Kavli Affiliate: Roger W. Romani
| First 5 Authors: Adam Ingram, Niek Bollemeijer, Alexandra Veledina, Michal Dovciak, Juri Poutanen
| Summary:
We report on an observational campaign on the bright black hole X-ray binary
Swift J1727.8$-$1613 centered around five observations by the Imaging X-ray
Polarimetry Explorer (IXPE). These observations track for the first time the
evolution of the X-ray polarization of a black hole X-ray binary across a hard
to soft state transition. The 2–8 keV polarization degree decreased from
$sim$4% to $sim$3% across the five observations, but the polarization angle
remained oriented in the North-South direction throughout. Based on
observations with the Australia Telescope Compact Array (ATCA), we find that
the intrinsic 7.25 GHz radio polarization aligns with the X-ray polarization.
Assuming the radio polarization aligns with the jet direction (which can be
tested in the future with higher spatial resolution images of the jet), our
results imply that the X-ray corona is extended in the disk plane, rather than
along the jet axis, for the entire hard intermediate state. This in turn
implies that the long ($gtrsim$10 ms) soft lags that we measure with the
Neutron star Interior Composition ExploreR (NICER) are dominated by processes
other than pure light-crossing delays. Moreover, we find that the evolution of
the soft lag amplitude with spectral state does not follow the trend seen for
other sources, implying that Swift J1727.8$-$1613 is a member of a hitherto
under-sampled sub-population.
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