Discovery of X-ray Polarization from the Black Hole Transient Swift J1727.8-1613

TL;DR

This paper reports the first detection of X-ray polarization from the black hole transient Swift J1727.8-1613, revealing an elongated corona structure aligned orthogonal to the jet, with high significance.

Contribution

It presents the first measurement of X-ray polarization in this source, providing new insights into the corona's geometry and its relation to jet orientation.

Findings

X-ray polarization degree of 4.1% with high significance

Polarization angle aligned with sub-mm jet direction

Corona is elongated orthogonal to the jet

Abstract

We report the first detection of the X-ray polarization of the bright transient Swift J1727.8-1613 with the Imaging X-ray Polarimetry Explorer. The observation was performed at the beginning of the 2023 discovery outburst, when the source resided in the bright hard state. We find a time- and energy-averaged polarization degree of 4.1%+/-0.2% and a polarization angle of 2.2+/-1.3 degrees (errors at 68% confidence level; this translates to about 20-sigma significance of the polarization detection). This finding suggests that the hot corona emitting the bulk of the detected X-rays is elongated, rather than spherical. The X-ray polarization angle is consistent with that found in sub-mm wavelengths. Since the sub-mm polarization was found to be aligned with the jet direction in other X-ray binaries, this indicates that the corona is elongated orthogonal to the jet.