Does Nature allow formation of ultra-compact black hole X-ray binaries via accretion-induced collapse of neutron stars?

Understand

The formation path to ultra-compact X-ray binaries (UCXBs) with black hole (BH) accretors is still unclear.

• In the classical formation scenario, it is difficult to eject the massive envelope of the progenitor star of the BH via common envelope process.
• Given that some neutron stars (NSs) in binary systems evidently have birth masses close to $\sim 2.0\;M_\odot$, we explore here the possibility that BH-UCXBs may form via accretion-induced collapse (AIC) of accreting NSs, assuming that these previously evolved in LMXBs to masses all the way up to the maximum limit of a NS.
• We demonstrate this formation path by modelling a few cases of NS-UCXBs with initial NS masses close to the maximum mass of a NS that evolve into BH-UCXBs after the NS accretes material from its He~WD companion.