MAXI J1957+032: an accreting neutron star possibly in a triple system

TL;DR

This paper characterizes MAXI J1957+032 as an accreting neutron star system, likely in a triple system with a white dwarf donor, based on optical observations and its unusual outburst properties.

Contribution

It provides the first optical identification of the system's counterpart and proposes a novel triple system model with a white dwarf donor, challenging previous assumptions.

Findings

Identified the optical counterpart as a late-K/early-M dwarf star.

Suggested the donor is likely a carbon-oxygen white dwarf, not a late-type star.

Proposed the system is a triple with a wide orbit around an inner neutron star-white dwarf binary.

Abstract

I present an optical characterization of the Galactic X-ray transient source MAXI J1957$+$032. This system flares by a factor of $\gtrsim10^{4}$ every few-hundred days, with each flare lasting $\sim5$ days. I identify its quiescent counterpart to be a late-K/early-M dwarf star at a distance of $5\pm2$ kpc. This implies that the peak $0.5-10$ keV luminosity of the system is $10^{36.4\pm0.4}$ erg s$^{-1}$. As found by Mata Sanchez et al., the outburst properties of MAXI J1957$+$032 are most consistent with the sample of accreting millisecond pulsars. However, the low inferred accretion rate, and the lack of evidence for a hydrogen-rich accretion flow, are difficult to reconcile with the late-K/early-M dwarf counterpart being the mass donor. Instead, the observations are best described by a low-mass hydrogen- and possibly helium-poor mass donor, such as a carbon-oxygen white dwarf, forming a tight interacting binary with a neutron star. The observed main-sequence counterpart would then likely be in a wide orbit around the inner binary.