First spectro-polarimetric study of the neutron star low-mass X-ray binary GX 9+1

ABSTRACT We present the spectro-polarimetric study of the bright atoll source GX 9+1, using the simultaneous Imaging X-ray Polarimetry Explorer (IXPE), Neutron star Interior Composition Explorer (NICER) and the Nuclear Spectroscopic Telescope Array (NuSTAR) observations. The source was observed to remain in the soft state, with no changes in state throughout the observation period. The source does not show significant polarization in the 2-8 keV energy range. However, a significant polarization (3.3$\sigma$) was detected in the 2–3 keV range, with a polarization degree of 3.3 $\pm$ 0.8 per cent and a polarization angle of 11 $\pm$ 7°. We used the simultaneous energy spectra from NICER (0.7–11 keV), NuSTAR (3.0–30.0 keV) and IXPE (2–8 keV) to study the spectral properties of the source during observations. The observed spectrum of the source can be well described by a combination of a multicolour disc blackbody emission from the accretion disc (diskbb model in xspec) and thermal Comptonized component (nthComp in xspec) with NS blackbody seed photons. No evidence for the presence of reflection features have been identified in 0.7–30.0 keV X-ray spectrum of GX 9+1 during the spectral analysis. The spectral properties of GX 9+1 during the observation are consistent with those of other bright atoll-sources in the soft state. However, the high polarization degree observed in the low-energy band does not align with previous IXPE observations of other atoll-sources. This observed low-energy polarization in the source is attributed to the polarization of the soft disc component. We discuss the results from the spectro-polarimetric studies in the context of various accretion disc and coronal geometries of the source.