Spectral-Timing Analysis of Cygnus X-1 with NICER

The soft X-ray coverage and high time resolution of NICER allow us to constrain the accretion flow of black hole X-ray binaries by combining spectral and timing techniques. We analyze the energy-dependent variability of Cyg X-1, extending previous monitoring campaigns to low X-ray energies across all states, which has not been possibly previously. We show that the hard state power spectrum can be decomposed into two main Lorentzians associated with the accretion disk and corona. We discover a low-energy timing phenomenon that is prominently seen in between these components at around 1Hz. It can be characterized by an abrupt lag change of hard (>2keV) with respect to soft (<1.5keV) photons, accompanied by a drop in coherence. The feature is fundamentally connected to the Lorentzians in the power spectrum. As Cyg X-1 softens, the feature follows the Lorentzians, which are known to shift to higher frequencies. We show that the feature is not only a property of Cyg X-1 but is also clearly detected in the low hard state of transient low-mass X-ray binaries, suggesting a general behavior of accreting black hole binaries in this region of the hardness-intensity diagram. We discuss that the feature is not compatible with the typical signature of reverberation and propose ideas for its origin. Finally, we use the coherence to probe the connection of the variability between low and high energies. In the hard and intermediate state, the variability is highly coherent, indicating that a process is at work, possibly propagating fluctuations, which connects the disk and coronal variability. We show that this connection fundamentally changes in the soft state, where strong red noise at high energies is incoherent to fluctuations in the soft X-rays.