The disc instability model for X-ray transients: Evidence for truncation and irradiation

Abstract

Abstract.We study the prospect of explaining the outbursts of Soft X-ray Transients (SXTs) by the thermalviscous instability in a thin disc. This instability is linked to hydrogen ionization and is significantly changed when irradiation of the disc by X-rays from the inner regions is included. We present the first numerically reliable, physically consistent calculations of the outburst cycles which include the effects of accretion disc irradiation. The decay from outburst is governed by irradiation, as pointed out by King & Ritter (1998), leading to slow exponential decays. At the end of the outburst, the disc is severely depleted, which lengthens the time needed to rebuild mass to the critical density for an outburst. Despite this, the long recurrence times and quiescent X-ray luminosities of SXTs still require the inner disc to be replaced by another type of flow in quiescence, presumably a hot advection dominated accretion flow (ADAF). We include the effects of such truncation and show that the resulting lightcurves are conclusively similar to those of SXTs like A0620-00. We conclude that the two-α disc instability model provides an adequate description of the outbursts of SXTs when both truncation and irradiation are included. The values for the viscosities in outburst and in quiescence are comparable to those used in CVs. We discuss the model in the context of present observations.

The Reflares and Outburst Evolution in the Accreting Millisecond Pulsar Sax J1808.4–3658: A Disk Truncated Near Co-Rotation?

“…Reflares are not a feature that can be easily justified within the disk instability model (DIM; see the extended reviews of Smak 1984;Menou et al 2000;Dubus et al 2001;Lasota 2001;Kotko et al 2012). Reflares are observed in numerical DIM simulations and they are created by back and forth propagation of cold and hot waves that re-combine and ionize the accretion disk cyclically.…”

“…Reflares are observed in numerical DIM simulations and they are created by back and forth propagation of cold and hot waves that re-combine and ionize the accretion disk cyclically. However, these reflares are considered a "deficiency" of the DIM model rather than a feature, since they cannot reproduce almost any of the properties observed in dwarf novae and soft X-ray transients (Dubus et al 2001;Lasota 2001). Furthermore reflares require a large reservoir of matter to be still in the disk once the main outburst is over, whereas the DIM predicts that the accretion disk should be almost devoid of matter toward the end of an outburst (Dubus et al 2001;Arai et al 2009).…”

Physics of Accretion Flows around Compact Objects

“…Doubtless it is a strong argument in favour of ADAFs and black holes. There has been a debate about where the neutron-star quiescent luminosity comes from, what part of it, if any, is due to accretion (see [18] and references therein; [17]) but in BH SXTs the quiescent luminosity is certainly due to accretion and that it is unlikely that quiescent discs around neutron stars are not truncated [19] (or truncated but not leaky). One should stress that the idea of plotting the luminosity as a function of period is not based on the assumption that BH SXTs and NS SXTs have the same transfer rates at a given orbital period.…”

“…SAX 1808-365 [24] or an ultra-compact X-ray binary (UCXB), in which case the neutron star companion would be a low-mass helium or carbon-oxygen white dwarf [25]. When transient (very short-period systems are rather persistent [26], such compact binaries exhibit short ( ∼ > 10 -∼ > 100 days), exponentially decaying outbursts as expected from small, X-ray irradiated accretion discs [19]. In all these very compact transient systems the neutron star is a millisecond pulsar (MSP).…”

Characterising Anomalous Transport in Accretion Disks from X-ray Observations

“…In addition, 1/f power spectra (inverse power-law frequency dependence with unspecified index) -suggestive of highly-correlated, possibly self-organised critical, behaviour -have been observed in diverse accretion systems (reviewed by, for example, Dendy et al 1998). It is also well established that instabilities occuring in accretion disks (Dubus et al 2001 and references therein) can give rise to anomalous statistics. However, these statistics will differ from those of fully-developed turbulence, and hence we seek to constrain such models using the differencing and rescaling technique described below.…”