X-raying the Birth of Binary Neutron Stars and Neutron Star-Black Hole Binaries

TL;DR

This paper explores how fallback accretion after ultra-stripped supernovae can lead to binary neutron star or neutron star-black hole systems that appear as binary ultraluminous X-ray sources, suggesting observational strategies for detection.

Contribution

It introduces a model of fallback accretion forming mini disks around nascent compact objects, predicting observable X-ray signatures of forming compact binaries.

Findings

Fallback accretion forms mini disks with super-Eddington rates.

Such systems can appear as binary ultraluminous X-ray sources.

Potential for detecting nascent binaries via X-ray observations within 100 Mpc.

Abstract

We consider fallback accretion after an ultra-stripped 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 $0.1\mbox{-}1\, \mathrm{day}$ 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 (ULX), and a fraction of the X rays can emerge through the USSN ejecta. We encourage follow-up observations of USSNe within $\lesssim 100\,\rm Mpc$ and $\sim 100\mbox{-}1,000\,\mathrm{day}$ 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.