X-ray constraint for the unseen companion of V723 Mon: it is a mass-gap black hole rather than binary neutron stars

TL;DR

This study uses X-ray and radio emission analysis to determine that the unseen companion of V723 Mon is most likely a mass-gap black hole rather than a binary neutron star system.

Contribution

It introduces a method to distinguish between a black hole and binary neutron stars using X-ray and radio emission modeling based on wind-fed accretion.

Findings

Single black hole scenario is most favored.

Predicted radio synchrotron emission peaks at 0.1-1 GHz with ~1 mJy flux.

Future observations could detect the predicted radio emission.

Abstract

Recently, the red giant V723 Mon is reported to have an unseen companion with a mass of $3.04\pm0.06M_{\odot}$, but question remains about whether it is a single (thus the so-called mass-gap) black hole or an inner binary of two more ordinary compact objects (neutron stars or white dwarfs). In this work, we estimate the X-ray emission by considering the wind-fed accretion from V723 Mon onto the compact companion. We analyze three different scenarios of the dark companion, i.e., a single black hole, binary neutron stars and binary of a neutron star and a white dwarf. We show that the single black hole is the most favored scenario. We also calculate the synchrotron emission from the bow shock caused by the interaction of the compact companion with the wind. We find this emission peaks at $\sim$ 0.1-1 GHz, with a flux density $\sim$ 1 mJy, which is expected to be detected by observations with higher angular resolution in the future.