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

Abstract

X-ray irradiation heating of accretion discs in black hole X-ray binaries (BHXBs) plays a key role in regulating their outburst cycles. However, despite decades of theoretical and observational efforts, the physical mechanism(s) responsible for irradiating these discs remains largely unknown. We have built an observation-based methodology to estimate the strength of irradiation of BHXB discs by combining multiwavelength X-ray and optical/infrared (OIR) data throughout transient outbursts. We apply this to ∼15 yr of activity in the Galactic BHXB GX339–4. Our findings suggest that the irradiation heating required by the optical data is large in this system. Direct illumination of the outer disc does not produce sufficient irradiation, but this should also produce a thermal-radiative wind which adds to the irradiation heating by scattering flux down on to the disc. However, analytic estimates of X-ray illumination from scattering in the wind are still not sufficient to produce the observed heating, even in combination with direct illumination. Either the analytic thermal-radiative wind models are underestimating the effect of the wind, or there are additional scattering mechanisms at work, such as magnetically driven outflows, acting to increase the OIR flux. While wind-driven irradiation is likely a common feature among long-period BHXBs, fully understanding the driving mechanism(s) behind such a wind will require radiation hydrodynamic simulations.

Using Optical Spectroscopy to Map the Geometry and Structure of the Irradiated Accretion Discs in Low-mass X-ray Binaries: The Pilot-Study of MAXI J0637−430

“…Figure 5 displays C( ) and irr ( ) compared to the irradiation properties derived from the broad-band (XRT+UVOT) SED fitting of the available Swift spectral data (see Section 4.1.2). We find that (i) the mid-UV (UVM2,UVW2) UVOT bandpasses best trace the irradiated disc, and (ii) a deviation of, at most, a factor of 2 − 3 between the far-UV light-curve and broad-band SED derived C, well within the expected uncertainty of the Tetarenko et al (2020) method.…”

“…Fitting the data in logspace, with a Bayesian Markov Chain Monte-Carlo algorithm (see Tetarenko et al 2020 for full details on this method), we find a clear power-law correlation of the form:…”

“…Recently, Tetarenko et al (2020) developed a methodology that uses multi-wavelength time-series data to track how properties of the Xray irradiation heating the discs in LMXB systems evolve over time (on daily to weekly timescales). We have applied their methodology to the X-ray (Swift/XRT), and optical/UV (Swift/UVOT) data available for J0637 (see Section 3.1).…”

Models of ultraluminous X-ray transient sources

“…Winds are required here to avoid the decrease in the illumination factor f irr when the accretion rate exceeds the Eddington value, but they also significantly modify the outburst duration and could have a major impact on angular momentum transport if the wind is magnetized. Unfortunately, although they have an important effect on the whole outburst cycle, the physics of outflows from accretion discs are still not fully understood (Dubus et al 2019;Tetarenko et al 2020).…”

“…However, their approach can apply to sub-Eddington rates only and we consider systems with a larger orbital separation than theirs. In addition, when the model is applied to an observed system, the conclusion is that scattering in the wind is still not sufficient to produce the observed heating, even in combination with direct illumination (Tetarenko et al 2020). Here, since discs are observed to be irradiated, we adopt a simple (even simplistic) "pragmatic" approach in order to produce models that can be tested by observations, as has been the case for X-ray transient models tested by Tetarenko et al (2018a,b).…”