2015ApJ...807...52A

We present near-simultaneous/Hubble Space Telescope () observations of the very faint (L_x< 10³⁶ erg/s) X-ray transient source M15 X-3, as well as unpublished archival observations of M15 X-3. The observations constrain the luminosity of M15 X-3 to be <10³⁴ erg/s in all observed epochs. The X-ray spectrum shows evidence of curvature, and prefers a fit to a broken power law with break energy keV, and power-law indices of and 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 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.