X-Ray Polarization Variability during a Few Hz Quasiperiodic Oscillations in Swift J1727.8-1613 Observed with IXPE

Abstract We report the polarization properties of low-frequency quasiperiodic oscillations (LFQPOs) observed in Swift J1727.8−1613 with the Imaging X-ray Polarimetry Explorer (IXPE). LFQPOs have been observed in various objects, but their mechanisms remain unclear despite multiple proposed models. In order to understand the origin of the quasiperiodic oscillation mechanism, we investigated the short time variability of polarized X-rays. We analyzed three data sets of Swift J1727.8−1613, which were observed during the bright outburst and exhibited the LFQPO by IXPE. We employed a stacking analysis to understand the time variability of polarization during LFQPOs. We found evidence that the polarization degree is higher in the high-flux phase than in the low-flux phase. In addition, the polarization angle during the high-flux phase differed from that during the low-flux phase, although the trend was not entirely the same across the three observations. The variations in spectral hardness and polarization degree correlated with the intensity of LFQPO suggest geometrical changes in the Comptonization region or the involvement of complex Comptonization processes. X-ray polarization observation in wide band provides further understanding of the geometry and magnetic field structure of the accretion flows near the black hole.