Speaker
Description
The exceptionally low mass $0.77_{-0.17}^{+0.2} M_{\odot}$ inferred of the central compact object (CCO) XMMU J173203.3--344518 within the SNR HESS J1731--347, of age $\approx 4.5$ kyr, challenges the standard core-collapse scenario of NS formation. The observed (likely post-AGB) star of $\approx 0.6 M_\odot$, at $0.3$ pc from XMMU J1732, also within the SNR, enriches the scenario. To address this puzzle, we advance the possibility that the gravitational collapse of a rotating iron core of an evolved star can produce a light NS. We estimate the structure of the rotating pre-SN iron core of $\approx 1.2 M_\odot$ and examine its gravitational collapse. We show that the angular momentum conservation during the collapse of an iron core rotating at $\approx 45\%$ of the Keplerian limit leads to an $\approx 0.9 M_\odot$ stable newborn NS. Assuming magnetic dipole radiation for a $10^{11}$ G field, the CCO must rotate relatively slowly with the upper limit of its rotation frequency being $\approx 530$ Hz. Thus, the CCO mass and radius measurement probes the non-rotating NS mass-radius relation in the low-mass region. We show that a canonical NS thermal evolution agrees with the XMMU J1732 observed surface temperature of $\approx 2\times 10^6$ K, assuming $4.5$ kyr as its age. We propose the pre-SN evolved star, likely an ultra-stripped core of $\approx 4.2 M_\odot$, formed a tidally locked binary of $\approx 1.43$ days orbital period with the post-AGB star. The SN produces a mass loss of $\approx 3 M_\odot$, imparting a kick velocity $<670$ km s$^{-1}$, disrupting the binary. This scenario agrees with the observed projected offset of $0.3$ pc between XMMU J1732 and IRAS 17287--3443. Therefore, our findings support the possibility of CCOs originating in binaries, the relevant role of rotation in core-collapse events, and the CCO XMMU J1732 being the lightest NS ever observed.
