HESS J1731−347

HESS J1731−347 (also known as SNR G353.6−0.7)^[1] is a young supernova remnant (SNR) located in the southern Milky Way Galactic plane. It was discovered in very-high-energy (VHE) gamma rays by the High Energy Stereoscopic System (H.E.S.S.) telescope array and is notable for its shell-like morphology in TeV gamma rays, making it one of the few SNRs exhibiting such a structure in this energy regime. The remnant is associated with non-thermal X-ray emission indicative of particle acceleration and contains a central compact object (CCO) that challenges conventional models of compact star formation due to its unusually low mass and small radius. HESS J1731−347 is considered a key site for studying cosmic ray production in young SNRs, potentially interacting with adjacent molecular clouds to enhance gamma-ray emission.^[2][3][4]

The remnant is estimated to be 2–6 thousand years old, with a distance of approximately 3.2–5.2 kiloparsecs (kpc) from Earth, placing it in the Scutum–Centaurus Arm of the Galaxy. Its physical radius is roughly 10–15 km, and it likely originated from a core-collapse supernova of a progenitor star with a mass of 8–25 solar masses.^[4][5]

HESS J1731−347 was first detected in 2007 as an unidentified VHE gamma-ray source during the H.E.S.S. Galactic Plane Survey. It was formally identified as a shell-type SNR in 2011, based on observations revealing a faint, asymmetric ring of TeV emission with a best-fit radius of 0.27° ± 0.02°. Other observations are done by Suzaku, XMM-Newton and Fermi Gamma-ray Space Telescope.^[6][4]

At the geometric center lies the CCO XMMU J173203.3−344518^[7] The CCO's parameters strain standard neutron star formation models, which predict minimum masses >1.1 M_☉ from iron-core collapse.^[8][9] Other proposed explanations include either a low-mass neutron star^[10] formation by low-entropy explosions, fallback accretion, or parity doublet models, reconciling with nuclear equation-of-state (EoS) constraints like symmetry energy slope L ≈ 46 MeV and radius around 12.2 km for 1.4 M_☉ stars^[11] or strange quark star allowing stable low-mass configurations while supporting maximum masses ~2.1–2.2 M_☉ consistent with PSR J0740+6620 and GW170817.^[12][13] Formation via neutron-to-strange conversion in the supernova core could eject ~0.3–0.4 M_☉, leaving a subsolar remnant.^[14][15] or a hybrid star with quark core.^[16]