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Astronomers unveil spectro-polarimetric properties of X-ray binary GX 9+1

Using the X-ray Polarimetry Explorer (IXPE) and Neutron star Interior Composition Explorer (NICER), Indian astronomers have observed an X-ray binary system known as GX 9+1. Results of the observational campaign, presented in a paper Feb. 4 on the arXiv pre-print server, shed more light on the spectro-polarimetric properties of this system.

X-ray binaries (XRBs) are composed of a normal star or a white dwarf transferring mass onto a compact neutron star or a black hole. Based on the mass of the companion star, astronomers divide them into low-mass X-ray binaries (LMXB) and high-mass X-ray binaries (HMXB).

Neutron star (NS) LMXBs are further divided into atoll and Z sources. The so-called atoll-type systems share some characteristics with black hole LMXBs as they have similar X-ray spectra and timing properties. However, they differ in their radio properties, in that atoll sources are 27 times less radio luminous.

GX 9+1 is a bright, persistent Galactic NS LMXB at a distance of about 16,300 light years. It was discovered in 1965 and is classified as an atoll source. Previous observations of GX 9+1 have found that it has spent most of its time in the soft state since its discovery.

Although GX 9+1 has been known for over half a century, many of its parameters remain hardly constrained. That is why a team of astronomers led by Valliattu Prakash of the Indian Space Research Organization (ISRO) decided to investigate this source with IXPE and NICER.

"We present the first spectro-polarimetric study of the bright atoll source GX 9+1, using the simultaneous Imaging X-ray Polarimetry Explorer, and Neutron star Interior Composition Explorer observations," the researchers wrote in the paper.

The observations revealed that GX 9+1 remained in the soft state, with no changes in state throughout the observation period, which commenced in August 2024. However, significant variations in count rate were identified in the light curve during these observations.

The study found that GX 9+1 showcases a significant polarization in the 2–3 keV energy range, with a polarization degree of about 3.3% and a polarization angle of approximately 11 degrees. The astronomers added that no significant polarization was detected in the 2–8 keV range.

According to the paper, the observed spectrum of GX 9+1 could be well modeled using a combination of Comptonized blackbody emission from the neutron star surface and Comptonized continuum with disk photons as input seed photons. The authors of the study conclude that, in general, the spectral properties of this system during the observations are consistent with those of other known bright atoll-sources in the soft state.

Summing up the results, the astronomers note that the polarization signatures in GX 9+1 favor the presence of an optically thick wedge shaped corona above the accretion disk. They add that the findings also suggest the existence of a transition shell with polar caps removed, close to the neutron star surface.