Description
In this talk, I will present Hubble Space Telescope (HST) and AstroSat ultraviolet (UV) spectroscopy of the black hole X-ray binary MAXI J1820+070 during its 2018 outburst. Our observations span three accretion states—luminous hard, hard-intermediate, and soft state—providing a rare multi-state UV view of this system. The source exhibits remarkably low extinction, E(B-V) = 0.2 ± 0.05, making it an exceptional laboratory for studying UV emission in LMXBs. It displays surprisingly similar spectra across states, challenging standard irradiated disc models. I will discuss how all UV emission lines are double-peaked, with higher-ionisation lines (e.g., N V, C IV) showing broader profiles than lower-ionisation transitions—a clear signature of their accretion disc origin. Notably, no blue-shifted absorption features are detected, despite previous reports of outflows in optical/NIR during the hard state. I will also highlight how line ratios point to near-solar abundances, supporting a low-mass companion. Finally, I will present our time-resolved HST analysis, revealing a broken power-law variability spectrum and a tentative ~18s quasi-periodic oscillation (QPO)—consistent with signals seen in other bands. These results suggest that UV emission in LMXBs is more complex than simple reprocessing, potentially involving additional physical processes.
This talk will emphasise why MAXI J1820+070 is a benchmark system for UV studies of accretion and how our findings motivate future multi-wavelength campaigns to unravel the underlying physics.
