Kavli Affiliate: Herman L. Marshall
| First 5 Authors: Fabio La Monaca, Alessandro Di Marco, Juri Poutanen, Matteo Bachetti, Sara E. Motta
| Summary:
The Imaging X-ray Polarimetry Explorer measured with high significance the
X-ray polarization of the brightest Z-source Sco X-1, resulting in the nominal
2-8 keV energy band in a polarization degree of 1.0(0.2)% and a polarization
angle of 8(6){deg} at 90% of confidence level. This observation was strictly
simultaneous with observations performed by NICER, NuSTAR, and Insight-HXMT,
which allowed for a precise characterization of its broad-band spectrum from
soft to hard X-rays. The source has been observed mainly in its soft state,
with short periods of flaring. We also observed low-frequency quasi-periodic
oscillations. From a spectro-polarimetric analysis, we associate a polarization
to the accretion disk at <3.2% at 90% of confidence level, compatible with
expectations for an electron-scattering dominated optically thick atmosphere at
the Sco X-1 inclination of 44{deg}; for the higher-energy Comptonized
component, we obtain a polarization of 1.3(0.4)%, in agreement with
expectations for a slab of Thomson optical depth of ~7 and an electron
temperature of ~3 keV. A polarization rotation with respect to previous
observations by OSO-8 and PolarLight, and also with respect to the radio-jet
position angle, is observed. This result may indicate a variation of the
polarization with the source state that can be related to relativistic
precession or to a change in the corona geometry with the accretion flow.
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