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

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{10}^{36}$ erg s${}^{-1}$) X-ray transient source M15 X-3, as well as unpublished archival Chandra observations of M15 X-3.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 $E_{\mathrm{b}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{k}}=2.7_{-0.6}^{+0.4}$ keV, and power law indices of ${\mathrm{\Gamma }}_1=1.3_{-0.2}^{+0.1}$ and $\Gamma_{2} = 1.9^{+0.2}_{-0.2}$overasinglepowerlaw\mathrm{.}WefitournewF438W($B$), 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 $0.440_{-0.060}^{+0.035}$ $M_{\odot }$ 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.