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

TL;DR

This study systematically analyzes NICER data to detect and understand reverberation lags in black hole X-ray binaries, revealing their evolution during state transitions and implications for coronal geometry.

Contribution

It provides the first comprehensive detection of reverberation lags in 8 sources, expanding the sample and analyzing their evolution during outbursts, offering new insights into accretion physics.

Findings

Reverberation lags increase and dominate at lower frequencies during state transitions.

Lag evolution indicates the corona may be the base of a jet that expands or ejects.

In the hard state, lags shorten even as QPO frequencies rise.

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 8 sources, increasing the total sample from 3 to 11, and study the evolution of reverberation lag properties as the sources evolve in outbursts. We find that in all of the 9 sources with more than 1 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 which 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 QPOs move to higher frequencies, but then in the state transition, while the QPOs continue to higher frequencies, the lags get longer. We discuss implications for the coronal geometry and physical models of QPOs in light of this new finding.