  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.

High Energy Astrophysical Phenomena;Solar and Stellar Astrophysics