8-12 juin, 2014


Challenging accretion models for Very Faint X-ray Transients with M15 X-3

Robin Arnason (University of Alberta)

R. M. Arnason (1), G. R. Sivakoff (1), C. O. Heinke (1), H. Cohn (2), & P. Lugger (2) (1) University of Alberta (2) Indiana University

Very faint X-ray transients (VFXTs) are an unusual class of low-luminosity X-ray binaries (XRBs) that are generally difficult to explain with conventional accretion models. To explain their low inferred mass accretion rates, astronomers typically invoke accretion following atypical paths of binary evolution, including accretion from a planetary or brown dwarf companion, accretion in an ultracompact system, or accretion from a primordial companion by an intermediate-mass black hole. Combining archival RXTE, Chandra-ACIS, and Chandra-HRC observations, we find that M15 X-3, an LMXB in the M15 globular cluster, is persistently observed (over the 18 years of available data) at the low luminosities characteristic of VFXTs. We present the results of near-simultaneous optical (Hubble WFC3) and X-ray (Chandra-ACIS) observations of M15 X-3. By modelling the system's optical emission, we determine the companion is a low-mass (0.4-0.5 solar mass) main sequence star, which rules out the above explanations, when combined with the implied size of the accretion disc. Additionally, we discuss the X-ray properties of the system, focusing on evidence of a "break" in its X-ray spectrum at ~2.7 keV.
(doit être confirmé par le SOC)