The First X-Ray Polarimetry of the Typical Accretion Disk Corona Source 4U 1822-37

Abstract

We report the first Imaging X-ray Polarimetry Explorer (IXPE) observations of the typical accretion disk corona (ADC) source 4U 1822-37, covering 44 orbital cycles, together with simultaneous Swift-XRT data. The polarization degree (PD) of this source is found to be higher than typical low-mass X-ray binary (LMXB) values and exhibits a significant energy dependence. Within the 2–5 keV and 5–8 keV bands, PD remains constant, with average values of 5.6 ± 0.8% and 11.6 ± 1.7%, respectively, and the difference is significant (>3σ). Orbital phase-resolved analysis reveals that during the companion eclipse, the PD in the 2–5 keV band drops from a significantly non-zero value to a level consistent with zero, while the polarization angle (PA) remains unchanged. In contrast, the PD in the 5–8 keV band shows a marginal rising trend, reaching 21.3 ± 6.1%, and the PA exhibits a periodic rotation of 30○ before the eclipse. In addition, the PA rotates by 40○ with increasing neutral absorption on a three-day timescale. These results suggest that the soft and hard emission in the 2–8 keV band may originate from regions with different spatial distributions, with the high PD implying a scattering contribution. The PA rotation is likely linked to scattering geometry asymmetries caused by a bulge or clumps.