Kavli Affiliate: Nicola Omodei
| 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
$3.1%pm0.7%$ 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. There is marginal (95.3% confidence) evidence for an
increase of PD with energy. We fit the spectra with the absorbed sum of a black
body plus a cutoff power-law component and separately with the absorbed sum of
a multitemperature blackbody accretion disk and thermal Comptonization
component. The polarization is predominantly derived from the Comptonization
component in the second model, while the origin of the polarization cannot be
distinguished in the first model. The relatively large PD of the source (up to
$6%pm2%$ in the 6-8 keV band) 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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