Abstract
We present near-simultaneous /Hubble Space Telescope () observations of the very faint (Lx < 1036 erg s−1) X-ray transient source M15 X-3, as well as unpublished archival observations of M15 X-3.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−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 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.
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