Variable QPO lags and reduced coherence between the disc and corona in MAXI J1820+070

Abstract: Quasi-periodic oscillations (QPOs) are observed in the hard state of many black hole X-ray binaries. Although their origin is unknown, they are strongly associated with the corona, of which the geometry is also subject to discussion. We present a thorough spectral-timing analysis of QPOs and broadband noise in the high-inclination BHXRB MAXI J1820+070, using the rich NICER data set of the source in the bright hard state of its outburst in 2018. We find that there is a large QPO hard lag between soft energy bands with significant disc emission and harder coronal power-law bands, which is absent when measuring lags between energy bands dominated by the coronal emission. The QPO lags between a soft band (with significant disc emission) and harder coronal power-law bands vary significantly with power-law flux, on time-scales of (tens of) seconds or a few QPO cycles, especially at QPO frequencies $\lesssim0.3$ Hz. At the same time, the QPO is found to be related to a decreased coherence between energy bands with significant disc emission and harder bands both at and below the QPO frequency, suggesting the QPO mechanism filters out part of the variability. Similar patterns in the frequency-dependent lags and coherence are observed in the BHXRB MAXI J1803-298, which is a (dipping) high-inclination source, but not in the low-inclination source GX 339-4. We suggest that these findings may be evidence of changes in the vertical extent of the corona on time-scales slightly longer than the QPO cycle.