On the Bolometric Quiescent Luminosity and Luminosity Swing of Black Hole Candidate and Neutron Star Low‐Mass X‐Ray Transients

Low mass X-ray transients (LMXRTs) hosting black hole candidates (BHCs)display on average a factor of ~100 larger swing in the minimum (quiescent) tomaximum (outburst) X-ray luminosity than neutron stars (NSs), despite the factthat the swing in the mass inflow rate is likely in the same range. Advectiondominated accretion flows (ADAFs) were proposed to interpret such a difference.The residual optical/UV emission of quiescent LMXRTs, after subtraction of thecompanion star spectrum, is produced by synchrotron radiation in the (latestversion) of ADAF and therefore is part of the ADAF's luminosity budget. Wedemonstrate that, once the residual optical/UV emission is taken into account,the bolometric luminosity swing of BHCs is consistent with that of NSs. Weexplore here an alternative scenario to ADAFs in which very little massaccretion onto the collapsed star takes place in the quiescence intervals. Theresidual optical/UV emission of BHCs are expected to derive from the energyreleased by the matter transferred from the companion star at radii comparableto the circularisation radius. The quiescent X-ray luminosity originates eitherfrom accretion onto the BH at very low rates and/or from coronal activity inthe companion star or in the outer disk. For comparably small mass inflowrates, the NSs in these systems are likely in the radio pulsar regime. In theinteraction of the radio pulsar relativistic wind with matter transferred fromthe companion star, a shock forms, the power law-like emission of which powersboth the harder X-ray emission and most of the residual optical/UV. The soft,thermal-like X-ray component may arise from the cooling of the NS surface. Thisscenario matches well both the X-ray and bolometric luminosity swing of LMXRTs.(ABRIDGED).Comment: 13 pages (including 2 postscript figures - use emulateapj macro). Accepted for publication in Ap