A Low-Mass Main-Sequence Star and Accretion Disk in the Very Faint Transient M15 X-3

TL;DR

This study combines X-ray and optical observations to characterize the faint transient M15 X-3, revealing a low-mass main-sequence companion and accretion disk, and suggesting magnetospheric inhibition as the likely accretion mechanism.

Contribution

It provides the first detailed multi-wavelength analysis of M15 X-3, identifying its companion star and constraining the accretion process in very faint transients.

Findings

Companion star is a ~0.44 solar mass main sequence star.

X-ray spectrum prefers a broken power-law model.

System likely involves magnetospheric inhibition of accretion.

Abstract

We present near-simultaneous Chandra/HST observations of the very faint ($L_{x} < 10^{36}$ erg s$^{-1}$) X-ray transient source M15 X-3, as well as unpublished archival Chandra observations of M15 X-3. The Chandra observations constrain the luminosity of M15 X-3 to be $< 10^{34}$ erg s$^{-1}$ in all observed epochs. The X-ray spectrum shows evidence of curvature, and prefers a fit to a broken power-law with break energy $E_{\rm break} = 2.7^{+0.4}_{-0.6}$ keV, and power law indices of $\Gamma_{1} = 1.3^{+0.1}_{-0.2}$ and $\Gamma_{2} = 1.9^{+0.2}_{-0.2}$ over a single power law. We fit our new F438W ($B$), F606W (broad $V$), and F814W ($I$) HST data on the blue optical counterpart with a model for an accretion disk and a metal-poor main sequence star. From this fit, we determine the companion to be consistent with a main sequence star of mass $0.440^{+0.035}_{-0.060}$ $M_{\odot}$ in a $\sim$4-hour orbit. X-ray irradiation of the companion is likely to be a factor in the optical emission from the system, which permits the companion to be smaller than calculated above, but larger than $0.15$ $M_{\odot}$ at the $3\sigma$ confidence level. M15 X-3 seems to be inconsistent with all suggested hypotheses explaining very faint transient behavior, except for magnetospherically inhibited accretion.