Optical and ultraviolet monitoring of the black hole X-ray binary MAXI J1820+070/ASASSN-18ey for 18 months
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 its outburst periods. We present extensive photometry in the X-ray, ultraviolet, and optical bands, as well as densely cadenced optical spectra, covering the phase from the beginning of the optical outburst to ~550 days. During the rebrightening process, the optical emission precedes the X-ray by 20.80 ± 2.85 days. The spectra are characterized by blue continua and emission features of Balmer series, He I, and He II lines and a broad Bowen blend. The pseudo equivalent widths (pEWs) of emission lines are found to show anticorrelations with the X-ray flux measured at comparable phases, due to increased suppression by the optical continuum. At around the X-ray peak, the full widths at half-maximum (FWHMs) of Hβ and He II λ4686 tend to stabilize at 19.4 and 21.8 Å, corresponding to the line-forming regions at radii of 1.7 and 1.3 R⊙ within the disc. We further analysed the absolute fluxes of the lines and found that the fluxes of Hβ and He II λ4686 show positive correlations with the X-ray flux, favouring the suggestion that the irradiation model is responsible for the optical emission. However, the fact that X-ray emission experiences a dramatic flux drop at t ~ 200 days after the outburst, while the optical flux only shows a little variation, suggests that additional energy such as viscous energy may contribute to the optical radiation, in addition to X-ray irradiation.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- July 2021
- DOI:
- 10.1093/mnras/stab1162
- arXiv:
- arXiv:2104.10370
- Bibcode:
- 2021MNRAS.504.4226S
- Keywords:
-
- accretion;
- accretion discs;
- stars: black holes;
- X-rays: binaries;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- Accepted for publication in MNRAS