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

Kavli Affiliate: Herman L. Marshall

| First 5 Authors: Alexandra Veledina, Fabio Muleri, Michal Dovciak, Juri Poutanen, Ajay Ratheesh

| Summary:

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.

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