Understanding X-ray irradiation in low-mass X-ray binaries directly from their light-curves

Abstract

The X-ray light-curves of the recurring outbursts observed in low-mass X-ray binaries provide strong test beds for constraining (still) poorly understood disc-accretion processes. These light-curves act as a powerful diagnostic to probe the physics behind the mechanisms driving mass inflow and outflow in these binary systems. We have thus developed an innovative methodology, combining a foundation of Bayesian statistics, observed X-ray light-curves, and accretion disc theory. With this methodology, we characterize the angular-momentum (and mass) transport processes in an accretion disc, as well as the properties of the X-ray irradiation-heating that regulates the decay from outburst maximum in low-mass X-ray transients. We recently applied our methodology to the Galactic black-hole low-mass X-ray binary population, deriving from their lightcurves the first-ever quantitative measurements of the α-viscosity parameter in these systems (Tetarenko et al. 2018). In this paper, we continue the study of these binaries, using Bayesian methods to investigate the X-ray irradiation of their discs during outbursts of strong accretion. We find that the predictions of the discinstability model, assuming a source of X-ray irradiation proportional to the central accretion rate throughout outburst, do not adequately describe the later stages of BH-LMXB outburst light-curves. We postulate that the complex and varied light-curve morphology observed across the population is evidence for irradiation that varies in time and space within the disc, throughout individual transient outbursts. Lastly, we demonstrate the robustness of our methodology, by accurately reproducing the synthetic model light-curves computed from numerical codes built to simulate accretion flows in binary systems.

Thermally driven disc winds as a mechanism for X-ray irradiation heating in black hole X-ray binaries: the case study of GX339–4

“…It controls the outburst decay towards quiescence (and thus the overall outburst duration) and sets the limit on the amount of mass able to be accreted during outburst (thus impacting the overall outburst recurrence timescales) (King & Ritter 1998;Dubus et al 2001). As a result, the light curves of X-ray binary outbursts display characteristic profile shapes, that encode within them distinct observational signatures of the X-ray irradiation source heating the disc in the system (King & Ritter 1998;Kim et al 1999;Dubus et al 2001;Tetarenko et al 2018a).…”

“…Recently, Tetarenko et al (2018a) analyzed the X-ray lightcurves for a large sample of BH-LMXB outbursts. They derived estimates for C by comparing the observed X-ray lightcurve profiles to the predictions of the DIM+irradiation, assuming a source of X-ray irradiation proportional to the central mass accretion rate (Ṁin) throughout outburst.…”

The curious case of Swift J1753.5−0127: a black hole low-mass X-ray binary analogue to Z cam type dwarf novae

“…We find that the J1753 mini-outburst can be well fit by a pure linear-shaped decay on a timescale of τ l = 71 ± 7 days, and does not require the exponential+linear decay profile as in many Galactic BH-LMXB outbursts (Tetarenko et al 2018a). A pure linear profile allows us to calculate an upper limit on C irr by assuming f t , the flux level at which the transition occurs between the viscous (exponential-shaped) and irradiation-controlled (linear-shaped) decay stages in the light curve, is the maximum observed flux (Tetarenko et al 2018a). We find C irr < 4.2 × 10 −2 (for an assumed D = 7.74 +5.00 −2.13 kpc).…”

“…As there is a gap in the data between MJD ∼ 57810 − 57838, during which time an exponential (viscous) decay may have taken place prior to the linear decay, we ple other BH-LMXBs as well (e.g. Gandhi et al 2010;Tetarenko et al 2018a). April 4-5, 2014;56751-56752), fit with the irradiated disc model with a temperature distribution T ∝ R −n characterized by n = 1/2 (left), n = 3/7 (middle), and a non-irradiated disc model with a temperature distribution characterized by n = 3/4 (right).…”

“…In another recent study, Tetarenko et al (2018a) demonstrated how the extremely diverse light curve morphology observed across the BH-LMXB population provides evidence for a likely temporally and spatially varying irradiation source heating the discs in these systems. In this paper, we build on their progress, presenting an alternative method to tackle this complex, multi-scale problem using the unusual BH-LMXB Swift J1753.5-0127 as a case study.…”