On the 'Failure' of the Standard Model for Soft X-Ray Transients in Quiescence

It is currently believed that the ``standard'' accretion disk theory under-predicts the observed X-ray luminosity from Soft X-ray Transients (SXT) in quiescence by as much as 4 to 6 orders of magnitude. This failure of the standard model is considered to be an important argument for the existence of the alternative mode of accretion -- Advection Dominated Accretion Flows (ADAF) in astrophysics, since these flows allow a much higher level of X-ray emission in quiescence, in agreement with the observations. Here we point out that, in stark contrast to steady-state standard disks, such disks in quiescence (being non-steady) produce most of the X-ray emission very far from the last stable orbit. Taking this into account, these disks can accommodate the observed X-ray luminosities of SXTs rather naturally. Our theory predicts that Fe K-alpha lines from standard accretion disks in quiescence should be narrow even though the cold disk goes all the way down to the last stable orbit.
Version accepted by ApJ Letters. Please note the change in the Authors from the previous version: Prof. Svensson made a very substantial contribution to the paper through discussions and changes to the text [made after SN submitted the first version of the paper to ApJ and astro-ph]