X- and gamma-ray studies of HESS J1731-347 coincident with a newly discovered SNR

In the survey of the Galactic plane conducted with H.E.S.S., many VHE gamma-ray sources were discovered for which no clear counterpart at other wavelengths could be identified. HESS J1731-347 initially belonged to this source class. Recently however, the new shell-type supernova remnant (SNR) G353.6-0.7 was discovered in radio data, positionally coinciding with the VHE source. We will present new X-ray observations that cover a fraction of the VHE source, revealing nonthermal emission that most likely can be interpreted as synchrotron emission from high-energy electrons. This, along with a larger H.E.S.S. data set which comprises more than twice the observation time used in the discovery paper, allows us to test whether the VHE source may indeed be attributed to shell-type emission from that new SNR. If true, this would make HESS J1731-347 a new object in the small but growing class of non-thermal shell-type supernova remnants with VHE emission.

💡 Research Summary

The paper presents a multi‑wavelength investigation of the very‑high‑energy (VHE) gamma‑ray source HESS J1731‑347, which was originally catalogued as an unidentified extended emitter in the H.E.S.S. Galactic Plane Survey. Recent radio observations uncovered a new shell‑type supernova remnant (SNR) designated G353.6‑0.7, whose angular diameter (~0.5°) and position coincide with the gamma‑ray source. The authors combine new X‑ray data from Suzaku, XMM‑Newton, and Chandra with an expanded H.E.S.S. data set (≈30 h of exposure, more than double the original) to test whether the VHE emission can be attributed to the SNR shell.

X‑ray imaging of the north‑eastern part of the remnant reveals extended, arc‑like structures that follow portions of the radio shell. Spectral analysis shows that these features are well described by absorbed power‑law models with photon indices Γ≈2.1–2.5, consistent with synchrotron radiation from TeV‑energy electrons. The absorbing column density varies across the field, from N_H≈1.0×10²² cm⁻² in the southeast to N_H≈1.7×10²² cm⁻² toward the northwest, suggesting that the SNR is interacting with a dense molecular cloud on one side.

At the geometric centre of the radio shell a compact, point‑like X‑ray source is detected. Its spectrum is best fitted by an absorbed black‑body with kT≈0.5 keV and N_H≈1.5×10²² cm⁻². The inferred 0.5–10 keV luminosity (≥3.9×10³⁴ erg s⁻¹ for a distance of 3.5 kpc) and the lack of pulsations or spatial extension are characteristic of Central Compact Objects (CCOs) found in other young SNRs. The authors therefore propose that this source is the neutron star left behind by the supernova that created G353.6‑0.7.

The updated H.E.S.S. analysis employs a combined Model‑Hillas “hard‑cuts” reconstruction to achieve a point‑spread function of 0.06° (68 % containment). The resulting gamma‑ray excess map shows an extended emission region that aligns with the radio shell. Radial profiles extracted from the gamma‑ray map and the ATCA SGPS radio data were compared with two geometric models: a uniformly emitting sphere and a thin shell. The shell model (radius 0.25° ± 0.02°, thickness 0.05° ± 0.03°) provides a marginally better fit (χ²/dof = 5.7/4) than the sphere (χ²/dof = 11.6/5), but the improvement is only at the 2.1σ level, so a definitive morphological classification awaits deeper observations.

To constrain the distance, the authors examined ¹²CO data from the CfA survey. The CO spectrum toward the region of highest X‑ray absorption shows peaks at LSR velocities of –17 km s⁻¹ and –81 km s⁻¹. Integrating the CO emission from 0 to –17 km s⁻¹ yields a column density that matches the X‑ray‑derived N_H, indicating that the –17 km s⁻¹ cloud lies in front of the SNR. Using a Galactic rotation model, this velocity corresponds to a near‑side distance of 3.5 kpc, establishing a lower limit for the SNR distance. With the angular radius from the shell model, the physical radius is ≥15 pc. Assuming a typical supernova explosion energy (10⁵¹ erg) and an ambient density of 0.1 cm⁻³, Sedov‑Taylor scaling gives an age of roughly 5 × 10³ yr, implying that G353.6‑0.7 is older than the few hundred‑year‑old shell‑type VHE SNRs (e.g., RX J1713.7‑3946) but still capable of accelerating particles to TeV energies.

In summary, the combined radio, X‑ray, and gamma‑ray evidence strongly supports the identification of HESS J1731‑347 as a shell‑type SNR with non‑thermal X‑ray emission and a central compact object. The gamma‑ray morphology is consistent with a shell, though not yet statistically conclusive. If future high‑resolution gamma‑ray observations confirm the shell, HESS J1731‑347 would become the most distant SNR with a spatially resolved VHE shell, providing a valuable laboratory for studying particle acceleration in an evolved supernova remnant interacting with a dense molecular environment.