Rapid Spectral Changes of Cygnus X-1 in the Low/Hard State with Suzaku

TL;DR

This study investigates rapid spectral variability in Cygnus X-1's low/hard state using Suzaku data, revealing symmetric intensity changes and energy-dependent spectral evolution on 0.1 s timescales, including first-time observations above 100 keV.

Contribution

First-time detailed analysis of rapid spectral changes above 100 keV in Cygnus X-1, revealing energy-dependent spectral evolution during shot events.

Findings

Intensity changes are symmetric in time.

Spectral parameters like y-parameter and electron temperature decrease during shots.

Spectral changes are more prominent above 100 keV.

Abstract

Rapid spectral changes in the hard X-ray on a time scale down to ~0.1 s are studied by applying "shot analysis" technique to the Suzaku observations of the black hole binary Cygnus X-1, performed on 2008 April 18 during the low/hard state. We successfully obtained the shot profiles covering 10--200 keV with the Suzaku HXD-PIN and HXD-GSO detector. It is notable that the 100-200 keV shot profile is acquired for the first time owing to the HXD-GSO detector. The intensity changes in a time-symmetric way, though the hardness does in a time-asymmetric way. When the shot-phase-resolved spectra are quantified with the Compton model, the Compton y-parameter and the electron temperature are found to decrease gradually through the rising phase of the shot, while the optical depth appears to increase. All the parameters return to their time-averaged values immediately within 0.1 s past the shot peak. We have not only confirmed this feature previously found in energies below ~60 keV, but also found that the spectral change is more prominent in energies above ~100 keV, implying the existence of some instant mechanism for direct entropy production. We discuss possible interpretations on the rapid spectral changes in the hard X-ray band.