GBM Monitoring of Cyg X-1 During the Recent State Transition

TL;DR

This paper reports on Fermi GBM observations of Cygnus X-1's transition from a hard to a soft spectral state, capturing flux changes and spectral evolution in the 8-1000 keV range.

Contribution

It provides the first detailed monitoring of Cyg X-1's state transition using GBM, including flux variations and spectral changes across the transition.

Findings

Flux decrease in hard X-ray and gamma-ray bands during transition

Detection of long flares in the soft state

Preliminary spectral evolution from hard to soft state

Abstract

Cygnus X-1 is a high-mass x-ray binary with a black hole compact object. It is normally extremely bright in hard x-rays and low energy gamma rays and resides in the canonical hard spectral state. Recently, however, Cyg X-1 made a transition to the canonical soft state, with a rise in the soft x-ray flux and a decrease in the flux in the hard x-ray and low energy gamma-ray energy bands. We have been using the Gamma-Ray Burst Monitor on Fermi to monitor the fluxes of a number of sources in the 8--1000 keV energy range, including Cyg X-1. We present light curves of Cyg X-1 showing the flux decrease in hard x-ray and low energy gamma-ray energy bands during the state transition as well as the several long flares observed in these higher energies during the soft state. We also present preliminary spectra from GBM for the pre-transition state, showing the spectral evolution to the soft state, and the post-transition state.