Optical and Ultraviolet Monitoring of the Black Hole X-ray Binary MAXI J1820+070/ASASSN-18ey for 18 Months

TL;DR

This study presents comprehensive multi-wavelength monitoring of the black hole X-ray binary MAXI J1820+070 over 18 months, revealing optical-X-ray delays, spectral features, and emission line behaviors that inform models of accretion and irradiation.

Contribution

It provides the first extensive optical, ultraviolet, and X-ray dataset covering the entire outburst and rebrightening phases of MAXI J1820+070, with detailed spectral and flux analysis.

Findings

Optical emission precedes X-ray by about 21 days during rebrightening.

Emission line widths suggest formation regions within the accretion disk at 1.3-1.7 R_sun.

Flux correlations support irradiation as a key driver of optical emission, with additional energy sources indicated.

Abstract

MAXI J1820+070 is a low-mass black hole X-ray binary system with high luminosity in both optical and X-ray bands during the outburst periods. We present extensive photometry in X-ray, ultraviolet, and optical bands, as well as densely-cadenced optical spectra, covering the phase from the beginning of optical outburst to $\sim$550 days. During the rebrightening process, the optical emission preceded the X-ray by 20.80 $\pm$ 2.85 days. The spectra are characterized by blue continua and emission features of Balmer series, He I, He II lines and broad Bowen blend. The pseudo equivalent width (pEW) of emission lines are found to show anticorrelations with the X-ray flux measured at comparable phases, which is due to the increased suppression by the optical continuum. At around the X-ray peak, the full width at half maximums (FWHMs) of H$_{\beta}$ and He II $\lambda$4686 tend to stabilize at 19.4 Angstrom and 21.8 Angstrom, which corresponds to the line forming region at a radius of 1.7 and 1.3 R_sun within the disk. We further analyzed the absolute fluxes of the lines and found that the fluxes of H$_{\beta}$ and He II $\lambda$4686 show positive correlations with the X-ray flux, favoring that the irradiation model is responsible for the optical emission. However, the fact that X-ray emission experiences a dramatic flux drop at t$\sim$200 days after the outburst, while the optical flux only shows little variations suggests that additional energy such as viscous energy may contribute to the optical radiation in addition to the X-ray irradiation.