The NICER “Reverberation Machine”: A Systematic Study of Time Lags in Black Hole X-Ray Binaries

Abstract

We perform the first systematic search of all NICER archival observations of black hole (and candidate) low-mass X-ray binaries for signatures of reverberation. Reverberation lags result from the light travel time difference between the direct coronal emission and the reflected disk component, and therefore their properties are a useful probe of the disk-corona geometry. We detect new signatures of reverberation lags in eight sources, increasing the total sample from three to 11, and study the evolution of reverberation lag properties as the sources evolve in outbursts. We find that in all of the nine sources with more than one reverberation lag detection, the reverberation lags become longer and dominate at lower Fourier frequencies during the hard-to-soft state transition. This result shows that the evolution in reverberation lags is a global property of the state transitions of black hole low-mass X-ray binaries, which is valuable in constraining models of such state transitions. The reverberation lag evolution suggests that the corona is the base of a jet that vertically expands and/or gets ejected during state transition. We also discover that in the hard state, the reverberation lags get shorter, just as the quasiperiodic oscillations (QPOs) move to higher frequencies, but then in the state transition, while the QPOs continue to higher frequencies, the lags get longer. We discuss the implications of the coronal geometry and physical models of QPOs in light of this new finding.

An Electron-scattering Time Delay in Black Hole Accretion Disks

“…As J1820 got brighter, the high-frequency soft lag associated with thermal disk reprocessing increased in duration, too long to be explained by a light-travel time delay alone (Wang et al 2021). A systematic analysis of ∼10 BH (and candidate) XRBs confirms this as a generic trend of the hard-tointermediate state transition, interpreted as coronal expansion/ ejection (Wang et al 2022). A thermalization time delay contribution to the soft reverberation lag would lessen the role of coronal expansion/ejection.…”

“…BH XRBs exhibit high-frequency soft reverberation lags, associated with the thermal component of the reflection spectrum, whose durations increase from 1 to ∼10 ms during the hard-to-intermediate state transition (Wang et al 2022). To explain these long-duration soft lags, we hypothesize an electron-scattering time delay for the accretion disk atmosphere to reprocess the coronal irradiation.…”

“…Interestingly, the high-frequency soft lag initially gets shorter in the rising hard state (e.g., De Marco et al 2015) 9 , before getting longer during the intermediate state transition and evolving from 1 to ∼10 ms (Wang et al 2021(Wang et al , 2022. Some groups interpret the initially decreasing soft lag as coronal contraction (Kara et al 2019;Wang et al 2022), whereas others argue for a decreasing inner disk truncation radius (De Marco et al 2015Marco et al , 2021Mahmoud et al 2019). Because the thermalization delay is probably negligible for low mass accretion rates, we cannot settle this hard state debate.…”

“…However, XRB reverberation lag models based solely on light-travel time delays systematically underpredict the high-frequency soft lag, which universally increases ∼tenfold during the hard-to-intermediate state transition (Wang et al 2022). 3 Interpretations of these long-duration soft lags appeal to coronal expansion/ejection to increase the corona-to-disk light-travel time (Wang et al 2021(Wang et al , 2022, under the assumption that the coronal irradiation gets reprocessed instantaneously within the disk atmosphere.…”

Black holes: Timing and spectral properties and evolution

“…Both papers suggest this result indicates that the hard-to-soft transition is marked by the corona expanding vertically and launching a jet knot that propagates along the jet stream at relativistic velocities. Most recently, [180] performed an automated NICER search for time lags in black hole LMXBs, and discovered that the longer reverberation lags during the state transition is not only observed in MAXI J1820+070, but is common behaviour in black hole transients (Fig. 7-right).…”