Kavli Affiliate: Herman L. Marshall
| First 5 Authors: Adam Ingram, Niek Bollemeijer, Alexandra Veledina, Michal Dovciak, Juri Poutanen
| Summary:
We report on a 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). This is the first time it has been possible to
trace 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 slowly
decreased from $sim$4% to $sim$3% across the five observations, but
remained in the North-South direction throughout. Using the Australia Telescope
Compact Array (ATCA), we measure the intrinsic 7.25 GHz radio polarization to
align in the same direction. Assuming the radio polarization aligns with the
jet direction (which can be tested in the future with resolved jet images),
this implies that the X-ray corona is extended in the disk plane, rather than
along the jet axis, for the entire hard intermediate state. This in turn
implies that the long ($gtrsim$10 ms) soft lags that we measure with the
Neutron star Interior Composition ExploreR (NICER) are dominated by processes
other than pure light-crossing delays. Moreover, we find that the evolution of
the soft lag amplitude with spectral state differs from the common trend seen
for other sources, implying that Swift J1727.8$-$1613 is a member of a hitherto
under-sampled sub-population.
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