and has faint
emission. At very low effective area, the level of the artifacts are
comparable to data noise. We emphasize that, at this rate, ARF contributions
where we see the dip artifact are effectively zero ($\sim$10-9 counts/s) and
the artifacts do not have an effect on any of our results.

Figure 14: All cross-ARFs for Region 15 (black), 16 (purple) and 17 (pink)
from Obs ID 70701001002 (upper panel), and only the cross-ARFs of artifacts
(lower panel), namely cross-ARFs: 15-10, 15-4, 16-2, 16-9, 16-14, 16-17, 17-1,
17-2, 17-12, and 17-16.