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

Abstract

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 $1.7\pm 0.4~{{\ \rm per\ cent}}$ over the 2–8 keV energy band, with a polarization angle of 63° ± 7°. The polarization is found to be energy-dependent, with a 3σ polarization degree consistent with null polarization in 2–4 keV, and 3.2 per cent 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 3σ upper limit of $\sim 11~{{\ \rm per\ cent}}$ on the polarization degree of the thermal component, and constrain that of the Comptonized component to $\sim 3~{{\ \rm per\ cent}}$. We comment on the possible corona geometry of the system based on our results.