Bridging X-ray Polarization with Timing & Spectroscopic Parameters of a galactic black hole: Swift J1727.8-1613

TL;DR

This paper reports a discovery of correlated energy-dependent time lag and polarization degree in Swift J1727.8-1613 during its 2023 outburst, linking spectral, timing, and polarimetric properties of a galactic black hole.

Contribution

It presents the first measurement of energy-dependent time lag and polarization degree correlation in a galactic black hole, suggesting additional mechanisms beyond inverse Comptonization.

Findings

Energy-dependent time lag measured around QPOs.

Degree of polarization correlates with QPO quality factor.

Processes other than inverse Comptonization contribute to polarization and time lags.

Abstract

We report the discovery of a correlated energy-dependent time lag and degree of polarization for Swift J1727.8-1613 during its 2023 outburst. The energy-dependent time lag is measured around the type-C quasi-periodic oscillations (QPO) observed by IXPE on 2023-09-07, while the degree of polarization is obtained from energy-resolved polarimetric measurements. The Spearman correlation coefficient was found to be 0.8, with a null hypothesis probability of 4.2\%. Furthermore, the correlation value drops as the quality factor, or Q value, of the observed QPO frequencies decreases. The spectral properties of Swift J1727.8-1613 are analyzed using simultaneous Insight/HXMT data. Thereafter, we present model-independent theoretical arguments to show that processes other than inverse Comptonization also contributes to both the observed polarization and time lags. This correlation may therefore point to additional mechanisms contributing to the connection between the spectral, temporal, and polarimetric properties of black hole binaries in their hard state.