A new SNR with TeV shell-type morphology: HESS J1731-347

Abstract: The recent discovery of the radio shell-type supernova remnant (SNR), G353.6-0.7, in spatial coincidence with the unidentified TeV source HESS J1731-347 has motivated further observations of the source with the High Energy Stereoscopic System (H.E.S.S.) Cherenkov telescope array to test a possible association of the gamma-ray emission with the SNR. With a total of 59 hours of observation, representing about four times the initial exposure available in the discovery paper of HESS J1731-347, the gamma-ray morphology is investigated and compared with the radio morphology. An estimate of the distance is derived by comparing the interstellar absorption derived from X-rays and the one obtained from 12CO and HI observations. The deeper gamma-ray observation of the source has revealed a large shell-type structure with similar position and extension (r~0.25{\deg}) as the radio SNR, thus confirming their association. By accounting for the H.E.S.S. angular resolution and projection effects within a simple shell model, the radial profile is compatible with a thin, spatially unresolved, rim. Together with RX J1713.7-3946, RX J0852.0-4622 and SN 1006, HESS J1731-347 is now the fourth SNR with a significant shell morphology at TeV energies. The derived lower limit on the distance of the SNR of 3.2 kpc is used together with radio and X-ray data to discuss the possible origin of the gamma-ray emission, either via inverse Compton scattering of electrons or the decay of neutral pions resulting from proton-proton interaction.

Insights from the Discovery of a New SNR with TeV Shell-Type Morphology: HESS J1731-347

The research paper under review presents detailed observations and analyses of the previously unidentified TeV source HESS J1731-347, now associated with the radio-detected supernova remnant (SNR) G353.6-0.7. This study, conducted by the HESS Collaboration, reveals significant findings concerning its gamma-ray morphology, energetic emissions, and a stronger confirmation of its classification as a shell-type SNR at TeV energies.

Gamma-ray Observations and Morphological Analysis

The paper details extensive observational data acquired over 59 hours using the HESS array, which provides a comprehensive gamma-ray profile of the source. The observations were steered towards testing the association between the radio SNR and the TeV emission, resulting in the confirmation of a large shell-type gamma-ray structure with a radius of approximately 0.25° consistent with the radio observations. The data rule out a simple spherical emission model and are best represented by a thin, unresolved shell, supporting a shell-type morphology akin to only a handful of known SNRs at TeV energies.

Spectral Insights and Distance Estimation

The authors investigated the energy spectrum within an effective threshold of 240 GeV, determining a spectral photon index that aligns with a power-law distribution across the energy range. Also highlighted is a spatially coherent emission in gamma-rays and radio, further solidifying the link between these two wavelengths. An innovative approach was applied to estimate distance by contrasting the interstellar absorption inferred from X-ray data with CO and HI observations, establishing a lower limit of approximately 3.2 kpc.

Multi-wavelength Emission and Energy Considerations

The analysis extends into a multi-wavelength view, integrating radio, X-ray, and gamma-ray data. The lack of thermal X-ray emission suggests a predominantly non-thermal, synchrotron origin, indicating acceleration of electrons to very high energies. The comparison reveals congruence in morphology at X-ray and gamma-ray wavelengths, a common feature among prominent shell-type SNRs like RX J1713.7-3946, thus putting HESS J1731-347 into context with its peers in the study of cosmic ray acceleration.

Speculative Discussions and Future Directions

With gamma-ray luminosity estimates reinforcing its status as a bright TeV source, the discussion contemplates the origin of the observed emission—whether leptonic or hadronic in nature. Theoretical modeling suggests that while a pure leptonic scenario fits the observed flux densities, hadronic contributions cannot be dismissed given the uncertainties in ambient density and magnetic field strength. The possible identification of a central compact object in X-ray observations introduces intriguing considerations regarding the supernova explosion mechanics and progenitor characteristics.

Conclusion

In conclusion, the paper's insights into HESS J1731-347 offer vital contributions to our understanding of gamma-ray SNRs, emphasizing their complex nature and invaluable role in elucidating high-energy astrophysical processes. Future studies, potentially utilizing refined modeling and deeper multi-wavelength observations, will be essential in resolving remaining ambiguities and further explicating the structure and behavior of such intriguing astrophysical phenomena.