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

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 eruption of MAXI J1820+070 with the Hubble Space Telescope (HST) and AstroSat observatory. The extinction towards the source turns out to be low - $\rm E_{B-V} = 0.2 \pm 0.05$ - 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 display 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 donor mass at birth was low enough to avoid CNO processing ($\rm M_{2,i} \lesssim 1.0 - 1.5 {\mathrm M_{\odot}}$). 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 (QPO) that has been seen in other spectral bands.

💡 Research Summary

We present the first multi‑epoch ultraviolet (UV) spectroscopic study of the black‑hole low‑mass X‑ray binary (LMXB) MAXI J1820+070 across three distinct accretion states during its 2018 outburst: a luminous hard state, a hard‑intermediate state, and a soft state. The source suffers from unusually low interstellar extinction (E(B‑V)=0.2±0.05), which enables high‑quality UV coverage from both the Hubble Space Telescope (HST) and AstroSat. HST observations were performed in time‑tag mode, providing both high‑resolution spectra (R≈45 800 in the far‑UV for the first epoch) and sub‑second timing information. The first HST visit (HS1) captured the source a few days after the outburst peak while still in a bright hard state; the second (HS2) occurred three months later, just before the hard‑to‑soft transition. AstroSat supplied complementary spectra in the hard and soft states.

A striking result is the remarkable similarity of the UV continuum across all three states. Contrary to the standard reprocessing picture—where a hard‑state corona irradiates the outer disc, boosting the UV flux, and a soft‑state disc dominates the emission—the observed continua cannot be reproduced by any physically plausible irradiated‑disc model, even after varying the irradiation efficiency, outer disc radius, and temperature profile. This suggests that the UV emission is not simply the reprocessed component of the X‑ray luminosity, challenging a cornerstone assumption of disc‑instability and irradiation models for LMXBs.

All spectra display double‑peaked emission lines, a classic signature of a rotating accretion disc. High‑ionisation lines (N V λ1240, Si IV λ1400, C IV λ1550) have larger peak‑to‑peak separations (≈1500–2000 km s⁻¹) than low‑ionisation lines, indicating that the higher‑ionisation material originates at smaller disc radii. No obvious wind signatures (blue‑shifted absorption or P‑Cygni profiles) are present in the UV, despite the detection of blue‑shifted absorption in optical and near‑infrared lines during the hard state. This discrepancy points to a multi‑phase or stratified outflow, where the UV‑tracing wind may be absent, highly ionised, or geometrically hidden, while cooler wind components manifest at longer wavelengths.

Line‑ratio diagnostics (e.g., N V/C IV, Si IV/C IV) are consistent with solar‑like abundances, implying that the donor star’s initial mass was modest (M₂,i ≲ 1.0–1.5 M⊙) and that CNO processing has not significantly altered the composition of the transferred material. This provides an evolutionary constraint on the binary’s history.

Time‑resolved UV data allow the construction of power spectral densities (PSDs) for the hard and hard‑intermediate states. All PSDs are well described by a broken power‑law: a low‑frequency slope of ≈‑1 breaking to ≈‑2 at ≈0.05 Hz (≈20 s). Superimposed on this continuum, the authors tentatively detect a quasi‑periodic oscillation (QPO) near 18 s, matching the low‑frequency QPOs previously reported in X‑ray and optical bands. The presence of a similar QPO in the UV supports models where variability propagates inward through the disc, imprinting correlated fluctuations across wavebands, and where geometric precession (e.g., Lense‑Thirring) or accretion‑flow instabilities generate coherent oscillations.

In summary, the paper delivers several key insights: (1) UV continua show minimal state dependence, contradicting simple irradiation‑reprocessing expectations; (2) double‑peaked line profiles confirm a disc origin and reveal ionisation‑dependent radial stratification; (3) the lack of UV wind signatures despite their presence at longer wavelengths suggests a complex, multi‑phase outflow geometry; (4) line ratios indicate normal elemental abundances and constrain the donor’s initial mass; (5) UV variability exhibits a broken‑power‑law PSD with a possible 18‑s QPO, linking UV timing behaviour to that seen at higher energies. These findings call for revised theoretical models of disc irradiation, wind launching, and variability propagation in black‑hole X‑ray binaries, and they highlight the unique diagnostic power of UV spectroscopy when combined with multi‑wavelength monitoring.