Can the central compact object in HESS J1731--347 be indeed the lightest neutron star observed?

Abstract

The exceptionally low mass of $0.77_{-0.17}^{+0.2}{M}_{\odot }$ 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-AGB star IRAS 17287 -- 3443 ($\approx 0.6M_{\odot }$), also within the SNR, enriches the scenario. To address this puzzle, we advance the possibility that the gravitational collapse of a rotating pre-SN iron core ($\approx 1.2M_{\odot }$) could result in a low-mass NS. We show that angular momentum conservation during the collapse of an iron core rotating at $\approx 45\%$ of the Keplerian limit results in a mass loss of $\approx 0.3M_{\odot }$, producing a stable newborn NS of $\approx 0.9M_{\odot }$. 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 ($\approx 2\times 10^6$ K) aligns with canonical thermal evolution for its $\approx 4.5$ kyr age. We propose the pre -- SN star, likely an ultra-stripped core of $\approx 4.2M_{\odot }$, formed a tidally locked binary with IRAS 17287 -- 3443, having a 1.43-day orbital period. The supernova led to a $\approx 3M_{\odot }$ mass loss, imparting a kick velocity $\lesssim 670$ 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.

Accepted for publication in ApJ, comments are welcome!