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 hosting black hole candidates display on average a factor of ~100 larger swing in the minimum (quiescent) to maximum (outburst) X-ray luminosity than neutron star systems do, despite the fact that the swing in the mass inflow rate is likely in the same range. Advection-dominated accretion flows, ADAFs, were proposed to interpret such a difference, because the advected energy disappears beyond the event horizon in black hole candidates but must be radiated away in neutron star systems. The residual optical/UV emission of quiescent low-mass X-ray transients, after subtraction of the companion star spectrum, was originally ascribed to optically thick emission from the outer accretion disk regions, where matter accumulates. Difficulties with this interpretation led to a revised ADAF model in which the bulk of the residual optical/UV emission in quiescence does not originate in the outermost disk regions but is instead produced by synchrotron radiation in the ADAF, and therefore is part of the ADAF's luminosity budget. We demonstrate that, once the residual optical/UV emission is taken into account, the bolometric luminosity swing of black hole candidates is consistent with that of neutron star systems. Therefore, ascribing the bulk of the residual optical/UV flux to the ADAF removes much of the evidence on which ADAF models for low-mass X-ray transients were originally developed, namely, the higher luminosity swing in black holes than in neutron stars. We also find that, for the neutron star spin periods (a few milliseconds) and magnetic fields (~108-109 G) inferred from some low-mass X-ray transients, the mass-to-radiation conversion efficiency of recently proposed ADAF/propeller models is considerably higher than would be required in order to match the observations, once the contribution from accretion onto the magnetospheric boundary is taken into account. Motivated by these findings, we explore here an alternative scenario to ADAFs in which very little mass accretion onto the collapsed star (if any) takes place in the quiescence intervals, whereas a sizeable fraction of the mass being transferred from the companion star (if not all) accumulates in an outer disk region. As in some pre-ADAF models, the residual optical/UV emissions of black hole candidate systems are expected to derive from the gravitational energy released by the matter transferred from the companion star at radii comparable to the circularization radius. The quiescent X-ray luminosity originates from accretion onto the black hole candidates at very low rates and/or from coronal activity in the companion star or in the outer disk. For comparably small mass inflow rates, it can be concluded that the neutron stars in these systems are likely in the radio pulsar regime. In the interaction of the radio pulsar relativistic wind with matter transferred from the companion star, a shock forms, the power law-like emission of which powers both the harder X-ray emission component and most of the residual optical/UV observed in quiescence. The soft, thermal-like X-ray component may arise from the cooling of the neutron star surface in between outbursts or, perhaps, heating of the magnetic polar caps by relativistic particles in the radio pulsar magnetosphere. This scenario matches well both the X-ray and bolometric luminosity swing of black hole candidate as well as neutron star systems, for comparable swings of mass inflow rates toward the collapsed object.