Coronal X‐Ray Emission from the Stellar Companions to Transiently Accreting Black Holes

Abstract

Many neutron stars and black holes are in binaries where the mass transfer rate onto the compact object is highly variable. X-ray observations of these transients in quiescence (LX < 1034 ergs s-1) have found that the binaries harboring black holes are much fainter than those that contain a neutron star. Narayan and collaborators postulated that the faint X-ray emission from black hole binaries was powered by an advection- dominated accretion flow (ADAF). The subsequent ADAF modeling requires that an appreciable fraction of the constant Roche-lobe overflow into the outer disk proceeds into the black hole during A robust and nearly uniform quenching mechanism must then be hypothesized for the neutron star binaries, as comparably large accretion rates would lead to luminosities in excess of 1036 ergs s-1 in quiescence. We explore an alternative explanation for the quiescent X-ray emission from the black hole systems: coronal emission from the rapidly rotating optical companion. This is commonly observed and well studied in other tidally locked binaries, such as the RS CVn, Algol, and By Dra systems. We show that two of the three X-ray detected black hole binaries (A0620-00 and GRO J1655-40) exhibit X-ray fluxes entirely consistent with coronal emission. The X-ray spectra of these objects should be best fitted with thermal Raymond-Smith models rich in lines when coronal emission predominates, a prediction that will be tested with Chandra and XMM-Newton observations. One black hole system (V404 Cyg) is too X-ray bright to be explained as coronal emission and remains a candidate for ADAFs in quiescence. The quiescent X-ray emission from all the neutron star binaries is far too bright for coronal emission. It might be that all soft X-ray transients have variable accretion rates in quiescence and that the basal quiescent X-ray flux is set by either coronal emission from the companion or—when present—by thermal emission from the neutron star. We have also searched for other indicators of coronal activity in these companion stars. For example, we show that the lithium abundances in the black hole systems are comparable to those in the RS CVns. Indeed, both the X-ray flux and lithium abundance from the K star in the binary V471 Tau match that of A0620-00. Though the production mechanisms for lithium in active coronae is still under debate, our work makes it clear that there is no longer a need for mechanisms that involve the compact object.