Ultraviolet spectroscopy of the black hole X-ray binary MAXI J1820+070 across a state transition

ABSTRACT We present ultraviolet (UV) spectroscopic observations covering three distinct accretion states of the low-mass X-ray binary (LMXB) MAXI J1820+070: the luminous hard state, a hard-intermediate state, and the soft state. Our observations were obtained during the 2018 outburst of MAXI J1820+070 with the Hubble Space Telescope (HST) and AstroSat observatory. The extinction towards the source turns out to be low – $E_{B-V} = 0.20 \pm 0.05$ – making it one of the best UV accretion laboratories among LMXBs. Remarkably, we observe only moderate differences between all three states, with all spectra displaying similar continuum shapes and emission lines. Moreover, the continua are not well-described by physically plausible irradiated disc models. All of this challenges the standard reprocessing picture for UV emission from erupting LMXBs. The UV emission lines are double-peaked, with high-ionization lines displaying higher peak-to-peak velocities. None of the lines displays obvious outflow signatures, even though blue-shifted absorption features have been seen in optical and near-infrared lines during the hard state. The emission line ratios are consistent with normal abundances, suggesting that the initial donor mass was low enough to avoid CNO processing ($M_{2,i} \lesssim 1.0\!-\!1.5\, {\mathrm{M}_{\odot }}$). Finally, we study the evolution of UV variability in our time-resolved HST observations (hard and hard-intermediate states). All UV power spectra can be modelled with a broken power law, superposed on which we tentatively detect the $\simeq 18$ s quasi-periodic oscillation that has been seen in other spectral bands.