2022ApJ...935...86K

We consider fallback accretion after an ultrastripped supernova (USSN) that accompanies formation of a binary neutron star (BNS) or a neutron star-black hole binary (NS-BH). The fallback matter initially accretes directly to the nascent NS, while it starts to accrete to the circumbinary disk, typically 0.1-1 day after the onset of the USSN explosion. The circumbinary disk mass further accretes, forming mini disks around each compact object, with a super-Eddington rate up to a few years. We show that such a system constitutes a binary ultraluminous X-ray source, and a fraction of the X-rays can emerge through the USSN ejecta. We encourage follow-up observations of USSNe within ≲100 Mpc and ∼100-1000 days after the explosion using Chandra, XMM Newton, and NuSTAR, which could detect the X-ray counterpart with time variations representing the properties of the nascent compact binary, e.g., the orbital motion of the binary, the spin of the NS, and/or the quasiperiodic oscillation of the mini disks.