2025ApJ...978....1Z

The exceptionally low mass of $0.7{7}_{-0.17}^{+0.2}{M}_{\odot }$ for the central compact object (CCO) XMMU J173203.3-344518 (XMMU J1732) in the supernova remnant (SNR) HESS J1731-347 challenges standard neutron star (NS) formation models. The nearby post-asymptotic giant branch star IRAS 17287-3443 (≈​​​​​​0.6M_⊙), also within the SNR, enriches the scenario. To address this puzzle, we advance the possibility that the gravitational collapse of a rotating presupernova (SN) iron core (≈1.2M_⊙) could result in a low-mass NS. We show that angular momentum conservation during the collapse of an iron core rotating at ≈45% of the Keplerian limit results in a mass loss of ≈0.3M_⊙, producing a stable newborn NS of ≈0.9M_⊙. Considering the possible spin-down, this indicates that the NS is now slowly rotating, thus fulfilling the observed mass-radius relation. Additionally, the NS's surface temperature (≈2 × 10⁶ K) aligns with canonical thermal evolution for its ≈4.5 kyr age. We propose the pre-SN star, likely an ultrastripped core of ≈4.2M_⊙, formed a tidally locked binary with IRAS 17287-3443, with a 1.43 day orbital period. The SN led to a ≈3M_⊙ mass loss, imparting a kick velocity ≲670 km s^–1, which disrupted the binary. This scenario explains the observed 0.3 pc offset between XMMU J1732 and IRAS 17287-3443 and supports the possibility of CCOs forming in binaries, with rotation playing a key role in core collapse, and the CCO XMMU J1732 being the lightest NS ever observed.