Polarized X-rays Correlated with Short--Timescale Variability of Cygnus X-1

TL;DR

This study analyzes short-timescale polarization variability in Cygnus X-1's X-ray emission, revealing correlations with flux peaks that suggest changes in accretion disk geometry near the black hole.

Contribution

First systematic investigation of polarized X-ray variability on a few-second timescale in Cygnus X-1's low/hard state using IXPE data, linking polarization changes to flux peaks.

Findings

Polarization degree decreases during flux peaks.

Polarization angle shifts slightly around peaks.

Results imply accretion disk contraction with increased luminosity.

Abstract

We systematically investigate the variability of polarized X-rays on a timescale of a few seconds in the low/hard state of the black hole binary Cygnus X-1. The correlation between polarization degrees and angles with X-Ray intensity was analyzed using data collected by the Imaging X-ray Polarimetry Explorer (IXPE) in June 2022. Given that X-Ray variability in the low/hard state of Cygnus X-1 is non-periodic, flux peaks were aggregated to suppress statistical fluctuations. We divided the temporal profiles of these aggregated flux peaks into seven time segments and evaluated the polarization for each segment. The results reveal that the polarization degree was 4.6\%$\pm$1.2 and 5.3\%$\pm$1.2 before and after the peak, respectively, but decreased to 3.4\%$\pm$1.1 and 2.7\%$\pm$1.1 in the segments including and immediately following the peak. Furthermore, the polarization angle exhibited a slight shift from approximately 30$^{\circ}$ to $\sim$40$^{\circ}$ before and after the peak. These findings suggest that the accretion disk contracts with increasing X-Ray luminosity, and the closer proximity of the X-Ray emitting gas to the black hole may lead to reduced polarization.