HESS J1731-347: TeV SNR & Compact Object

• HESS J1731-347 is a shell-type supernova remnant identified by its TeV gamma-ray emission and non-thermal X-rays, coinciding with radio remnant G353.6-0.7.
• Deep H.E.S.S. and multiwavelength observations have resolved a detailed shell morphology and revealed interactions with dense gas, providing insights into cosmic-ray acceleration.
• The central compact object, with its unusually low mass and radius, challenges standard neutron-star models and serves as a critical benchmark for dense-matter equation-of-state studies.

Searching arXiv for recent and foundational papers on HESS J1731-347. Searching arXiv for HESS J1731-347 compact-object and multiwavelength studies. HESS J1731-347, identified with the radio supernova remnant G353.6-0.7, is a shell-type supernova remnant emitting TeV gamma rays and non-thermal X-rays, with the compact X-ray source XMMU J173203.3-344518 at its geometric center. Deeper H.E.S.S. observations established that the TeV emission forms a shell with essentially the same angular size and position as the radio remnant, while later X-ray, radio, GeV, and dense-gas studies made the system a key case for both shock-acceleration physics and compact-star equation-of-state work. Its importance increased sharply after the central compact object was inferred to have $M=0.77^{+0.20}_{-0.17}\,M_\odot$ and $R=10.4^{+0.86}_{-0.78}\,\mathrm{km}$ under specific atmosphere and distance assumptions, values that are unusually small for a neutron star (Collaboration et al., 2011, Sagun et al., 2023).

1. Shell supernova remnant and source identification

HESS J1731-347 was transformed from an unidentified extended TeV source into a firmly established TeV shell-type supernova remnant by deeper H.E.S.S. observations. With 59 hours of observation, the gamma-ray morphology was shown to be a shell with best-fit radius $r=0.27^\circ\pm0.02^\circ$, consistent with the radio shell radius of about $0.25^\circ$, and with a shell-thickness upper limit $\Delta r<0.12^\circ$ at 90% confidence. The source was detected at $22\sigma$, and the shell model was preferred over a filled sphere at $3.9\sigma$. The corresponding H.E.S.S. spectrum, extracted from a circular region of radius $0.3^\circ$, was fit by a power law with $$\Gamma = 2.32 \pm 0.06_{\rm stat} \pm 0.20_{\rm syst}$$, normalization $$N_0=(6.1\pm0.5_{\rm stat})\times10^{-12}\ {\rm cm^{-2}\ s^{-1}\ TeV^{-1}}$$ at $R=10.4^{+0.86}_{-0.78}\,\mathrm{km}$0 TeV, and integrated $R=10.4^{+0.86}_{-0.78}\,\mathrm{km}$1 TeV energy flux $R=10.4^{+0.86}_{-0.78}\,\mathrm{km}$2 (Collaboration et al., 2011).

The remnant belongs to the small class of shell-type supernova remnants detected at very high energies. The shell is spatially coincident with the radio remnant G353.6-0.7, and the presence of the central compact object strengthens the SNR interpretation. In the H.E.S.S. field, the adjacent source HESS J1729-345 and the bridge-like emission between the two sources later became part of the environmental discussion, especially in escaping-cosmic-ray scenarios (Capasso et al., 2016).

The shell also has complete X-ray coverage with XMM-Newton. That full-shell analysis confirmed that HESS J1731-347 is a non-thermal shell-type remnant with shell-averaged photon index $R=10.4^{+0.86}_{-0.78}\,\mathrm{km}$3, total $R=10.4^{+0.86}_{-0.78}\,\mathrm{km}$4 keV shell flux $R=10.4^{+0.86}_{-0.78}\,\mathrm{km}$5, and a strongly variable absorption column across the remnant. The shell is intrinsically asymmetric in X-rays, with brighter eastern filaments and fainter western emission toward the Galactic plane (Doroshenko et al., 2017).

2. Radio structure, X-ray absorption, and ambient medium

Low-frequency radio imaging with the GMRT established clear 325 and 610 MHz counterparts of the remnant and showed that the shell is filamentary rather than uniformly bright. The integrated flux density at 325 MHz is $R=10.4^{+0.86}_{-0.78}\,\mathrm{km}$6 Jy. Four bright filaments detected at both 325 and 610 MHz have spectral indices ranging from $R=10.4^{+0.86}_{-0.78}\,\mathrm{km}$7 to $R=10.4^{+0.86}_{-0.78}\,\mathrm{km}$8, consistent overall with non-thermal synchrotron emission. The brightest radio structures lie mainly in the south-east and east, whereas the north-east TeV peak corresponds to comparatively faint radio emission; this radio–TeV anti-correlation was interpreted in terms of spatially varying magnetic field strength in a leptonic inverse-Compton scenario, especially if acceleration is synchrotron-loss limited (Nayana et al., 2017).

The X-ray absorption pattern is a second major environmental diagnostic. Across the shell, $R=10.4^{+0.86}_{-0.78}\,\mathrm{km}$9 varies from about $r=0.27^\circ\pm0.02^\circ$0 to $r=0.27^\circ\pm0.02^\circ$1, increasing toward the western and south-western sectors. Full-shell XMM-Newton analysis found that the X-ray emission is suppressed toward the Galactic plane, and interpreted this as evidence for lower shock velocities there, likely due to interaction with nearby molecular material. The same study found a statistically significant correlation between the X-ray-derived absorption and integrated CO emission for $r=0.27^\circ\pm0.02^\circ$2, reinforcing the previously suggested lower distance limit of about $r=0.27^\circ\pm0.02^\circ$3 kpc and favoring a distance of a few kiloparsecs rather than one beyond the Galactic center (Doroshenko et al., 2017).

Molecular-line work sharpened the distance and cloud-association picture. Mopra CO, CS, and SGPS HI data were used to compare cumulative foreground gas columns with X-ray absorption columns in multiple regions across the remnant. The inverse-variance-weighted intercept velocity was found to be $r=0.27^\circ\pm0.02^\circ$4, favoring association with the Scutum-Crux arm at $r=0.27^\circ\pm0.02^\circ$5 kpc. At that distance, dense infrared-dark gas traced by CS(1–0) is coincident with the northern part of HESS J1731-347, the nearby HII region G353.43-0.37, and the direction of HESS J1729-345, strengthening the idea that the remnant and the surrounding molecular complex are physically linked (Maxted et al., 2017).

3. GeV detection and high-energy emission scenarios

Independent Fermi-LAT analyses reported a GeV counterpart with a hard spectrum smoothly joining the TeV data. One analysis using 8 years of Pass 8 data over $r=0.27^\circ\pm0.02^\circ$6 GeV–$r=0.27^\circ\pm0.02^\circ$7 TeV found a detection at approximately $r=0.27^\circ\pm0.02^\circ$8; the shell-template fit yielded photon index $r=0.27^\circ\pm0.02^\circ$9 and energy flux $0.25^\circ$0 between 1 GeV and 2 TeV. Morphology was not decisively resolved in the full band, but the LAT spectral energy distribution was reported to connect very well to the H.E.S.S. spectrum, supporting the association with the shell remnant (Condon et al., 2017). A separate 9-year LAT study found a slightly extended source with preferred disk radius $0.25^\circ$1, $0.25^\circ$2, photon index $0.25^\circ$3, and integral photon flux $0.25^\circ$4 over $0.25^\circ$5 GeV (Guo et al., 2017).

Before those GeV detections, a four-year LAT analysis had produced only upper limits. Those limits were strong enough that one-zone hadronic models with energetic proton spectral slope greater than $0.25^\circ$6 could be ruled out. In the corresponding broadband fits, the preferred one-zone leptonic model had electron spectral slope $0.25^\circ$7 and magnetic field about $0.25^\circ$8, while hadronic fits required unusually hard proton spectra and larger magnetic fields. The paper therefore placed HESS J1731-347 in the same broad phenomenological class as RX J1713.7-3946, RX J0852.0-4622, and RCW 86, where inverse-Compton-dominated interpretations are favored (Yang et al., 2014).

The GeV field is nevertheless not uniform. A focused LAT reanalysis isolated a compact soft GeV component coincident with the western molecular cloud region MC-core. In that decomposition, the rest of the SNR had $0.25^\circ$9, while the compact western component had $\Delta r<0.12^\circ$0. That contrast was interpreted through a shock-cloud collision scenario in which the western shock stalls in dense gas, suppressing X-ray synchrotron emission while allowing GeV cosmic rays to leak into the cloud; the rest of the shell remained consistent with a one-zone leptonic interpretation (Cui et al., 2019). The broader H.E.S.S. surroundings study also reported that the TeV bridge region toward HESS J1729-345 shows the best correspondence with dense gas in the velocity interval $\Delta r<0.12^\circ$1 to $\Delta r<0.12^\circ$2, again pointing to a $\Delta r<0.12^\circ$3 kpc association and to escaping-cosmic-ray illumination of nearby clouds (Capasso et al., 2016).

4. The central compact object and the low-mass anomaly

The central compact object in HESS J1731-347 became a major compact-star test case after Doroshenko et al. inferred, under a uniform-temperature carbon-atmosphere interpretation at distance $\Delta r<0.12^\circ$4 kpc, a mass and radius of

$\Delta r<0.12^\circ$5

together with a redshifted surface temperature

$\Delta r<0.12^\circ$6

and an age of about $\Delta r<0.12^\circ$7 kyr. This would make it simultaneously one of the lightest and smallest compact objects ever detected. The same paper emphasized three tensions: most viable neutron-star EOSs do not naturally produce such a small radius at such a low mass; standard core-collapse calculations tend to predict a lowest gravitational mass closer to $\Delta r<0.12^\circ$8; and unpaired quark matter would cool too fast to match the temperature datum (Sagun et al., 2023).

That observational inference is explicitly model dependent. The analysis depends on the assumptions of a uniform-temperature carbon atmosphere and a source distance of $\Delta r<0.12^\circ$9 kpc, and later papers repeatedly note that future work should reassess hot and cold spots, atmosphere composition, and distance systematics. The low mass therefore functions less as an established population datum than as a stress test for neutron-star, hybrid-star, strange-star, and dark-matter-admixed-star models (Sagun et al., 2023).

The object is also unusual because its low mass makes it probe the EOS in a density regime not far above saturation. A recent Bayesian EOS study treating it as a neutron star emphasized that a HESS-like object constrains mainly the symmetry-energy sector and the radii of low-to-intermediate-mass stars rather than the deepest core physics; under that assumption the inferred HESS source properties became $22\sigma$0, $22\sigma$1, and central pressure $22\sigma$2 at 90% confidence (Char et al., 2024).

5. Dense-matter interpretations of the compact object

The most conservative interpretation keeps the object hadronic. In a joint analysis of thermal evolution and mass-radius constraints, HESS J1731-347 was found to be consistent within $22\sigma$3 with a very low-mass neutron star built from a soft hadronic EOS, provided pairing and envelope physics are chosen appropriately; in the same study, strange stars, hybrid stars with an early deconfinement transition, and dark-matter-admixed neutron stars were also found to be consistent within $22\sigma$4, while a stiff purely hadronic EOS such as BigApple was disfavored (Sagun et al., 2023). Other hadronic studies reached more specific parameter statements. A parity-doublet analysis argued that the object can be explained as the lightest neutron star for $22\sigma$5, with the favored region characterized by large $22\sigma$6 and small $22\sigma$7, in practice $22\sigma$8 (Gao et al., 2024). A density-dependent relativistic mean-field study based on the DDVT model concluded that vector-meson tensor couplings soften the outer-core EOS and that a soft crust is crucial; in its softest realizations the model yields $22\sigma$9, sufficiently small to describe HESS J1731-347 while remaining consistent with GW170817 and NICER constraints (Huang et al., 2023).

Hybrid-star interpretations split according to the transition construction and the assumed quark phase. A model-agnostic study built $3.9\sigma$0 hybrid EOSs from generalized piecewise polytropes plus a constant-speed-of-sound quark sector and argued that many of them can explain the HESS object as a slow stable hybrid star, with the slow-conversion assumption extending the dynamically stable branch beyond the usual turning-point limit (Mariani et al., 2024). Another hybrid analysis, motivated by the tension between PREX-II and HESS, found that constant-bag vector-MIT constructions capable of reaching the HESS region are strongly disfavored by the $3.9\sigma$1 pulsar constraint, whereas a Gaussian density dependence of the bag constant yields HESS-compatible hybrid EOSs that can satisfy a conservative $3.9\sigma$2 bound, though PSR J0030+0451 remains problematic at $3.9\sigma$3 and only marginally consistent at $3.9\sigma$4; in that framework, combining HESS with PSR J0952-0607 suggested a strong phase transition below $3.9\sigma$5 (Laskos-Patkos et al., 2023). A more specific CFL-based study concluded that pure, absolutely stable color-flavor-locked quark matter can explain HESS J1731-347 while also satisfying heavy-pulsar and GW170817 constraints, with favored parameters approximately $3.9\sigma$6 and $3.9\sigma$7, whereas the particular hybrid MDI-APR1 + CFL models constructed with a Maxwell transition can reproduce the low-mass HESS object but fail the $3.9\sigma$8 pulsar constraint (Kourmpetis et al., 2024).

Strange-star interpretations remain prominent because self-bound matter naturally allows compact low-mass configurations. A “minimal consistency checks” study concluded that HESS J1731-347 fits within the same quark-star models used to explain the NICER masses and radii of PSR J0030+0451 and PSR J0740+6620, and further argued that a simple cooling scenario with superconducting quarks and small pairing gaps provides an overall good explanation of the surface temperature (Horvath et al., 2023). A separate quark-star study using two vBag and two CFL benchmark EOSs found that all four pass through the HESS confidence region while still supporting heavy compact stars above $3.9\sigma$9, and computed the ten lowest radial frequencies for a $0.3^\circ$0 HESS-like strange star; the fundamental mode lies around $0.3^\circ$1–$0.3^\circ$2 kHz (Rather et al., 2023).

Model dependence extends even within purely hadronic interpretations. A recent CCT relativistic mean-field study with a Gibbs construction for a low-density two-phase nucleon-lepton system argued that the same mechanism that makes low-mass stars unusually compact also implies a mixed phase extending up to densities normally identified with crustal matter. In that specific model, HESS J1731-347-like stars would have a crust about 50% thicker and crustal mass and moment of inertia several times larger than in more standard constructions (Kubis et al., 9 Jul 2025).

6. Scientific significance and unresolved questions

HESS J1731-347 is significant because it combines two otherwise separate research programs. As a shell-type TeV supernova remnant with non-thermal X-rays, GeV emission, and dense-gas structure, it is a laboratory for particle acceleration, cloud interaction, and cosmic-ray escape. As a remnant whose central compact object may have $0.3^\circ$3 and $0.3^\circ$4 km, it probes compact-star matter in a regime where the outer core, crust, and near-saturation symmetry energy are unusually important (Maxted et al., 2017, Char et al., 2024).

The main controversies are observational rather than terminological. For the shell, the primary open issue is still the relative contribution of leptonic and hadronic gamma-ray channels. Hard GeV spectra smoothly connected to the TeV data favor inverse-Compton-dominated emission for much of the shell, but the western cloud-contact region and the external source HESS J1729-345 remain natural locations for hadronic components powered by shocked or escaped cosmic rays (Condon et al., 2017, Cui et al., 2019). For the compact object, the main uncertainty is whether the carbon-atmosphere, uniform-temperature, $0.3^\circ$5 kpc interpretation of the X-ray spectrum is correct. If that inference is revised upward in mass or radius, the pressure on conventional neutron-star models weakens; if it is confirmed, the case for unusually soft hadronic matter, hybrid matter, or self-bound quark matter remains strong (Sagun et al., 2023).

The broader implication is not that one unique nature has been established. Rather, HESS J1731-347 has become a benchmark object against which multiple microphysical frameworks are now tested: soft hadronic EOSs, parity-doublet and tensor-coupled mean-field models, model-agnostic hybrid constructions, CFL strange-matter scenarios, and even dark-matter admixture. The present literature therefore treats HESS J1731-347 less as a settled classification and more as a stringent astrophysical constraint linking supernova-remnant phenomenology, dense QCD matter, and compact-star formation.