# 2FHL J1745.1–3035: A Newly Discovered, Powerful Pulsar Wind Nebula Candidate

S. Marchesi Dipartimento di Fisica e Astronomia (DIFA), Università di Bologna,
via Gobetti 93/2, I-40129 Bologna, Italy  Department of Physics and Astronomy,
Clemson University, Kinard Lab of Physics, Clemson, SC 29634-0978, USA  INAF -
Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero
Gobetti, 93/3, 40129, Bologna, Italy  J. Eagle NASA Goddard Space Flight
Center, Greenbelt, MD 20771, USA  M. Ajello Department of Physics and
Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC 29634-0978,
USA  D. Castro Harvard-Smithsonian Center for Astrophysics, Cambridge, MA
02138, USA  A. Domínguez IPARCOS and Department of EMFTEL, Universidad
Complutense de Madrid, E-28040 Madrid, Spain  K. Mori Columbia Astrophysics
Laboratory, Columbia University, New York, NY 10027, USA  L. Tibaldo IRAP,
Université de Toulouse, CNRS, CNES, UPS, 9 Avenue Colonel Roche, 31028
Toulouse, Cedex 4, France  J. Tomsick Space Sciences Laboratory, University of
California, Berkeley, 7 Gauss Way, Berkeley, CA 94720-7450, USA  A. Traina
INAF - Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via
Piero Gobetti, 93/3, 40129, Bologna, Italy  Dipartimento di Fisica e
Astronomia (DIFA), Università di Bologna, via Gobetti 93/2, I-40129 Bologna,
Italy  C. Vignali Dipartimento di Fisica e Astronomia (DIFA), Università di
Bologna, via Gobetti 93/2, I-40129 Bologna, Italy  INAF - Osservatorio di
Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti, 93/3, 40129,
Bologna, Italy  R. Zanin Cherenkov Telescope Array Observatory gGmbH, Via
Piero Gobetti, 93/3, 40129, Bologna, Italy

# 2FHL J1745.1–3035: A Newly Discovered, Powerful Pulsar Wind Nebula Candidate

S. Marchesi Dipartimento di Fisica e Astronomia (DIFA), Università di Bologna,
via Gobetti 93/2, I-40129 Bologna, Italy  Department of Physics and Astronomy,
Clemson University, Kinard Lab of Physics, Clemson, SC 29634-0978, USA  INAF -
Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero
Gobetti, 93/3, 40129, Bologna, Italy  J. Eagle NASA Goddard Space Flight
Center, Greenbelt, MD 20771, USA  M. Ajello Department of Physics and
Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC 29634-0978,
USA  D. Castro Harvard-Smithsonian Center for Astrophysics, Cambridge, MA
02138, USA  A. Domínguez IPARCOS and Department of EMFTEL, Universidad
Complutense de Madrid, E-28040 Madrid, Spain  K. Mori Columbia Astrophysics
Laboratory, Columbia University, New York, NY 10027, USA  L. Tibaldo IRAP,
Université de Toulouse, CNRS, CNES, UPS, 9 Avenue Colonel Roche, 31028
Toulouse, Cedex 4, France  J. Tomsick Space Sciences Laboratory, University of
California, Berkeley, 7 Gauss Way, Berkeley, CA 94720-7450, USA  A. Traina
INAF - Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via
Piero Gobetti, 93/3, 40129, Bologna, Italy  Dipartimento di Fisica e
Astronomia (DIFA), Università di Bologna, via Gobetti 93/2, I-40129 Bologna,
Italy  C. Vignali Dipartimento di Fisica e Astronomia (DIFA), Università di
Bologna, via Gobetti 93/2, I-40129 Bologna, Italy  INAF - Osservatorio di
Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti, 93/3, 40129,
Bologna, Italy  R. Zanin Cherenkov Telescope Array Observatory gGmbH, Via
Piero Gobetti, 93/3, 40129, Bologna, Italy

###### Abstract

We present a multi-epoch, multi-observatory X-ray analysis for 2FHL
J1745.1–3035, a newly discovered very high energy Galactic source detected by
the Fermi Large Area Telescope (LAT) located in close proximity to the
Galactic Center (l=358.5319∘; b=–0.7760∘). The source shows a very hard
$\gamma$-ray photon index above 50 GeV, $\Gamma_{\gamma}$=1.2$\pm$0.4, and is
found to be a TeV-emitter by the Fermi–LAT. We conduct a joint XMM-Newton,
Chandra and NuSTAR observing campaign, combining archival XMM-Newton
observations, to study the X-ray spectral properties of 2FHL J1745.1–3035 over
a time-span of over 20 years. The joint X-ray spectrum is best-fitted as a
broken power law model with break energy E${}_{b}\sim$7 keV: the source is
very hard at energies below 10 keV, with $\Gamma_{\rm 1}\sim$0.6, and
significantly softer in the higher energy range measured by NuSTAR with
$\Gamma_{\rm 2}\sim$1.9. We also perform a spatially resolved X-ray analysis
with Chandra, finding evidence for marginal extension (up to an angular size
$r\sim$5′′), a result that supports a compact pulsar wind nebula scenario.
Based on the X-ray and $\gamma$-ray properties, 2FHL J1745.1–3035 is a
powerful pulsar wind nebula candidate. Given its nature as an extreme TeV
emitter, further supported by the detection of a coincident TeV extended
source HESS J1745-303, 2FHL J1745.1–3035 is an ideal candidate for a follow-up
with the upcoming Cherenkov Telescope Array.

[0.45em]0.2em0.2em

##  1 Introduction

The Galactic plane is rich with efficient accelerators producing cosmic rays
(CRs, both leptons and hadrons), neutrinos (via hadronic interactions), and
energetic $\gamma$-ray photons. Very high-energy (VHE, $>$50 GeV)
$\gamma$-rays provide a direct view of some of the most extreme environments
in the Galaxy: indeed, $\gamma$-rays represent an excellent probe of non-
thermal astrophysical processes, since they are produced by the interaction of
relativistic particles. In particular, pulsar wind nebulae (PWNe) are some of
the brightest sources in the VHE sky and represent an ideal laboratory for
studying phenomena around objects with extreme densities and magnetic field
strengths, as well as how relativistic particle winds interact with ambient
media. The spinning neutron star, or pulsar, left behind after a supernova
(SN) explosion may generate a relativistic wind of electrons and positrons
that interact with unheated supernova ejecta, developing a standing shock wave
where the electrons and positrons are injected and accelerated into the non-
thermal expanding bubble of diffuse plasma, defining the PWN (Gaensler &
Slane, 2006; Malyshev et al., 2009). Since PWNe have well-defined central
energy sources (i.e., the pulsar) and many are close enough to be spatially
resolved, they allow for studying in great depth both relativistic winds and
the shocks that result from these winds colliding with the ambient medium. The
radio to X-ray emission of PWNe is caused by synchrotron radiation of
relativistic electrons in pulsar winds shocked in the ambient medium (Kennel &
Coroniti, 1984; Kargaltsev & Pavlov, 2008). Additionally, the same population
of relativistic electrons responsible for the PWN synchrotron emission can
scatter off local photon fields, resulting in inverse Compton (IC) emission at
$\gamma$-ray energies: notably, PWNe are the dominant class of TeV
$\gamma$-ray Galactic sources, as observed by Cherenkov telescopes (for
example, 39 % of the 31 sources identified by the H.E.S.S. Galactic plane
survey are PWNe; H. E. S. S. Collaboration et al., 2018). Therefore, a
complete picture of the electron population in PWNe, and a full understanding
of the mechanisms underlying VHE emitters, can be achieved only by
constraining simultaneously the synchrotron component, using X-ray
observations, and the IC one, with $\gamma$-ray data (e.g., Renaud, 2009;
Kargaltsev et al., 2013; Eagle, 2022).

The Fermi–LAT 2FHL catalog (Ackermann et al., 2016) contains 12 VHE sources in
the Galactic plane ($|b|<10^{\circ}$) having a $\gamma$-ray photon index
$\Gamma_{\gamma}$$<$1.8 and currently lack any association. Such a hard photon
index reliably rules out an extragalactic origin for these objects, since at
energies $>$50 GeV the 2FHL blazars generally exhibit a soft spectrum, with an
average photon index of $\Gamma_{\gamma}\sim 3.4$. In blazars, this energy
range is generally observed as the descending part of the inverse Compton (IC)
peak in the spectral energy distribution (SED). Prior reports have already
performed and reported on the analysis for 2 of the 12 unassociated Galactic
2FHL targets. The first study focused on 2FHL J0826.1–4500, which was found to
be a candidate shock-cloud interaction on the western edge of the Vela
supernova remnant (SNR; Eagle et al., 2019). 2FHL J1703.4–4145 was the second
source of the sample analyzed and was similarly found to be the byproduct of a
shock-cloud interaction at the edge of a supernova remnant, SNR G344.7–0.1
(Eagle et al., 2020).

In this work, we present the results of a multi-observatory X-ray monitoring
of a third source in the 2FHL VHE subsample, 2FHL J1745.1–3035. The work is
organized as follows: in Section 2 we present the Fermi–LAT source properties,
and in Section 3 we present the different X-ray datasets available for this
target and the data reduction process and analysis performed. In Section 4 we
present the results of the X-ray spectral fits, both for the single-epoch
observations and for the overall, multi-epoch spectrum, and we analyze the
Chandra image of the source, searching for evidence of extended emission.
Finally, in Section 5 we discuss the properties of 2FHL J1745.1–3035 by
combining the information derived from the X- and $\gamma$-ray data. We
summarize the results of the paper in Section 6. Through the rest of the work,
errors are quoted at the 90 % confidence level, unless otherwise stated.

##  2 2FHL J1745.1–3035

While inspecting the properties of the new VHE sources reported in the 2FHL
catalog, 2FHL J1745.1–3035 stands out as one of the most intriguing. First of
all, the source is located in close proximity to the Galactic Center
(l=358.5319∘; b=–0.7760∘). From a $\gamma$-ray perspective, this source is the
second brightest111The brightest being 2FHL J1703.4–4145, which was already
analyzed in Eagle et al. (2020). of the 12 new unassociated VHE sources in the
2FHL sample ($F_{>50~{}{\rm GeV}}$= (2.69$\pm 0.63)$$\times$10-11 erg s-1
cm-2), and presents a very hard spectrum in the $\gamma$-rays above 50 GeV (as
shown in Figure 1), having photon index $\Gamma_{\gamma}$=1.2$\pm 0.4$. In
fact, Fermi–LAT detected a 2FHL J1745.1–3035 photon at 940 GeV, which implies
that this source is a TeV emitter. Notably, such a hard $\gamma$-ray photon
index safely rules out an extragalactic origin for 2FHL J1745.1–3035, since no
$\gamma$-ray detected extragalactic source has similar $\Gamma_{\gamma}$, and
challenges our understanding of the emission mechanism that causes it.

Figure 1: Fermi–LAT spectral energy distribution of 2FHL J1745.1–3035 (violet
stars). In the Fermi–LAT data above 50 GeV, the source has an impressively
hard $\gamma$-ray photon index, $\Gamma_{\gamma}$=1.2$\pm$0.4, and is detected
up to $\sim$1 TeV, making this object a candidate for the most powerful
Galactic accelerator. We also plot, for reference, the SEDs for 4FGL
J1745.8–3028e (green circles) and HESS J1745–303 (orange pentagons). We
discuss in the text the discrepancy between the 2FHL and H.E.S.S. data.

As shown in Figure 2, the 2FHL source is overlapping within the radius
($r$=0.53∘) of an extended object reported in two other Fermi–LAT catalogs:
namely, the Third Catalog of Hard Fermi-LAT Sources (3FHL, which reports
objects detected in the 10 GeV–2 TeV range; Ajello et al., 2017, source ID:
3FHL J1745.8-3028e) and the Fourth Fermi Large Area Telescope Catalog (4FGL,
which reports objects detected in the 50 MeV–1 TeV range; Abdollahi et al.,
2020a, source ID: 4FGL J1745.8-3028e). The 3FHL source is associated with the
4FGL one. Both the 3FHL and the 4FGL source are modeled with a disk profile
with radius $r$= 0.53 $\pm$ 0.02 $\pm$ 0.26∘ (where the first error is
statistical and the second is systematic), as originally reported in the Fermi
Galactic Extended Source Catalog (FGES; Ackermann et al., 2017). There is
however no source formally associated222The association procedure is based on
a statistical Bayesian cross-correlation of a $\gamma$-ray catalog with other
catalogs, as explained in detail in Ackermann et al. (2016). to 2FHL
J1745.1–3035 in the 4FGL and 3FGL catalogs. The 4FGL/3FHL Fermi source has a
TeV-detected counterpart of unknown origin, HESS J1745–303, in the H.E.S.S.
Galactic plane survey (H. E. S. S. Collaboration et al., 2018). As shown in
Figure 1, the H.E.S.S. source has a SED significantly different from the 2FHL
one, a discrepancy that can be explained with the offset between the two
sources, which may indicate different physical origins.

HESS J1745–303 has already been studied in several works in the literature.
Aharonian et al. (2008) performed a multi-wavelength (including XMM-Newton)
study of the region where the H.E.S.S. source is detected. They reported that
no counterpart alone could fully explain the VHE emission detected by
H.E.S.S.: however, at least part of the TeV emission might be linked to either
a supernova-remnant/molecular-cloud (SNR/MC) association or to a high-spin-
down-flux pulsar, or a combination of the two. Bamba et al. (2009) reported
the results of a Suzaku observation of a part of the H.E.S.S. region, but
found no significant non-thermal X-ray emission. Hui et al. (2011) instead
analyzed the combined H.E.S.S. and 28–month Fermi–LAT SED, but did not find a
conclusive explanation for the VHE emission origin.

In summary, the complexity of the observed $\gamma$-ray field and the extreme
properties of 2FHL J1745.1–3035 make this source particularly interesting.
Specifically, the $\gamma$-ray maps hint at a scenario where the emission
observed and reported in the 2FHL catalog might not be entirely caused by the
same object, or processes, causing the emission detected by H.E.S.S. (and
likely in the 4FGL catalog). To better characterize this intriguing 2FHL
source, we therefore performed an extended, multi-X-ray observatory follow-up
campaign.

Figure 2: Tricolor $\gamma$-ray image of the region where 2FHL J1745.1–3035 is
located. We plot in red the 2FHL counts ($>$50 GeV), in green the 3FHL counts
($>$10 GeV), and in blue the H.E.S.S. flux in 0.03–100 TeV band. We report in
the figure the 2FHL, 3FHL, and H.E.S.S. source locations: since the 3FHL
(white circle) and H.E.S.S. (green circle) sources are found to be extended,
we plot a circle with a radius equal to the best-fit extensions reported in
the catalogs. For the 2FHL source (cyan circle), which is found to be point-
like, we use the 95 % confidence level radius.

##  3 X-ray Data reduction and analysis

We report in Table 1 a summary of the five X-ray observations of 2FHL
J1745.1–3035 that have been performed between March 2001 and March 2023. In
the following subsections, we describe in detail the data reduction and
analysis for each instrument.

Instrument | Sequence | Start Time | Exposure | Net count rate
---|---|---|---|---
| ObsID | (UTC) | ks | 10-2 counts s-1
XMM-Newton | 0103261301 | 2001-03-21T23:12:15 | 7.6; 7.6; 4.4 | 1.07$\pm$0.12; 1.29$\pm$0.13; 1.99$\pm$0.22
XMM-Newton | 0782170601 | 2017-04-03T11:56:57 | 9.6; 9.6; 6.5 | 1.53$\pm$0.13; 1.28$\pm$0.12; 5.11$\pm$0.29
XMM-Newton | 0886010401 | 2021-03-16T19:06:37 | 21.3; 21.3; 16.5 | 0.74$\pm$0.06; 0.96$\pm$0.07; 2.67$\pm$0.14
Chandra | 23573 | 2022-05-20T08:52:56 | 22.8 | 0.59$\pm$0.05
NuSTAR | 60301026002 | 2023-03-27T19:01:09 | 111 | 0.68$\pm$0.04; 0.74$\pm$0.05

Table 1: Summary of the X-ray observations of the 2FHL J1745.1–3035
counterpart. The XMM-Newton count rates are computed in the 0.6–10 keV band,
the Chandra ones in the 0.6–8 keV band, and the NuSTAR ones are computed in
the 3–70 keV band. The XMM-Newton exposures and count rates are the MOS1, MOS2
and pn ones, in this order; the NuSTAR exposures and count rates are the FPMA
and FPMB, in this order. All count rates are the net, background-subtracted
ones. Exposures are computed after removing high-background periods.

###  3.1 XMM-Newton

The source is in the field of view of three XMM-Newton observations.

  1. 1.

The first observation we studied is a 16 ks one, taken in 2017 and aimed
specifically at finding an X-ray counterpart for 2FHL J1745.1–3035
(observation ID: 0782170601; PI: M. Ajello). In the XMM-Newton cameras’ field
of view, the source was found at a distance from the 2FHL centroid (and
therefore at an off-axis angle) $\sim$3.5′: the J2000 coordinates of the XMM-
Newton counterpart centroid are R.A. = 17:45:07.99 and Dec = –30:39:06.17.
This source is reported in the fourth release of the XMM-Newton source
catalog333http://xmmssc.irap.omp.eu/Catalogue/4XMM-DR13/4XMM_DR13.html, 4XMM-
DR13 (Zolotukhin et al., 2017; Traulsen et al., 2020; Webb et al., 2020), with
source ID 4XMM J174507.9-303906 (http://xmm-
catalog.irap.omp.eu/source/201032613010001). We report in Figure 3 an image of
the XMM-Newton pn 0.3–10 keV observation with the Fermi–LAT 90 % confidence
positional uncertainty overlaid: it can be seen that the X-rays allow us to
reliably identify a bright counterpart of the 2FHL source (magenta circle in
Figure 3). As a safety check, we nonetheless verified the properties of the
faint source that can be noticed in the upper right part of the image. This a
soft X-ray source with no emission above 2 keV, which has a bright (GAB=12.5)
counterpart in the Gaia DR3 catalog (https://gea.esac.esa.int/archive/; source
ID: Gaia DR3 4056681582705849600 Gaia Collaboration et al., 2016, 2023), where
the source is flagged as a F-G class star (T${}_{\rm eff}\sim$6000 K) with 100
% probability. We thus safely rule it out as a possible counterpart of the
2FHL source. The detection of a bright, compact X-ray counterpart strongly
supports a scenario where the 2FHL–detected emission does not have the same
origin of the H.E.S.S. one, which would explain why the two objects have such
different SEDs. We also note that the following Chandra and NuSTAR
observations have been performed targeting the XMM-Newton–detected object.
Finally, we used the Aladin web tool444aladin.cds.unistra.fr to search for
possible counterparts of the X-ray source, but none was identified in the
optical images down to magnitudes $R_{\rm AB}\sim$22\. As reported in Table 1,
this observation was affected by strong flares, therefore the observation MOS
(pn) net exposure time is of $\sim$9.5 ks ($\sim$6.5 ks).

  2. 2.

The detection of an X-ray source in the XMM-Newton 2017 image allowed us to
perform a search in the XMM-Newton archive aimed at finding other observations
of the object. We find two such observations: the first one was taken in 2001,
is 8 ks long, and targeted the pulsar PSR B1742-30 (also known as PSR
J1745-3040; observation ID: 0103261301; PI: F. Jansen). In this observation,
our source is found at an off-axis angle $\sim$10′. We performed a cross-match
between the X-ray coordinates of our target and the Australia Telescope
National Facility (ATNF) Pulsar Catalog (Manchester et al., 2005)555The
updated catalog is available at
http://www.atnf.csiro.au/research/pulsar/psrcat/. The closest source to our
X-ray target is indeed PSR B1742-30. Based on the information in the ATNF
Pulsar Catalog, PSR B1742-30 is at a $\sim$200 pc distance from the Earth and
has an estimated age of 546 kyr. While a displacement of 10′ (which is 0.6 pc)
could be reasonably expected as a consequence of a pulsar kick, a pulsar that
old would be expected to be accompanied by a bow-shock nebula at the pulsar
position in X-rays, while the relic nebula left behind would not be as compact
as the source that we observe either in the X-rays nor in the 2FHL catalog.
Furthermore, the pulsar E-dot as reported in the ATNF Pulsar Catalog is
$\dot{E}$ = 8.5 $\times$ 1033 erg s-1, a value significantly lower than those
commonly measured in pulsars powering PWNe ($\sim$1036 erg s-1; see, e.g.,
Mattana et al., 2009; Acero et al., 2013). We therefore conclude that the
source we detect is likely not associated with the known pulsar PSR B1742-30,
and would instead be a newly discovered pulsar.

  3. 3.

More recently, in 2021, our X-ray source was serendipitously observed during a
23 ks observation of the Milky Way plane (observation ID: 0886010401; PI: G.
Ponti). In this observation, the object we are studying is located at an off-
axis angle $\sim$9′.

For all of the observations, the XMM-Newton standard data reduction procedure
has been performed using the SAS software, version 19.0.0. To extract the
source spectra we used a circular region with radius $r_{\rm src}$=20′′, while
the background spectra were extracted from a circle having radius $r_{\rm
bkg}$=45′′ located in proximity of the source and visually inspected to avoid
contamination from bright targets. Finally, given the different exposure times
of the three observations, we adopted a different spectral binning for each of
them: the 2001 and 2017 spectra are binned with 7 counts per bin, while in the
2021 observation we binned the MOS spectra with 10 counts per bin and the pn
spectrum with 15 counts per bin.

Figure 3: XMM pn CCD 0.3–10 keV image, centered on the 2FHL J1745.1–3035
Fermi–LAT position. The black circle shows the LAT 90% confidence positional
uncertainty ($r=5.4^{\prime}$). The X-ray counterpart is uniquely detected
within the LAT uncertainty region.

###  3.2 Chandra

2FHL J1745.1–3035 was observed in May, 2022 with a $\sim$25 ks Chandra ACIS-I
observation (PI: S. Marchesi). The dataset, obtained by the Chandra X-ray
Observatory, is contained in the Chandra Data Collection (CDC) cdc.191
https://doi.org/10.25574/cdc.191 (catalog doi:10.25574/cdc.191). We performed
a standard data reduction using the CIAO software (Fruscione et al., 2006),
version 4.14. We then followed the standard astrometric correction procedure
reported in the Chandra
threads666https://cxc.cfa.harvard.edu/ciao/threads/reproject_aspect/ to
correct the astrometry of our Chandra image by using as a reference the
USNO-A2.0 astrometric standards catalog (Monet, 1998). We find and apply
offset corrections $\Delta$RA = RAChandra \- RAUSNO = 0.471′′ and $\Delta$Dec
= DecChandra \- DecUSNO = 0.278′′. We detect the Chandra counterpart of 4XMM
J174507.9-303906 at the position R.A. = 17:45:08.03 and Dec = –30:39:06.76,
just 0.7′′ away from the 4XMM source centroid.

We report in Figure 4 a zoom-in of 4XMM J174507.9-303906 in the Chandra 2–7
keV image (top left), in the smoothed XMM-Newton PN 2–10 keV image (top
right), in the DSS2-red survey (bottom left; McLean et al., 2000) and in the
UKIRT Infrared Deep Sky Survey (UKIDSS) DR11PLUS J, H, and K band survey
(bottom left, center, and right; Warren et al., 2007). As it can be seen, no
clear counterpart of the X-ray source is detected in the optical, where the
two closest targets to the X-ray source are two objects both classified as
stars with 100 % probability in the Gaia DR3 catalog (these GAIA sources are
plotted as cyan circles in the figure). There are instead two faint near-
infrared sources in the proximity (distance $\lesssim$1′′) of the Chandra
centroid, as reported in the UKIDSS–DR6 Galactic Plane Survey (Lucas et al.,
2008)777https://vizier.cds.unistra.fr/viz-bin/VizieR-3?-source=II/316. We flag
these sources as U1 (UKIDSS ID: J174507.94+429192822.5, HAB = 18.1 $\pm$ 0.2;
KAB = 17.2 $\pm$ 0.2) and U2 (UKIDSS ID: J174508.02+429192824.8; KAB = 17.2
$\pm$ 0.2) in Figure 4. The faintness of these sources with respect to the
X-ray brightness of our target, and the high source density of this field in
the near infrared (as a reference, there are 28 UKIDSS sources within a 10′′
radius from the Chandra centroid) do not allow us to reliably associate either
of the UKIDSS sources to the X-ray object.

We used the CIAO tool specextract to extract the Chandra source spectrum from
a circle having radius $r$=3′′, while the background spectrum was extracted
from a $r$=25′′ circular region located nearby the source. The Chandra
spectrum was then binned with 10 counts per bin. In Section 4.3 we will
investigate in detail the presence of extended emission in the Chandra image,
and the subsequent implications on the nature of 2FHL J1745.1–3035.

Figure 4: Zoom-in of 4XMM J174507.9-303906 in the Chandra 2–7 keV image (top
left), in the XMM-Newton PN 2–10 keV image (top center), in the DSS2-red
survey (top right; McLean et al., 2000) and in the UKIDSS survey (bottom
right; Skrutskie et al., 2006) J, H, and K bands (bottom left, center, and
right, respectively). In the XMM-Newton image, we report the 4XMM catalog
position of 4XMM J174507.9-303906. For visual clarity, we overlay the smoothed
Chandra contours on the other images. The positions of two Gaia sources and
two UKIDSS sources are also shown as cyan and red circles, respectively.

###  3.3 NuSTAR

2FHL J1745.1–3035 was observed by NuSTAR in March, 2023. Our source is located
in close proximity of the Galactic Center (longitude 358.481127∘, latitude
-0.804527∘), which means that its NuSTAR observations are strongly affected by
stray light (Grefenstette et al., 2021). This is clearly visible in Figure 5,
where we also report the region files which we used to extract the source and
background spectra, with a radius of 30′′ and 45′′, respectively. Despite its
challenging location, the source is clearly detected in both NuSTAR cameras.
Due to the high background, we binned the spectra with 75 counts per bin, to
ensure an adequate signal-to-noise ratio.

Figure 5: NuSTAR FPMA (left) and FPMB (right) observations of 2FHL
J1745.1–3035: the white circle marks the position of the Fermi–LAT source, and
has a radius corresponding to the $\gamma$-ray 90 % positional uncertainty
($r$=5.4′). As it can be seen, stray light from the nearby Galactic Center is
strongly affecting the observation: for this reason, the background region
(dashed cyan circle) is selected as close as possible to the source
(highlighted with a yellow circle), to ensure a background characterization as
accurate as possible.

##  4 X-ray data analysis and interpretation

###  4.1 Single-epoch X-ray fits and variability

We perform single-epoch fits to search for potential variability in flux and
photon index, both commonly observed in the X-ray spectra of PWNe (e.g.,
Pavlov et al., 2001; Klingler et al., 2018; Guest & Safi-Harb, 2020). In
particular, X-ray variability in PWNe has already been observed over time-
scales that vary from months to several years (e.g., the PWN embedded in the
SNR Kes 75; Ng et al., 2008; Livingstone et al., 2011; Reynolds et al., 2018),
and it has been linked to physical processes in both the pulsar and the
surrounding nebula. Such processes include synchrotron cooling of the PWN
flow, resulting in a steepening of the X-ray photon index, or Kelvin–Helmoltz
instabilities in the shocked winds (Pavlov et al., 2001).

We note that we also searched for short-term variability within the
observations, but we did not find any significant evidence of it.
Interestingly, the 2017 and 2021 XMM-Newton observations of 4XMM
J174507.9-303906 are flagged as variable by the automatic pipeline used in the
4XMM catalog
(https://xcatdb.unistra.fr/4xmmdr13/xcatindex.html?detid=107821706010001 for
the 2017 observation and
https://xcatdb.unistra.fr/4xmmdr13/xcatindex.html?detid=108860104010001 for
the 2021 one). However, we performed a multi-band, multi-instrument analysis
of the 2017 observation and found no evidence of simultaneous variability in
the three XMM-Newton cameras (MOS1, MOS2 and PN) up to 6 keV. At energies
greater than 6 keV, a flare is detected in all three cameras, but the same
flare is still detected even at energies $>$10 keV, where the contribution
from the source becomes negligible and the emission is dominated by the
background. We thus conclude that in the 2017 observation the variability
reported in the 4XMM catalog can be ascribed to an episode of high background
activity that commonly affects XMM-Newton observations. As for the 2021
observation, we compared the MOS1, MOS2, and pn light curves in different
bands and once again found no evidence of consistent cross-instrument
variability. We report in the Appendix B a summary of the light curves we
produced for the 2017 and 2021 observations.

For each epoch of the X-ray observations, we fit the available data with an
absorbed power law model, where the column density of the absorber was left
free to vary. Since sources nearby the Galactic Center have been shown to be
potentially affected by dust scattering (see, e.g., Jin et al., 2018), we
performed a consistency check by fitting our data a first time using the XSPEC
XScat model (Smith et al., 2016), and then a second time using the standard
photoelectric absorption model phabs. Since the results are fully equivalent,
we report below those obtained using the standard phabs model.

When fitting XMM-Newton and NuSTAR observations, we also included cross-
normalization constants to account for possible differences between different
cameras. We also included in the model the absorption due to our own Galaxy,
$N_{\rm H,Gal}$=9.7$\times$1021 cm-2 (Kalberla et al., 2005). The best-fit
results for each epoch are reported in Table 2, while in Figure 6 we plot the
best-fit photon index, line-of-sight column density, and 2–10 keV flux we
measure in each epoch. The single-epoch spectra and the relative best-fit
models are reported in Appendix A.

As it can be seen, the photon index and line-of-sight column density do not
vary significantly in the three XMM-Newton observations, as well as in the
Chandra one. In all cases, the X-ray photon index is significantly hard
(varying in the range $\Gamma_{\rm X}$=[0.2–0.7]), and the X-ray emission is
absorbed by material having column density NH,los$\sim$4–6 $\times$1022 cm-2.
The NuSTAR best-fit results are instead different: most importantly, we
measure an X-ray photon index that is significantly softer than those measured
by the 0.5–10 keV instruments, being $\Gamma_{\rm
NuS}$=1.99${}_{-0.32}^{+0.37}$. The fit to the NuSTAR data also gives a
slightly larger line-of-sight column density than the one measured with
Chandra and XMM-Newton, which is NH,los,Nus=1.8${}_{-1.1}^{+1.4}$ $\times$1023
cm-2. Such a discrepancy can however be explained with the fact that NuSTAR
lacks the $<$3 keV coverage that is key to reliably constrain line-of-sight
column densities smaller than 1023 cm-2.

2FHL J1745.1–3035 shows some marginal evidence for flux variability in the
2–10 keV band. In particular, the 2022 Chandra and the 2023 NuSTAR data are
$\sim$50 % fainter than the three XMM-Newton observations taken between 2001
and 2021. However, the relatively large uncertainties on the flux measurements
prevent us from studying this tentative trend in further detail. We also note
that in the $\gamma$-rays no information on source flux variability is
reported in the 2FHL catalog. There is also no observational evidence for flux
variability in the 4FGL source associated with the H.E.S.S. one and reported
in the Fermi–LAT data release 3 catalog (DR3, Abdollahi et al., 2022), which
is an incremental version of the 4FGL catalog published in 2020 and is based
on the first 12 years of Fermi–LAT observations between 50 MeV and 1 TeV.
Specifically, the source variability index (computed as the sum of the
log(likelihood) difference between the flux fitted in each time interval and
the average flux over the full catalog interval) is 5.1. As a reference, a
value greater than 18.48888Such a value corresponds to a 99 % confidence level
in a $\chi^{2}$ distribution with seven degrees of freedom, see Abdollahi et
al. (2020b) over 12 intervals indicates $<$1 % chance of being a steady
source.

Figure 6: Best-fit X-ray photon index (left), line-of-sight column density
(center), and 2–10 keV flux (right) for each of the five X-ray observations of
2FHL J1745.1–3035. All observations are fitted with an absorbed single power
law model. Downward triangles are used when only a 90 % confidence upper limit
is measured.

Instrument | Observation date |  $\chi^{2}$/d.o.f. | Photon Index | NH,los | 2–10 keV flux
---|---|---|---|---|---
| (UTC) |  |  | 1022 cm-2 | 10-12 erg s-1 cm-2
XMM-Newton 1 | 2001-03-21 | 45.1/59 | 0.68${}_{-0.72}^{+0.87}$ | 6.4${}_{-3.3}^{+4.8}$ | 1.31${}_{-1.02}^{+0.15}$
XMM-Newton 2 | 2017-04-03 | 68.0/87 | 0.67${}_{-0.41}^{+0.46}$ | 6.1${}_{-1.9}^{+2.5}$ | 0.95${}_{-0.58}^{+0.05}$
XMM-Newton 3 | 2021-03-16 | 57.8/67 | 0.42${}_{-0.36}^{+0.39}$ | 4.2${}_{-1.4}^{+1.7}$ | 0.99${}_{-0.31}^{+0.07}$
Chandra | 2022-05-20 | 4.7/10 | 0.21${}_{-0.57}^{+1.34}$ |  $<$6.4 |  $<$0.35
NuSTAR | 2023-03-27 | 30.0/46 | 1.99${}_{-0.32}^{+0.37}$ | 18.2${}_{-10.7}^{+14.2}$ | 0.54${}_{-0.45}^{+0.04}$

Table 2: Best-fit results of the single-epoch X-ray spectral fits of 2FHL
J1745.1–3035. All spectra are fitted with an absorbed single power law model,
where NH,los is the line-of-sight column density of the absorber.
$\chi^{2}$/d.o.f. is the reduced $\chi^{2}$ of the fit, where d.o.f. are the
degrees of freedom. In the XMM-Newton (NuSTAR) data the 2–10 keV flux is
computed from the pn (FPMA) dataset.

###  4.2 Joint X-ray spectral fit

Since we did not find any significant line-of-sight column density variability
between the four soft X-ray observations, we jointly fit all X-ray spectra by
tying this parameter among the different observations. Since we instead found
evidence for potential X-ray flux variability, we added to the model a cross-
normalization component to take this intrinsic variability into account. We
then first fit the spectra with the same absorbed power law model we used in
the single-epoch analysis.

We measure a photon index $\Gamma_{\rm X}$=1.53${}_{-0.15}^{+0.16}$ and a
line-of-sight column density NH,los=9.9${}_{-1.1}^{+1.3}$ $\times$1022 cm-2.
We report in Figure 7, left panel, the single power law best-fit model:
significant residuals are visible in the ratio between the model and the data
above 10 keV. Furthermore, as expected, the best-fit photon index we measure
lies between the values we obtain in the soft X-rays ($\Gamma_{\rm X}\sim$0.5,
as shown in Table 2) and the one measured using NuSTAR, $\Gamma_{\rm
X}$=1.99${}_{-0.37}^{+0.32}$.

Given this observational evidence, we performed a new fit, this time using a
broken power law model: notably, the presence of a break at $\sim$5–15 keV in
the X-ray spectra of PWNe has been reported in several works (see, e.g., An et
al., 2014; Nynka et al., 2014; Madsen et al., 2015, 2020; An, 2019; Bamba et
al., 2022). Such a variation in $\Gamma_{\rm X}$ is an intriguing challenge to
our understanding of PWN emission mechanisms, since PWNe SED models do not
properly fit such an X-ray feature (see, e.g., Tanaka & Takahara, 2011).
Consequently, additional model components, such as for example different
electron injection spectra or radially dependent PWN parameters would be
required.

The parameterization of the broken-power law is

$\frac{dN}{dE}=\begin{cases}KE^{-\Gamma_{1}}&\text{if }E\leq E_{\rm b}\\\
KE_{\rm b}^{\Gamma_{2}-\Gamma_{1}}(E/1\,{\rm
keV})^{-\Gamma_{2}})&\text{otherwise}\end{cases}$ (1)

Where $\Gamma_{1}$ and $\Gamma_{2}$ are the low- and high-energy photon
indices, $K$ is the normalization parameter, and $E_{\rm b}$ is the energy of
the break.

The results of this fit are reported in Figure 7, right panel. As it can be
seen, the data are much better constrained across the whole energy range, and
in particular at energies E$>$10 keV. We also measure a significant
improvement in the fit, from $\chi^{2}$/d.o.f.=248.1/264 for the single power
law model to $\chi^{2}$/d.o.f.=217.0/262 for the broken power law one. The
photon index at energies lower than the break is $\Gamma_{\rm
1}$=0.56${}_{-0.41}^{+0.42}$, in good agreement with the photon indices we
measured in the Chandra and XMM-Newton spectra, while the second photon index
is $\Gamma_{\rm 2}$=1.87${}_{-0.24}^{+0.60}$, a value consistent with the one
measured in the NuSTAR spectra. Coherently, the best-fit break energy is
Eb=7.1${}_{-0.9}^{+3.0}$ keV, which is where the NuSTAR contribution becomes
the dominant one. In Figure 8 we report the confidence contours of the break
energy as a function of both photon indices: $\Gamma_{\rm 1}$ and $\Gamma_{\rm
2}$ are different at the $>$99 % confidence level, a result that further
favors the broken power law scenario over the single power law one. We also
note that the best-fit column density obtained with the broken power law fit,
NH,los=5.1${}_{-1.4}^{+1.4}$ $\times$ 1022 cm-2, is significantly larger than
the one expected on the line of sight from the neutral Hydrogen measurements
by Kalberla et al. (2005, NH,Gal=9.7$\times$1021 cm-2). Such a result supports
a scenario where the X-ray emission is taking place in a dense, gas-rich
environment.

Finally, as a consistency check, and in particular to partially rule out an
SNR origin for 2FHL J1745.1–3035, we fitted our data with the nei non-
equilibrium ionization collisional plasma model, which is commonly used when
modeling the X-ray spectra of SNRs (see, e.g., Harrus et al., 2001; Lazendic &
Slane, 2006; Katsuda et al., 2009; Prinz & Becker, 2013; Leahy et al., 2020;
Eagle et al., 2022). We report the best-fit results in Table 3: while the
best-fit statistic ($\chi^{2}$/d.o.f.=225.3/262) is significantly better than
the one obtained using the single power law model, and fairly consistent with
the one we obtain using the broken power law model, the best-fit temperature
($kT$=34.4${}_{-9.1}^{+24.4}$ keV) is fully unphysical. As a reference, Leahy
et al. (2020) analyzed the X-ray spectra of 43 SNR of different ages,
explosion energies, and circumstellar medium densities: none of them was found
having $kT\geq$4 keV. Such a result reliably rules out a thermal SNR scenario
for 2FHL J1745.1–3035.

Model |  $\chi^{2}$/d.o.f. | NH,los | $\Gamma_{\rm 1}$ | $\Gamma_{\rm 2}$ | Eb | $kT$ | $\tau$ | $Z$
---|---|---|---|---|---|---|---|---
|  | 1022 cm-2 |  |  | keV | keV | 1010 s cm-3 | $Z_{\odot}$
Single Power Law | 248.1/264 | 9.0${}_{-1.1}^{+1.3}$ | 1.53${}_{-0.15}^{+0.16}$ | – | – | – | – | –
Broken Power Law | 217.0/262 | 5.1${}_{-1.4}^{+1.4}$ | 0.56${}_{-0.41}^{+0.42}$ | 1.87${}_{-0.24}^{+0.60}$ | 7.1${}_{-0.9}^{+3.0}$ | – | – | –
nei | 225.3/262 | 8.9${}_{-1.4}^{+0.9}$ | – | – | – | 34.4${}_{-9.1}^{+24.4}$ |  $<$5.95 | 0.24${}_{-0.11}^{+0.11}$

Table 3: Best-fit results of the multi-epoch X-ray spectral fits of 2FHL
J1745.1–3035, for the three different models discussed in the text.
$\chi^{2}$/d.o.f. is the reduced $\chi^{2}$ of the fit, where d.o.f. are the
degrees of freedom; NH,los is the line-of-sight column density of the
absorber; $\Gamma_{\rm 1}$ is the photon index of the single power law model,
and the photon index before the break of the broken power law model; Eb and
$\Gamma_{\rm 2}$ are the energy break and the photon index at energies larger
than it in the broken power law model, respectively; finally, $kT$, $\tau$,
and $Z$ are the best-fit temperature, ionization time scale, and metallicity,
derived using the nei thermal model.

Figure 7: Multi-epoch X-ray spectra of 2FHL J1745.1–3035: the 2001, 2017, and
2021 XMM-Newton observations are plotted in orange, red, and cyan,
respectively; the 2022 Chandra observation is plotted in green; the 2023
NuSTAR observation is plotted in blue. The best-fit models (left: single power
law; right: broken power law) are plotted as black solid lines. Residuals
above 10 keV are clearly visible in the single power law best-fit model, while
a significant improvement is visible in the broken power law fit.  Figure 8:
68 % (solid line), 90 % (dashed line), and 99 % confidence levels for the
broken power law energy break as a function of the photon index before the
break, $\Gamma_{\rm 1}$ (red), and of the photon index after the break,
$\Gamma_{\rm 2}$ (blue).

###  4.3 Chandra spatially resolved analysis

The X-ray counterpart of 2FHL J1745.1–3035 does not show clear evidence for
extended emission in the NuSTAR image or in the XMM-Newton images, which
implies that the X- and $\gamma$-ray emitter must be compact. Extended PWN
emission is however commonly observed with Chandra, and indeed a first visual
evidence is visible in our Chandra ACIS-I observation as well. As shown in the
top panels of Figure 9, both the unbinned Chandra image and the smoothed one
present evidence for asymmetric extended emission, with a faint tail of
emission detected Southwards of the source center.

To quantitatively search for extended emission in 2FHL J1745.1–3035, we
perform a comparison between the 2–7 keV surface brightness radial
distribution and the one obtained from a simulated PSF in the same band,
computed using the ChaRT and MARX 5.5.1 tools (see e.g., Fabbiano et al.,
2017; Jones et al., 2020; Ma et al., 2020; Traina et al., 2021; Silver et al.,
2022, for a detailed description of this same approach in nearby active
galactic nuclei). We report in Figure 9, bottom left panel, the 2–7 keV
surface brightness radial profile of 2FHL J1745.1–3035, which we computed from
a set of 11 annuli, where the two radii increase by 0.5′′ in each consecutive
annulus, and the maximum external radius is 6.5′′, and the corresponding
expected PSF profile which one would observe for a point-like source. As it
can be seen, this quantitative analysis confirms the first qualitative
impression, since the source surface brightness exceeds the one predicted for
a point-like scenario up to $\sim$5′′. As a caveat, we note that it has been
shown (Primini et al., 2011), that that the ray tracing simulations generated
by MARX will have a roughly correct total intensity but tend to have PSF wings
that are broader than observations and a PSF core narrower than observed, an
effect that could in principle affect our simulation as well.

Finally, the low count statistic of the Chandra spectrum prevents us from
performing an energy dependent surface brightness profile, as well as a
spatially resolved spectral fit. Nonetheless, a simple tricolor image such as
the one we report in Figure 9, bottom right panel, gives a first indication of
the presence of two distinct sources of emission: a harder, unresolved one at
the center (likely the pulsar) and a fainter, softer diffuse emission in the
outskirts (in our scenario, the pulsar wind nebula). We note that Li et al.
(2008) analyzed the Chandra and XMM-Newton spectra of a sample of pulsars and
PWNe, and found tentative evidence for both direct correlation between the
X-ray photon index of the pulsar and its age (i.e., younger pulsars have
harder photon indices) and anti-correlation between the PWN photon index and
the pulsar age (i.e., younger PWNe have softer photon indices). Both these
trends support a young PWN scenario for 2FHL J1745.1–3035, based on the
tentative observational evidence we find in the Chandra image.

Figure 9: Top panels: unbinned (left) and smoothed (center) Chandra ACIS-I 2–7
keV image of 2FHL J1745.1–3035. In both images, we plot a circle centered on
the brightest pixel and having a radius corresponding at the Chandra ACIS on-
axis 90 % encircled energy fraction (0.98′′). Bottom panels: on the left we
show the surface brightness of our target (red) as a function of the radius:
for comparison we also show the expected trend for the Chandra PSF (blue),
normalized so that in the first bin the PSF surface brightness is equal to the
measured one, as well as the average background surface brightness (dashed
black line); the error bars represent the Poissonian error of the counts
inside each annulus. On the right, we report a smoothed tricolor (red: 0.5–2
keV; green: 2–4.5 keV; blue: 4.5–7 keV) image of 2FHL J1745.1–3035. The
fainter, diffuse emission is found to be softer than the central, unresolved
one.

##  5 Spectral energy distribution modeling and physical characterization

In order to investigate the origin of the observed high-energy emission, we
use the NAIMA Python package (v0.10.0, Zabalza, 2015), which computes the
radiation from a single non-thermal relativistic particle population. For the
particle distribution in energy, we assume a power law shape with an
exponential cut-off,

$f(E)=A\bigg{(}\frac{E}{E_{0}}\bigg{)}^{-\alpha}\exp\big{(}{-{\frac{E}{E_{c}}}}\big{)}\\\
$ (2)

We then test a combination of free parameters (namely the normalization $A$,
index $\alpha$, and energy cut-off $E_{c}$) that can best explain the
broadband spectrum. We consider three photon fields in all Inverse Compton
Scattering calculations in this section, the Cosmic Microwave Background
(CMB), a near infrared (NIR) and a far infrared (FIR) stellar photon field,
set to the GALPROP values measured from a Galactic radius of 6.5 kpc.

Due to the complex $\gamma$-ray emission detected in this region, it is
plausible that there is source confusion from more than one object generating
the observed $\gamma$-rays (as discussed in Section 2 and visually shown in
Figure 1). Indeed a reasonable broadband representation for the X- and
$\gamma$-ray data requires more than one particle population and the predicted
properties depend on the dominant $\gamma$-ray spectral properties we assume.
We are unable to determine the presence or level of contribution from low- and
high-energy components using the $\gamma$-ray data alone, so we explore two
combinations of data for modeling: 1) radio, X-ray, and $\gamma$-ray data
assuming the extended 4FGL and H.E.S.S. sources are the dominant $\gamma$-ray
counterparts and 2) combine the same radio and X-ray data to the point-like
2FHL data. We assume two scenarios of particle distributions in each case: two
leptonic populations or a lepto-hadronic population. In an effort to constrain
radio synchrotron contributions, we estimate an upper limit on the radio flux
from the 1.28 GHz MEERKAT Galactic Center radio observation999https://archive-
gw-1.kat.ac.za/public/repository/10.48479/fyst-hj47/index.html, (Heywood et
al., 2022). (Heywood et al., 2022). We discuss the results of the broadband
modeling in the following sections. The model parameters that can best
reproduce the two data combinations are provided in Table 4.

###  5.1 Leptonic Scenario

In the case of two-leptonic particle distributions, the radio–X-ray–2FHL data
combinations is reasonably reproduced (as shown in the left panel of Figure
10), while no combination of parameters can adequately describe the SED
composed by the radio, X-ray, 4FGL and H.E.S.S. data, and in particular the
4FGL data (Figure 11, left panel). This is due to the very low radio flux
upper limit that forces the electron index to be harder than what is inferred
by the 4FGL data ($\sim 2$). Introducing another fitting parameter such as the
index to the energy cut-off (i.e., an index after the break deviating from the
assumed value of 1 here) may be able to reconcile the poor fitting in the
Fermi band. An index after the cutoff that is smaller than one, for instance,
would soften the cut-off shape in the synchrotron peak for the low-energy
population (Population 1 in left panel of Figure 11). A final possibility
would be that the particle population is more complicated than what is
explored here.

As mentioned above, the radio, X-ray and 2FHL $\gamma$-ray data combination is
instead reasonably reproduced by the two-leptonic model (left panel of Figure
10). Assuming the two-leptonic scenario for both data combinations, the
predicted properties are consistent with an energetic, young and/or more
evolved PWN where the extended 4FGL and H.E.S.S. $\gamma$-ray data may be an
extended relic PWN and the point-like 2FHL emission may be a compact, young
PWN located close to the pulsar.

###  5.2 Lepto-Hadronic Scenario

In order to explore a hadronic scenario to the broadband data, we fit a pion
decay emission model to characterize the GeV and TeV data together combined
with the previously derived leptonic component to characterize the radio and
X-ray. As shown in the right panels of Figures 11 and 10, both data
combinations are adequately characterized in the lepto-hadronic scenario, and
in particular we are now able to efficiently describe the SED including the
4FGL data, something that could not be done with a purely leptonic model.

We note that the fits to both datasets require very large target densities:
more specifically, $n_{\rm H}=5000$ cm-3 in the 4FGL+H.E.S.S. dataset and
$n_{\rm H}=400$ cm-3 in the 2FHL dataset. The high $n_{\rm H}$ values are a
requirement if the total proton energy $W_{\rm p}$ is not to exceed the total
energy expected to be released from a typical supernova (SN) explosion $E_{\rm
SN}=10^{51}\,$erg. Even for the highest estimate $n_{\rm H}=5000$ cm-3 which
characterizes the 4FGL+H.E.S.S. data, $W_{\rm p}\sim 6\times 10^{50}$ erg
challenges a hadronic $\gamma$-ray origin. However, both $n_{\rm H}$ and
$W_{\rm p}$ may be smaller if there is a significant contribution from non-
thermal bremsstrahlung emission (true for both data combinations). A similar
representation is presented in Bamba et al. (2009), showing that non-thermal
bremsstrahlung radiation can dominate the high-energy emission.

In summary, the SED fitting analysis shows that the two-leptonic model
efficiently describes the radio, X-ray, and 2FHL $\gamma$-ray data
combination, and in particular provides the most accurate modelization of the
2FHL $\gamma$-ray spectrum. On the other hand, the lepto-hadronic
representation more efficiently describes the radio, X-ray, and 4FGL+H.E.S.S.
extended $\gamma$-ray emission, as shown in the right panel of Figure 11,
under the condition that the ambient particle density is high, $n_{\rm
H}=5000$ cm-3. We discuss the implications for the best-fit models in more
detail in the following section.

Figure 10: Broadband representations for the radio, X-ray, and 2FHL
$\gamma$-ray data combination. Left: Best-fit broadband model for the leptonic
scenario. Radio data is taken from Heywood et al. (2022), X-ray data is from
this work, and Fermi–LAT data from Ackermann et al. (2016); Ajello et al.
(2017). Right: Best-fit broadband model for the lepto-hadronic scenario.

Figure 11: Broadband representations for the radio, X-ray, 4FGL and H.E.S.S. $\gamma$-ray data combination. Left: Best-fit broadband model for the leptonic scenario. Radio data is taken from Heywood et al. (2022), X-ray data is from Section 4.2, Fermi–LAT data from Abdollahi et al. (2022), and H.E.S.S. from H. E. S. S. Collaboration et al. (2018). Right: Best-fit broadband model for the lepto-hadronic scenario. 4FGL+H.E.S.S. | Two-Leptonic | Lepto-hadronic
---|---|---
$W_{\rm e}$ or $W_{\rm p}^{a}$ (erg) | $4.5\times 10^{44}\,1.6\times 10^{48}$ |  $W_{\rm e}=4.5\times 10^{44}$ $W_{\rm p}=6.2\times 10^{50}$
$n_{\rm H}$ (cm-3) | $-\,-$ |  $-$ $5000$
$\alpha_{\rm e}$ or $\alpha_{\rm p}^{b}$ | 1.0 1.4 |  $\alpha_{\rm e}=1.0$ $\alpha_{\rm p}=2.4$
$E_{e,c}$ or $E_{p,c}^{c}$ (TeV) | 794 3.63 |  $E_{e,c}=794$ $E_{p,c}=71.0$
$B^{d}$ ($\mu$G) | 37.7 0.145 | 37.7
2FHL | Two-Leptonic | Lepto-hadronic
$W_{\rm e}$ or $W_{\rm p}^{a}$ (erg) | $1.5\times 10^{46}\,1.1\times 10^{48}$ |  $W_{\rm e}=4.5\times 10^{44}$ $W_{\rm p}=2.1\times 10^{48}$
$n_{\rm H}$ (cm-3) | $-\,-$ |  $-$ $400$
$\alpha_{\rm e}$ or $\alpha_{\rm p}^{b}$ | 1.9 1.3 |  $\alpha_{\rm e}=1.0$ $\alpha_{\rm p}=1.9$
$E_{e,c}$ or $E_{p,c}^{c}$ (TeV) | 617 22.4 |  $E_{e,c}=794$ $E_{p,c}=71.0$
$B^{d}$ ($\mu$G) | 3.0 0.25 | 37.7
Table 4: Summary of the best-fit model parameters for the models displayed in
Figures 11 (4FGL+H.E.S.S.) and 10 (2FHL). The maximum particle energy is fixed
to 2 PeV in all cases. a The total particle energy for electrons $W_{\rm e}$
or protons $W_{\rm p}$, b the index for electrons $\alpha_{\rm e}$ or protons
$\alpha_{\rm p}$, c the cutoff energy for electrons or protons.

###  5.3 Model Interpretation

In the previous sections, we attempted to determine the most likely origin of
the unique and puzzling high-energy source 2FHL J1745.1–3035 through detailed
broadband modeling. We tested two data combinations assuming two energetic
scenarios: 1) the 4FGL+H.E.S.S. extended $\gamma$-ray data is related to the
point-like X-ray source or 2) the point-like 2FHL $\gamma$-ray data is related
to the point-like X-ray source. In the previous sections, we determined that
the most favorable models correspond to the lepto-hadronic model for the
4FGL+H.E.S.S. dataset, while a two-leptonic model can better characterize the
2FHL dataset.

The lepto-hadronic model accurately characterizes the source responsible for
the 4FGL+H.E.S.S. observations, requiring a large target proton density
$n_{\rm H}=5000$ cm-3. If the distance assumed to the source corresponds to
the one reported in Bamba et al. (2009), which is, $d=8.5\,$kpc, then 2FHL
J1745.1–3035 is likely located within the Galactic Center and thus among dense
molecular material. In fact, HESS J1745–303 is spatially coincident with a
molecular cloud (Aharonian et al., 2008) consistent with the estimate $n_{\rm
H}=5000$ cm-3, making this scenario plausible.

The two-leptonic model best characterizes the emission responsible for 2FHL
observations and infers a magnetic field strength that is low compared to the
average value for the ISM ($\sim 2\,\mu$G, Han & Qiao, 1994). If we assume
that all of the considered data are associated in some way, a depicted
scenario is one where the extended $\gamma$-ray emission (4FGL+H.E.S.S.)
represents an older, diffuse, and softer spectral component whereas the point-
like $\gamma$-ray emission (2FHL) represents a younger, more compact, and
harder spectral component. This type of energy-dependent morphology is often
observed in evolved PWNe (e.g., Aharonian et al., 2006; H. E. S. S.
Collaboration et al., 2012), but could also be explained by an energetic
distant SNR where the highest-energy particles have already escaped and are
diffusing into the ambient medium (e.g., Eagle et al., 2020). Another possible
explanation includes both PWN and SNR contributions, whether they are
components of the same system or not. We remark that the models do not
consider any electron-to-proton ratio, acknowledging the possibility the
emission may be coming from separate, unrelated sources.

A final scenario that would be compatible with the hard X-ray spectrum we
measure is a compact accreting binary system where the compact object, either
a neutron star or black hole, is accreting material from a stellar companion.
SS 433 represents the most well-known example of such a system and is detected
from radio to TeV $\gamma$-rays with extended emission originating from the
outflowing jets of the accreting compact object (HAWC Collaboration et al.,
2018). Accreting compact binary systems like SS 433 can be extremely luminous
in the X-ray band ($\gtrsim 10^{39}$ erg s-1), indicating the the total jet
power is much higher (Sudoh et al., 2020). The X-ray spectrum is often modeled
as synchrotron emission as in the case for SS 433 (Safi-Harb et al., 2022),
and can dominate over the Inverse Compton emission if the magnetic field
strength is sufficiently high. Lastly, the X-ray emission of these systems can
be variable on the timescales of days to hours. The lack of both a clear
optical counterpart and of significant X-ray variability overt time-scales of
hours (i.e., within each observation) are however two elements that make this
scenario less likely with respect to the compact PWN one.

While the models presented above provide the best representations to the
broadband data, there are other limitations to be considered. The most
uncertain assumption is the distance used for the modeling, which we assumed
to be the same distance reported in Bamba et al. (2009) for the H.E.S.S.
source, which is $d=8.5\,$kpc. It is possible the source generating the
observed emission is closer, which would enable lower target densities and
lower total particle energies. Secondly, other particle distributions and
hence properties are possible. An alternative broadband representation
includes a non-thermal bremsstrahlung component which yields reasonable
results (Bamba et al., 2009). Finally, we note that a single population
(leptonic or hadronic) cannot explain the broadband data without far exceeding
$10^{51}\,$erg in total particle energies at a distance of 8.5 kpc.

We remark that 2FHL J1745.1–3035 is unlikely to be of extragalactic origin
based on the source selection from the 2FHL catalog, as discussed in Section
1. The spectral index for this source at energies above 50 GeV is much harder
than the average spectral index observed for blazars above 50 GeV.
Furthermore, 2FHL J1745.1–3035 is located along the Galactic plane and is
potentially associated with significant extended GeV and TeV emission (4FGL
and H.E.S.S. counterparts). Based on this evidence, we conclude that the most
plausible scenario for 2FHL J1745.1–3035 is to be the descendant of a Galactic
supernova explosion such as a PWN, SNR, or neutron star or black hole with a
stellar companion, but current observations and the presented models prevent
us from firmly establishing an origin.

##  6 Summary and conclusions

In this paper, we studied the X- and $\gamma$-ray properties of 2FHL
J1745.1-3035, a very high energy Galactic emitter originally detected by the
Fermi–LAT at energies $>$50 GeV. 2FHL J1745.1-3035 has a very hard
$\gamma$-ray spectrum, with photon index above 50 GeV
$\Gamma_{\gamma}$=1.2$\pm$0.4, and is found to be a TeV-emitter by the
Fermi–LAT. A multi-observatory X-ray campaign with XMM-Newton, Chandra and
NuSTAR allowed us to reliably locate the counterpart of 2FHL J1745.1-3035 and
study its properties. The source broadband X-ray spectrum is best-fitted with
a broken power law model with break energy Eb$\sim$7 keV. The X-ray spectrum
is very hard before the break, having photon index $\Gamma_{\rm
X,1}$=0.6$\pm$0.4, while it becomes significantly softer at higher energies,
with $\Gamma_{\rm X,2}$=1.9${}_{-0.2}^{+0.6}$. The X-ray source is compact,
with no evidence for extension in the XMM-Newton and NuSTAR images. However,
the subarcsecond Chandra angular resolution allowed us to detect significantly
extended emission up to a scale of $\sim$5′′.

We report below the main results of our analysis.

  1. 1.

The X-ray analysis favors a compact PWN origin for 2FHL J1745.1-3035: if this
is confirmed, it would be one of the hardest PWNe ever detected in the X-rays
and the hardest ever detected in the $\gamma$-ray (see Figure 5 in Kargaltsev
et al., 2013). Perhaps the only other Galactic source observed to have such a
hard X-ray spectrum is the Crab (e.g., Madsen et al., 2015; Zanin, 2017),
where the spectrum is associated with synchrotron radiation as in the case
developed here for 2FHL J1745.1–3035.

  2. 2.

The multi-band observational evidence, and in particular the X-ray one, could
be linked to the presence of a young nebula ($t\lesssim$103 yrs). In fact,
both the cut-off and the normalization of the synchrotron spectral curve
depend on the magnetic field strength, and the final emission spectrum depends
on a combination of the injected particle spectrum, the losses sustained by
the population of previously injected particles and the system evolution. Due
to these effects, the peak of the synchrotron curve drops in time from roughly
1 MeV at very early times to 1-10 keV when the reverse shock finally crushes
the nebula (see, e.g., Gelfand et al., 2009). At the present day, Fermi–LAT
has detected almost only old PWNe (e.g., Acero et al., 2013), thus making 2FHL
J1745.1-3035 uniquely interesting. However, we also note that the TeV to X-ray
luminosity ratio for this source is $\sim 5$ and is expected to increase with
time (Mattana et al., 2009; Kargaltsev et al., 2013). Some of the youngest
PWNe/SNRs have ratios $\lesssim 2$ while older systems have ratios larger than
2 (Yamazaki et al., 2006), but the value can vary depending on the system and
environment conditions.

  3. 3.

The PWN scenario is also supported by the lack of short-term variability in
the single-epoch observations, which is expected if a source is indeed a PWN,
while variability might be observed in other classes of compact, X-ray
emitting Galactic sources, such as X-ray binaries or cataclysmic variable
stars. We note that no observational evidence for a pulsar is reported in the
literature for the 4FGL source: in particular, a blind pulsation search was
performed by Hui et al. (2011) on the first 29 months of Fermi–LAT
observations, but no clear evidence for periodicity was detected.

  4. 4.

The broadband SED fitting we performed supports a scenario where the compact
X-ray source is linked to the 2FHL target and is well fitted by a two-leptonic
model, while the 4FGL+H.E.S.S. source requires the presence of an hadronic
component to be efficiently modeled. These results are consistent with a
scenario where the extended $\gamma$-ray emission (4FGL+H.E.S.S.) represents
an older, diffuse, and softer spectral component whereas the point-like
$\gamma$-ray emission (2FHL) represents a younger, more compact, and harder
spectral component.

  5. 5.

Given its TeV-emitter nature, 2FHL J1745.1-3035 represents an ideal follow-up
target for the upcoming Cherenkov Telescope Array (CTA Acharya et al., 2013;
Cherenkov Telescope Array Consortium et al., 2019). CTA will have a $\sim$2
times better angular resolution and a $\sim$5 times flux sensitivity than
H.E.S.S. at 1 TeV (Cherenkov Telescope Array Consortium et al., 2019), and is
therefore expected to resolve into individual point sources at least a
fraction of the diffuse emission detected by H.E.S.S.. The study of the
Galactic Center region is one of the Key Science Programs of the CTA
Observatory, and in particular 2FHL J1745.1-3035 will be covered by the
planned 300-hours Extended Survey. Furthermore, recent preparatory studies for
the ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana) Mini-
Array Observatory (Vercellone et al., 2022) suggest that observations at
$\sim$100–300 TeV can provide important information on the emission processes
in young, powerful PWNe, and in particular can put strong constraints on the
presence (or lack of) a hadronic component.

In summary, all the observational evidence shows 2FHL J1745.1-3035 is a rare
Galactic Fermi–LAT source, having the hardest $\gamma$–ray photon index in the
2FHL sample and an impressively hard X-ray photon index. Combining all of this
evidence hints at a scenario where 2FHL J1745.1-3035 could be a newly
detected, young PWN system.

The unique properties of this target make it an ideal source for follow-up
campaigns with facilities looking at the TeV sky, first and foremost CTA.
Furthermore, deeper Chandra observations would allow one to perform a multi-
region X-ray spectral fit to obtain a radial profile of the X-ray photon index
and flux, which, if confirmed to be a PWN, could be used to test different PWN
emission models to estimate the properties such as the age (see, e.g., Tibaldo
et al., 2017).

## Acknowledgments

This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS)
jointly developed by the ASI Space Science Data Center (SSDC, Italy) and the
California Institute of Technology (Caltech, USA). This work makes use of
Matplotlib (Hunter, 2007) and NumPy (Harris et al., 2020). This work has made
use of data from the European Space Agency (ESA) mission Gaia
(https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and
Analysis Consortium (DPAC,
https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has
been provided by national institutions, in particular the institutions
participating in the Gaia Multilateral Agreement. This publication makes use
of data from the UKIRT Infrared Deep Sky Survey (UKIDSS). The UKIDSS project
is defined in Lawrence et al. (2007). UKIDSS uses the UKIRT Wide Field Camera
Casali et al. (WFCAM; 2007). The photometric system is described in Hewett et
al. (2006), and the calibration is described in Hodgkin et al. (2009). The
pipeline processing and science archive are described in Hambly et al. (2008).
We thank Patrick Slane for the useful conversation and suggestions. S.M.
acknowledges financial support from the European Union – Next Generation EU
under the project IR0000012 - CTA+ (CUP C53C22000430006), announcement N.3264
on 28/12/2021: “Rafforzamento e creazione di IR nell’ambito del Piano
Nazionale di Ripresa e Resilienza (PNRR)”. A.D. is thankful for the support of
the Ramón y Cajal program from the Spanish MINECO, Proyecto
PID2021-126536OA-I00 funded by MCIN / AEI / 10.13039/501100011033, and
Proyecto PR44/21-29915 funded by the Santander Bank and Universidad
Complutense de Madrid.

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##  Appendix A Single epoch X-ray spectra

We report in this Appendix, in Figure 12, the single-epoch X-ray spectra of
2FHL J1745.1-3035. The best-fit models are obtained by fitting the data with
an absorbed power law, as discussed in Section 4.1.

Figure 12: Single epoch X-ray spectra of 2FHL J1745.1-3035. In the XMM-Newton
observations, MOS1, MOS2, and pn spectra are plotted in black, red, and green,
respectively; in the NuSTAR observations, FPMA and FPMB spectra are plotted in
black and in red, respectively. The best-fit model from an absorbed power law
is plotted as a solid line.

##  Appendix B Light curves of the 2017 and 2021 XMM-Newton observations

We report in this Appendix, in Figure 13, the XMM-Newton light curves in the
2–4 keV, 4–6 keV, and 6–10 keV, for the 2017 and 2021 observations of 4XMM
J174507.9-303906. We derived these light curves directly from the data: as it
can be seen, differently to what is reported in the 4XMM catalog, no clear
evidence of cross-instrument variability is visible in the 2021 observations;
as for the 2017 observation, significant variability is detected only at
energies $>$6 keV and can be attributed to a strong background flare.

Figure 13: XMM-Newton light curves in the 2–4 keV (top panels), 4–6 keV
(center), and 8–10 keV band (bottom) for the 2017 (left) and 2021 (right)
observations of 4XMM J174507.9-303906. MOS1 data-points are plotted in blue,
MOS2 in red, and PN in cyan. The MOS1 and MOS2 light curves have been rescaled
so that their average value matches the pn one.