2017ApJ...851..114R

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 10^36.4±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.