A Hidden Pulse: Uncovering a New Timing Signal in Cygnus X-1 with AstroSat

TL;DR

This paper uncovers a new timing signal in Cygnus X-1 using AstroSat data, revealing previously undetected variability components that offer insights into jet properties and accretion flow geometry.

Contribution

The study introduces a novel frequency-segmented analysis technique and identifies a new coherence dip at energies above 3 keV in Cygnus X-1.

Findings

Detection of a new coherence dip at >3 keV

Identification of previously unseen variability components

Insights into jet and accretion flow geometry

Abstract

The study of fast variability properties in X-ray binaries advances our understanding of the physical processes and geometric properties of the accretion flow around the compact object. In this work, we study the evolution of the timing properties of Cygnus X-1 with AstroSat/LAXPC, during the transition of the source from the hard to soft state in 2017. We use a novel frequency-segmented technique to fit simultaneously the cross spectra and parts of the power spectra and coherence function with a multi-Lorentzian model and predict the phase-lags and the complementary parts of the power spectra and coherence function. We study the evolution of the frequency and power of the main variability components that are present throughout all the states. In particular, we identify previously undetected variability components, one of which manifests as a narrow dip in the coherence function and a broad drop in the phase-lag spectrum at the same frequency. This dip in coherence, which we detected for the first time in Cygnus X-1 at energies above 3 keV, appears in a state in which the source shows high-amplitude radio variability and significant hard X-ray polarization. While the contribution of the compact jet in X-rays is debated in the literature, this study provides a new avenue for investigating jet properties as well as the geometry of the Comptonizing medium.