Speaker
Description
MAXI J1820+070 is a low-mass X-ray binary harboring a black hole. It entered a very bright outburst in 2018. We performed shot analyses of X-ray and optical sub-second flares observed during the low/hard state of that outbursts. The advantage of shot analyses is that the average flare shape can be preserved. We found that the timescale of shots was about 0.2 s and that optical shots were less spread than X-ray shots. The amplitude of X-ray shots was the highest at the onset of the outburst, and they faded at the transition to the intermediate state. We also detected the abrupt spectral hardening synchronized with this steep flaring event, which was not found in previous shot analyses for another black-hole binary Cyg X-1. The time evolution of optical shots was not similar to that of X-ray shots. These results suggest that accreting gas blobs triggered a series of magnetic reconnections at the hot inner accretion flow in the vicinity of the black hole, which enhanced X-ray emission and generated flaring events. The rapid X-ray spectral hardening would be caused by this kind of magnetic activity. Also, the synchrotron emission not only at the hot flow but also at the jet plasma would contribute to the optical rapid variability. We also found that the low/hard state exhibited six different phases in the hardness-intensity diagram and the correlation plot between the optical flux and the X-ray hardness. The amplitude and duration of X-ray shots varied in synchrony with these phases. This time variation may provide key information about the evolution of the hot flow, the low-temperature outer disk, and the jet-emitting plasma.
