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

TL;DR

MAXI J1820+070 is studied in ultraviolet light across three accretion states using HST and AstroSat, leveraging its low extinction to probe disc irradiation and line formation. The UV continuum is surprisingly similar across states and is better described by a viscous disc than by standard irradiated-disc models, challenging simple reprocessing pictures. Emission lines are double-peaked and trace higher-velocity, higher-ionization regions closer to the black hole, with no robust UV wind signatures detected; line ratios indicate a donor that has not undergone strong CNO processing. Time-resolved UV analysis reveals possible 18 s QPO signals in the hard state, consistent with multi-band QPO phenomena and suggesting reprocessing in the outer disc as a plausible origin. Collectively, the results imply a complex, geometry-dependent irradiation regime and emphasize the need for more realistic disc-atmosphere models to interpret UV emission in BHXTs.

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 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.

Figures (14)

• Figure 1: Left panel: Swift/BAT (hard X-rays) and NICER (soft X-rays) light curves of the outburst of MAXI J1820+070 in 2018. Right panel: NICER HID of the source (blue), averaged by day. In both cases, we have superimposed the timing of our UV observations for comparison. The times of our HST (pink) and AstroSat (light green) observations are noted by different colour, signifying the respective accretion state of the system. The two panels share the same label as they are referred to NICER observations. The transition between the two accretion states is evident in both panels.
• Figure 2: Modelling of the HST:HS2 spectrum to estimate the line-of-sight reddening to MAXI J1820+070. We employ the strength of the $\lambda$2175Å interstellar feature and Fitzpatrick's extinction law fitzpatrick99 to model the HST:HS2 spectrum and find the reddening value, which makes the near-UV bump to disappear. From this analysis, we derive an interstellar reddening value of $\rm E_{B-V}=0.2$, which is the one adopted in this paper.
• Figure 3: Modelling of the damped profile of the interstellar Ly$\alpha$ line in our HST:HS1 spectrum. The different fits are represented by different solid coloured lines, whose corresponding column density is specified in the legend. The light blue dashed line signifies the column density, derived from X-ray spectral fitting, as described by koljonen23. The optimal model, represented by a light green colour, corresponds to a column density of $\rm N_H=10^{21} cm^{-2}$.
• Figure 4: The dereddened time-averaged UV spectra of MAXI J1820+070 as the system evolves across three distinct stages of its outburst, accompanied by line identifications of the most prominent species. Our work covers three key points of the source's outburst: a) a luminous hard state, combining (quasi-)simultaneous HST/AstroSat observations b) HST observations of a lower luminosity hard state before the state transition and c) AstroSat observations of the soft state. The HST spectra cover the far- and near-UV regions (1150-3000Å) whereas the AstroSat observations cover only the 1200-1800Å regime. As a reference, the slope indices of both a standard Shakura-Sunyaev accretion disc ($\rm F_{\lambda, visc} \propto \lambda^{-7/3}$) and an irradiated disc ($\rm F_{\lambda, irr} \propto\lambda^{-1}$) are illustrated. For clarity, we cut the interstellar lines by 70$\%$ of their flux, compared to the original lines. The inset zooms in on the Civ $\lambda1550$Å profile, emphasizing the absence of an outflow.
• Figure 5: Continuum-subtracted line profiles of the most prominent UV resonance lines, observed during our HST:HS1 epoch. Each profile is fitted by a number of Gaussian components to account for each of their individual shapes (dashed lines). Our modelling is based on the fact that each atomic transition produces a distinguised blue and a red emission component. Here, we follow the blue and red colour distinction to demonstrate the relevant created profiles. The last panel shows the VLT/X-shooter $\rm H\alpha$ Balmer line, taken few days before the luminous hard state observation considered here, for comparison. Its profile demonstrates a hint of a shallow absorption blue wing, suggesting the presence of an outflow. Details of our Gaussian modelling are provided in Table \ref{['tab: line_profiles']}.