X-ray spectropolarimetry of the bright atoll Serpens X-1

TL;DR

This study combines X-ray spectral and polarimetric observations of Serpens X-1, setting upper limits on polarization and confirming reflection features, to understand the accretion geometry of this bright atoll source.

Contribution

First simultaneous X-ray spectral and polarimetric observations of Serpens X-1, providing new constraints on polarization and accretion geometry.

Findings

Upper limit of 2% polarization in 2-8 keV band.

Detection of four type-I X-ray bursts, with polarization upper limit of 80%.

Confirmation of reflection features, including a broad iron line.

Abstract

We present simultaneous X-ray polarimetric and spectral observations of the bright atoll source Ser~X-1 carried out with the Imaging X-ray Polarimetry Explorer (IXPE), NICER, and NuSTAR. We obtain an upper limit of 2% (99% confidence level) on the polarization degree in the 2--8 keV energy band. We detect four type-I X-ray bursts, two of which during the IXPE observation. This is the first time that has IXPE observed type-I X-ray bursts, and it allows us to place an upper limit on their polarization degree; however, due to the limited total number of counts in each burst, we obtain a relatively high upper limit (80%). We confirm the presence of reflection features in the X-ray spectrum, notably a broad iron line. Fitting the data with a relativistic reflection model, we derive a disk inclination of 25 deg. The spectral and polarization properties are comparable with other atolls observed by IXPE, suggesting a similar accretion geometry, and the relatively low polarization is consistent with the low inclination.