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# The origin of optical emission lines in the soft state of X-ray binary outbursts: the case of MAXI J1820+070

Mar 2023

The optical emission line spectra of X-ray binaries (XRBs) are thought to beproduced in an irradiated atmosphere, possibly the base of a wind, locatedabove the outer accretion disc. However, the physical nature of - and physicalconditions in - the line-forming region remain poorly understood. Here, we testthe idea that the optical spectrum is formed in the transition region betweenthe cool, geometrically thin part of the disc near the mid-plane and a hot,vertically extended atmosphere or outflow produced by X-ray irradiation. Wefirst present a VLT X-Shooter spectrum of XRB MAXI J1820+070 in the soft stateassociated with its 2018 outburst, which displays a rich set of double-peakedhydrogen and helium recombination lines. Aided by ancillary X-ray spectra andreddening estimates, we then model this spectrum with the Monte Carlo radiativetransfer code Python, using a simple biconical disc wind model inspired byradiation-hydrodynamic simulations of irradiation-driven outflows from XRBdiscs. Such a model can qualitatively reproduce the observed features; nearlyall of the optical emission arising from the transonic 'transition region' nearthe base of the wind. In this region, characteristic electron densities are onthe order of 10$^{12-13}$ cm$^{-3}$, in line with the observed flat Balmerdecrement (H$\alpha$/H$\beta \approx 1.3$). We conclude that strong irradiationcan naturally give rise to both the optical line-forming layer in XRB discs andan overlying outflow/atmosphere that produces X-ray absorption lines.

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