I present an optical characterization of the Galactic X-ray transient source MAXI J1957+032. This system flares by a factor of ≳104 every few-hundred days, with each flare lasting ∼5 days. I identify its quiescent counterpart to be a late-K/early-M dwarf star at a distance of 5±2 kpc. This implies that the peak 0.5−10 keV luminosity of the system is 1036.4±0.4 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.
@article{arxiv.1710.01333,
title = {MAXI J1957+032: an accreting neutron star possibly in a triple system},
author = {Vikram Ravi},
journal= {arXiv preprint arXiv:1710.01333},
year = {2017}
}