A Low-mass Main-sequence Star and Accretion Disk in the Very Faint X-Ray Transient M15 X-3

Abstract

We present near-simultaneous {\text{}}{Chandra}/Hubble Space Telescope ({\text{}}{HST}) observations of the very faint (L_x < 10³⁶ erg s^-1) X-ray transient source M15 X-3, as well as unpublished archival {\text{}}{Chandra} observations of M15 X-3. The {\text{}}{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 {{{Γ }}}₁={1.3}_-0.2^+0.1 and {{{Γ }}}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.