The hard to soft spectral transition in LMXBs - affected by recondensation of gas into an inner disk

TL;DR

This paper explores how the presence of an inner cool disk in low-mass X-ray binaries influences the luminosity at which they transition from hard to soft spectral states, highlighting the role of recondensation and irradiation effects.

Contribution

It introduces a model showing that an inner disk reduces irradiation and causes the hard-soft transition to occur at lower luminosities, supported by observations of GX 339-4.

Findings

Inner disks can persist during the hard state at certain luminosities.

Reduced irradiation due to an inner disk lowers the transition luminosity.

Observations of GX 339-4 support the model's predictions.

Abstract

Soft and hard spectral states of X-ray transient sources reflect two modes of accretion, accretion via a geometrically thin, optically thick disk or an advection-dominated accretion flow (ADAF). The luminosity at transition between these two states seems to vary from source to source, or even for the same source during different outbursts, as observed for GX 339-4. We investigate how the existence of an inner weak disk in the hard state affects the transition luminosity. We evaluate the structure of the corona above an outer truncated disk and the resulting disk evaporation rate for different irradiation. In some cases, recent observations of X-ray transients indicate the presence of an inner cool disk during the hard state. Such a disk can remain during quiescence after the last outburst as long as the luminosity does not drop to very low values (10^-4 to 10^-3 of the Eddington luminosity). Consequently, as part of the matter accretes via the inner disk, the hard irradiation is reduced. The hard irradiation is further reduced, occulted and partly reflected by the inner disk. This leads to a hard-soft transition at a lower luminosity if an inner disk exists below the ADAF. This seems to be supported by observations for GX 339-4.