Holger Stiele (NTHU), Albert Kong (NTHU)
We present a detailed timing study of the bright black hole X-ray transient MAXI J1820+070 (ASASSN-18ey), based on Swift and NICER observations obtained between March 2018 and October 2019. We computed the fundamental diagrams commonly used to study black hole transients, and studied the evolution of timing parameters. The light curves clearly show four outbursts. The observed properties are consistent with a bright black hole X-ray binary that evolves from the low-hard-state to the high-soft state and back to the low-hard-state and shows two reflares, where it remains in the hard state. In many observations the power density spectra showed type-C quasi-periodic oscillations with a characteristic frequencies below 1 Hz. This finding suggests that the source stayed in a state of low effective accretion for large parts of its outburst. The absence of other types of quasi-periodic oscillations hinders a precise determination of the state transitions, but from combining NICER and Swift/XRT data, we find that MAXI J1820+070 went from the hard-intermediate to the soft state in less than one day. In addition, we present the results of our study of covariance spectra and of multi-waveband studies based on Swift/UVOT data, and discuss implications.