A low-mass main-sequence star and accretion disk in the very faint X-ray transient M15 X-3

We present near-simultaneous $Chandra$/Hubble Space Telescope ($HST$) observations of the very faint (L_x < 10³⁶ erg s⁻¹) X-ray transient source M15 X-3, as well as unpublished archival $Chandra$ observations of M15 X-3.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 <10³⁴ 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 $E_{break}={2.7}_{-0.6}^{+0.4}$ keV, and power-law indices of ${\Gamma }_1={1.3}_{-0.2}^{+0.1}$ 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 ${0.440}_{-0.060}^{+0.035}$ M_⊙ in a ∼4 hr 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_⊙ at the 3σ confidence level. M15 X-3 seems to be inconsistent with all suggested hypotheses explaining very faint transient behavior, except for magnetospherically inhibited accretion.