Spectro-polarimetric view of bright atoll source GX 9+9 using IXPE and AstroSat

TL;DR

This study presents the first spectro-polarimetric analysis of the bright NS-LMXB GX 9+9, revealing energy-dependent polarization and constraining the polarization of its spectral components, offering insights into the system's corona geometry.

Contribution

First spectro-polarimetric observation of GX 9+9 combining IXPE and AstroSat data, providing polarization measurements and component polarization constraints.

Findings

Detected 1.7% polarization in 2-8 keV band.

Polarization is energy-dependent, with null in 2-4 keV and 3.2% in 4-8 keV.

Upper limit of 11% on thermal component polarization.

Abstract

We have carried out the first spectro-polarimetric study of the bright NS-LMXB GX 9+9 using IXPE and AstroSat observations. We report a significant detection of polarization of $1.7\pm 0.4\%$ over the $2-8$ keV energy band, with a polarization angle of $63^{\circ}\pm 7^{\circ}$. The polarization is found to be energy-dependent, with a $3\sigma$ polarization degree consistent with null polarization in $2-4$ keV, and $3.2\%$ in $4-8$ keV. Typical of the spectra seen in NS-LMXBs, we find that a combination of soft thermal emission from the accretion disc and Comptonized component from the optically thick corona produces a good fit to the spectra. We also attempt to infer the individual polarization of these components, and obtain a $3\sigma$ upper limit of $\sim 11\%$ on the polarization degree of the thermal component, and constrain that of the Comptonized component to $\sim 3\%$. We comment on the possible corona geometry of the system based on our results.