Kavli Affiliate: Herman L. Marshall
| First 5 Authors: M. Lynne Saade, Philip Kaaret, Andrea Gnarini, Juri Poutanen, Francesco Ursini
| Summary:
We present the first X-ray polarimetric study of the dipping accreting
neutron star 4U 1624$-$49 with the Imaging X-ray Polarimetry Explorer (IXPE).
We report a detection of polarization in the non-dip time intervals with a
confidence level of 99.99%. We find an average polarization degree (PD) of
$3.1pm0.7$% and a polarization angle of $81pm6$ degrees east of north in the
2-8 keV band. We report an upper limit on the PD of 22% during the X-ray dips
with 95% confidence. The PD increases with energy, reaching from $3.0pm0.9$%
in the 4-6 keV band to $6pm2$% in the 6-8 keV band. This indicates the
polarization likely arises from Comptonization. The high PD observed is
unlikely to be produced by Comptonization in the boundary layer or spreading
layer alone. It can be produced by the addition of an extended geometrically
thin slab corona covering part of the accretion disk, as assumed in previous
models of dippers, and/or a reflection component from the accretion disk.
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