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

It is currently believed that the "standard" accretion disk theory underpredicts the observed X-ray luminosity from soft X-ray transients (SXTs) in quiescence by as much as 4-6 orders of magnitude. This failure of the standard model is considered to be an important argument for the existence of an alternative mode of accretion, i.e., advection-dominated accretion flows in astrophysics; 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 nonsteady) 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 Kα 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.