Sco X-1 as seen by IXPE

TL;DR

This study reports a high-significance X-ray polarization measurement of Sco X-1 with IXPE in the 2–8 keV band during a strictly simultaneous multi-mission campaign with NICER, NuSTAR, and Insight-HXMT. The data reveal a continuum polarization of about $PD=1.0\pm0.2\%$ at $PA\approx8^\circ$, with disk polarization constrained to $<3.2\%$ and a hard Comptonized component around $PD\approx1.3\pm0.4\%$, while reflection may be polarized up to $PD\approx14\pm5\%$ under relxillNS modeling; the inner disk radius is near the NS surface ($\sim9\,R_g$). A notable result is a $\sim46^\circ$ rotation between the X-ray polarization angle and the radio jet direction, indicating possible corona-geometry evolution or relativistic precession across the source state. The measurements provide constraints on the emitting geometry of Z-sources and demonstrate the utility of simultaneous broad-band polarimetry for disentangling disk, boundary layer, and corona contributions.

Abstract

The X-ray polarization of Sco X-1 was measured for the first time with very high significance by the Imaging X-ray Polarimeter Explorer (IXPE). A polarization degree of 1.0% $\pm$ 0.2% at a PA of 8° $\pm$ 6° at 90% confidence level (CL) in the 2-8 keV energy band is obtained, while the source was in its soft state with short flaring periods. The source state was determined by a strictly simultaneous X-ray observation campaign jointly with IXPE, which involved NICER, NuSTAR, and Insight-HXMT, allowing for a broad-band spectrum characterization and study of quasiperiodic oscillations. The spectropolarimetric analysis yielded a polarization of <3.2% for the accretion disk and a polarization of 1.3% $\pm$ 0.4% for the hard Comptonized component. A constraint on the polarization of the reflection component, modeled using relxillNS, is obtained. All the results about the polarization degree match the theoretical expectations, while the polarization angle of 8° +/- 6° at 90% CL shows a rotation of 46° $\pm$ 9° with respect to the measured position angle of the radio jet and previous marginal results by PolarLight. This may suggest a variation in the polarization angle related to the source state, which is linked to the variation of corona geometry as reported by IXPE observations of Z sources, or possibly to relativistic precession.

Figures (7)

• Figure 1: A simplified sketch (not in scale) of the geometry of the inner disk, BL, SL and NS surface
• Figure 2: Top panel: IXPE light curve with the corresponding HR binned in 100 s intervals. The blue dotted line identifies the threshold for the high and low flux states. Bottom panel: IXPE HR is obtained as the ratio of the IXPE counting rates in the 4–8 and 2–4 keV energy bands.
• Figure 3: CCD of Sco X-1 from NuSTAR data archive (gray points). Overlapping colored points identify the simultaneous NuSTAR observation, with the color changing from red to yellow with the time elapsed since the observation began.
• Figure 4: Polar plot of the PD and PA for Sco X-1 in the 3--8 keV energy band obtained by ixpeobssim when the observation is divided in high and low flux state. The contours are at 50%, 90%, and 99% CL from the innermost to the outermost.
• Figure 5: PD (top panel) and PA (bottom panel) resolved in time during the observation of Sco X-1, overlapped to the IXPE light curve (gray points). The observation is divided into 8 equal time bins of ${\sim}1.9$ hours. Errors are reported at 68% CL. The colored dashed lines indicate the average-time PD and PA values in the whole observation in the 3--8 keV energy band, and the colored pink areas are the 68% CL on those values.