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Observation Date (UT) Observation Lat

Canonical Name:HESS J1731-347
TeVCat Name:TeV J1732-347
Other Names:SNR G353.6-0.7
Source Type:Shell
R.A.:17 32 03 (hh mm ss)
Dec.:-34 45 18 (dd mm ss)
Gal Long: 353.54 (deg)
Gal Lat: -0.67 (deg)
Distance: 3.2 kpc
Flux: (Crab Units)
Energy Threshold: GeV
Spectral Index:2.32
Extended:Yes
Size (X):0.18 (deg)
Size (Y):0.11 (deg)
Discovery Date:2007-07
Discovered By: H.E.S.S.
TeVCat SubCat:Default Catalog

Source Notes:

H.E.S.S. Galactic Plane Survey (HGPS, 2018):
A selection of information for each of the 78 sources in the HGPS is provided in TeVCat. For full details, visit the HGPS website.

Name: HESS J1731-347
Source Class: SNR
Identified Object: G353.6-0.7
R.A. (J2000): 263.01 deg (17 32 03)
Dec. (J2000): -34.76 deg (-34 45 18)
Spatial Model: Shell
Size: 0.270 +/- 0.020 deg
Spectral Model: power law
Integral Flux > 1 TeV: 2.01e-12 +/- 1.46e-13 cm-2 s-1
Pivot Energy, E0: 0.78 TeV
Diff. Flux at E0: 4.67e-12 +/- 1.90e-13 cm-2 s-1 TeV-1
Spectral Index: 2.32 +/- 0.06
HGPS Source Notes:
"This is one of the fourteen EXTERN sources in the HGPS catalog, i.e., VHE sources in the HGPS region previously detected by H.E.S.S. that were not reanalyzed in the paper:"
- "Given the difficulties with modeling complex source morphologies, we decided to restrict the HGPS analyses to a symmetrical Gaussian model assumption and exclude all firmly identified shell-like sources and the very complex GC region from reanalysis."
This is one of the 31 firmly-identified objects among the HGPS sources. One association is listed in Table A.9. "This is a list of astronomical objects, extracted from catalogs of plausible counterparts, which are are found to be spatially coincident with the HGPS source":
- G353.6−0.7 (SNR)

Source position and its uncertainty:

From Aharonian et al. (2008):
- RA: (J2000): 17 31 55
- Dec (J2000): -34 42 36
- The fit position has a statistical error of 0.05 deg

From Abramowski et al. (2011):
- RA: (J2000): 17 32 03
- Dec (J2000): -34 45 18
- no positional uncertainty is quoted

Source of the gamma-ray emission:

From Horvath et al. (2023):
- "Recently, Doroshenko and collaborators reported a very low-mass
compact star, a Central Compact Object named XMMU J173203.3-344518
inside the supernova remnant HESS J1731-347. Its tiny mass is at odds
with all calculations of minimum masses of neutron stars generated by
iron cores, therefore (and even if not compellingly) it has been
suggested to be a strange star. Besides the mass, a radius and surface
temperature have been extracted from data, and the whole body of
information should ultimately reveal if this object is truly
consistent with an exotic composition."
- "We conclude that XMMU J173203.3-344518 in the remnant HESS
J1731-347 fits into a ``strange star'' scenario that is also
consistent with heavier compact stars, which can also belong to the
same class and constitute an homogeneous type of self-bound objects
produced in Nature."

From Di Clemente, Drago & Pagliara (2022):
- "The analysis of the central compact object within the supernova
(SN) remnant HESS J1731-347 suggests that it has a small radius and,
even more interestingly, a mass of the order or smaller than one solar
mass (Doroshenko et al. 2022, Nature Astronomy). This raises the
question of which astrophysical process could lead to such a small
mass, since the analysis of various types of SN explosions indicate
that is it not possible to produce a neutron star (NS) with a mass
smaller than about 1.17 M_sun . Here we show that masses of the order
or smaller than one solar mass can be obtained in the case of strange
quark stars (QSs) and that it is possible to build a coherent
astrophysical scenario explaining not only"

From Doroshenko et al. (2022):
- "To constrain the equation of state of cold dense matter,
astrophysical measurements are essential. These are mostly based on
observations of neutron stars in the X-ray band, and, more recently,
also on gravitational wave observations. Of particular interest are
observations of unusually heavy or light neutron stars which extend
the range of central densities probed by observations and thus permit
the testing of nuclear-physics predictions over a wider parameter
space. Here we report on the analysis of such a star, a central
compact object within the supernova remnant HESS J1731-347. We
estimate the mass and radius of the neutron star to be
M =0.77 -0.17 +0.20 M_sun and R = 10.4 -0.78 +0.86 km, respectively,
based on modelling of the X-ray spectrum and a robust distance
estimate from Gaia observations. Our estimate implies that this object
is either the lightest neutron star known, or a `strange star' with a
more exotic equation of state."

From Tsaloukidis et al. (2022):
- "... we discuss the constraints on the radius and mass of the
recently observed compact object within the supernova remnant HESS
J1731-347. The estimations implies that this object is either the
lightest neutron star known, or a star with a more exotic equation of
state."

From Cui et al. (2019):
- "The SNR HESS J1731-347, which displays a non-thermal X-ray and TeV
shell structure, is believed to be still expanding inside the
low-density pre-SN bubble. A dense molecular clump, called MC-core, is
located at the western part of the SNR, and it is possibly embedded
inside this pre-SN bubble and presently colliding with the SNR.
Following the previous intriguing discoveries on HESS J1731-347, that
MC-core region has shown a soft GeV emission (S0) and a dim X-ray
emission up to 10 keV, we explored whether the SNR has collided with
MC-core at its west."
- "Our Fermi-LAT analysis has unveiled two GeV components of HESS J1731-347,
one located at the SNR center displaying a spectral index of
... 1.79 +/- 0.22 (stat) +/- 0.10 (sys),
and one located at MC-core displaying a spectral index of
... 2.42 +/- 0.22 (stat) +/- 0.10 (sys)."
- "A hadronic model involving a shock-cloud collision scenario is
built to explain the gamma-ray emission from this area. It consists of
three CR sources:
... run-away super-TeV CRs that have escaped from the fast shock,
... leaked GeV CRs from the stalled shock,
... and the local CR sea.
The X-ray and gamma-ray emission of the SNR excluding the shock-cloud
interaction region is explained in a one-zone leptonic model. Our
shock-cloud collision model explains well the GeV-TeV observations
from both cloud regions around HESS J1731-347, i.e. from the cloud in
contact with the SNR and from the more distant cloud which is
coincident with the nearby TeV source HESS J1729-345. We find however
that the leaked GeV CRs from the shockcloud collision do not
necessarily dominate the GeV emission from the clouds, due to a
comparable contribution from the local CR sea."
- "In summary, our results have further supported the recent finding
of S0 by Condon et al. (2017), that the GeV emission from HESS J1731-347
is likely to be dominated by two components, a soft component from S0
and a hard component from the SNR. After separating S0 from the SNR,
our GeV flux of the SNR is lower than those published ones, especially
for those data points with energies < 20GeV"

From Condon et al. (2017):
- the first high-significance GeV gamma-ray detection of HESS J1731-347
at GeV energies with eight years of Fermi-LAT Pass 8 data is reported:
- "Using 8 years of Fermi-LAT Pass 8 data at energies between 1 GeV
and 2 TeV, we detect emission at the position of HESS J1731-347 with a
significance of approx. 5 sigma and a spectral index of 1.66 +/- 0.16stat
+/- 0.12syst. The hardness of the index and the good connection with the
TeV spectrum of HESS J1731-347 support an association between the two
sources."

From Guo et al. (2017):
- the detection of this source at GeV energies using 9 years of data
from the Fermi Large Area Telescope is reported:
- "We find a slightly extended GeV source in the direction of
HESS J1731-347. The spectrum above 1 GeV can be fitted by a power-law
with an index of 1.77 +/- 0.14, and the GeV spectrum connects smoothly
with the TeV spectrum of HESS J1731-347. Either a hadronic-leptonic or
a pure leptonic model can fit the multi-wavelength spectral energy
distribution of the source. However, the hard GeV gamma-ray spectrum
is more naturally produced in a leptonic (inverse Compton scattering)
scenario, under the framework of diffusive shock acceleration."

From Maxted et al. (2017):
- Observations were carried out with the Mopra radio telescope,
targeting CO(1-0), CO(1-0) and CS(1-0) emission. The authors conclude:
- "when adopting a CO X-factor towards the mid-range of published
values, X-ray absorption column densities derived from the HESS
J1731−347 X-ray emission are consistent with column densities
foreground to the Scutum-Crux arm at a line of sight velocity of
approx. -15 km s-1 , suggesting a kinematic distance of approx. 3.2
kpc for HESS J1731−347."
- "Components of dense molecular gas at approx. 3.2 kpc are coincident
with the north of HESS J1731−347, HESS J1729−345 and a cloud
associated with the HII region G353.43−0.37, as evidenced by CS(1-0)
emission and infrared-dark features. The detection of dense gas
towards gamma-ray emission to the north of HESS J1731−347 is
suggestive of a runaway CR scenario and flags a new component of
target material mass to be included in future particle propagation
models."
- "This dense gas lends weight to the idea that HESS J1729−345 and
HESS J1731−347 are connected, perhaps via escaping cosmic rays."

From Nayana et al. (2017):
- "We compare the radio brightness profile with the VHE emission. It
is particularly striking that the peak in gamma-ray emission comes
from the filament which is faintest in the radio, and the brightest
filaments in the radio correspond to faint emission in gamma rays."
- "The faintest filament in gamma rays also shows a steep spectral index
of −1.11 +/- 0.22. This may be due to the effect of non-uniform
magnetic field strength, which is suggestive from the possible
evidence of synchrotron cooling in that region of the SNR."
- "In this framework, the anti-correlated emission can be explained if
the VHE emission is of leptonic origin for an isotropic injection.
This kind of an anti-correlated emission is reported for the first
time in VHE shell SNRs."

From Cui, Puhlhofer & Santangelo (2016):
- the source is modeled using a core-collapse SNR model
- "Through exploring the SNR evolution history with different
scenarios (8 M-solar, 15 M-solar, 20 M-solar, 25 M-solar progenitor
masses), we found that the SNR HESS J1731-347 is most likely still
expanding inside its progenitor main sequence wind bubble. With 20
M-solar and 25 M-solar progenitor mass scenarios, we successfully
reproduce relatively fast shocks > 2000 km/s at present time (∼ 6 kyr
and ∼ 3 kyr after the SN explosion), which is required by the
non-thermal X-ray emission detected from the SNR."

From Tian, W.W. et al. (2009):
- "argue that the extended TeV emission originates from the
interaction between the SNR shock and the adjacent CO clouds rather
than from a PWN."

From de Naurois (2011):
- classified as a shell-type SNR

From Abramowski et al. (2011):
- classified as a shell-type SNR

Source Extent:

From Abramowski et al. (2011):
- best fit radius is 0.27 +/- 0.02 deg
- emission compatible with thin, spatially unresolved shell
with upper limit thickness of 0.12deg

From Aharonian et al. (2008):
- semi-major axis: 0.18 +/- 0.07 deg
- semi-minor axis: 0.11 +/- 0.03 deg
- angle: -89 +/- 21 deg
...measured counter-clk relative to RA axis

Note on size of emission region from Abramowski et al. (2011):
"The flux measured here is lower than what has been derived initially in
Aharonian et al. (2008): (16.2 +/- 3.6stat +/- 3.2syst) e10−12 erg
cm−2 s−1 in the same energy band. However, the region of extraction in
the discovery paper was much larger (r = 0.6deg versus r = 0.3deg in
this paper), including HESS J1729−345 and possibly some surrounding
diffuse emission. A cross-check to derive the flux from the SNR only
using the same data set as used in Aharonian et al. (2008) and
following the original analysis method gave results consistent with
the complete data set presented here thus confirming that the flux
difference was mainly due to the choice of the integration region"

Spectral Properties:

From Abramowski et al. (2011):
- spectral index: 2.32 +/- 0.06stat +/- 0.2syst
- decorrelation energy: 0.783 TeV
- normalization: 4.67 +/- 0.19stat e-12 cm-2 s-1 TeV-1
- 1-10 TeV integrated flux: 6.91 +/- 0.75stat e-12 erg cm-2 s-1
- systematic error of 20% on the flux

Distance:

From Maxted et al. (2017):
- Observations were carried out with the Mopra radio telescope,
targeting CO(1-0), CO(1-0) and CS(1-0) emission. The authors conclude:
- "when adopting a CO X-factor towards the mid-range of published
values, X-ray absorption column densities derived from the HESS
J1731−347 X-ray emission are consistent with column densities
foreground to the Scutum-Crux arm at a line of sight velocity of
approx. -15 km s-1 , suggesting a kinematic distance of approx. 3.2
kpc for HESS J1731−347."

From Abramowski et al. (2011):
- a lower limit of 3.2 kpc is quoted


Seen by: H.E.S.S.
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