Ionization Instability Driven Outbursts in SXTs

Abstract

Abstract We study the hydrogen ionization instability mechanism in the context of low-mass X-ray binaries with a black hole as a central object. We make numerical calculations of the predicted outbursts’ light curves and compare them to the data observed by X-ray satellites. The comparison to the data is done for five sources observed by RXTE/ASM (XTE J1550−564, 4U 1630−472, XTE J1859+226, GX 339-4, XTE J1818−245) and one source observed by MAXI (MAXI J1659−152). The aim of this paper is to show that the hydrogen ionization instability operating in an accretion disk is responsible for the shape of outbursts observed in low-mass X-ray binaries. From the data fitting process, we put tight constraints on global source parameters such as black hole mass and disk accretion rate. The influence of chemical composition on the overall analysis is also shown. In the case of each outburst, we found the overall bolometric light curve shape that qualitatively matches the data. We were able to model the main outburst and secondary reflare often seen in the data, the latter one caused by the presence of metals in disk gas. In the case of 4U 1630−472, we analyzed two outbursts, which allowed us to put tight constraints on the black hole mass of 4 ± 0.5M ⊙ and on the accretion rate of yr−1.