Exploring the spreading layer of GX 9+9 using RXTE and INTEGRAL

TL;DR

This study analyzes RXTE and INTEGRAL spectra of GX 9+9, revealing unexpected behavior of the spreading layer and providing new insights into its temperature and luminosity relationship, along with implications for its distance.

Contribution

It introduces a model with a spreading layer on the neutron star surface, challenging traditional boundary layer assumptions and offering new spectral fitting insights.

Findings

Spreading layer temperature increases at lower luminosities.

The angular extent of the spreading layer depends linearly on luminosity.

Estimated distance to GX 9+9 is about 10 kpc, double the commonly assumed value.

Abstract

We have fitted ~200 RXTE and INTEGRAL spectra of the neutron star LMXB GX 9+9 from 2002-2007 with a model consisting of a disc blackbody and another blackbody representing the spreading layer (SL), i.e. an extended accretion zone on the NS surface as opposed to the more traditional disc-like boundary layer. Contrary to theory, the SL temperature was seen to increase towards low SL luminosities, while the approximate angular extent had a nearly linear luminosity dependency. Comptonization was not required to adequately fit these spectra. Together with the ~70 degree upper bound of inclination implied by the lack of eclipses, the best-fitting normalization of the accretion disc blackbody component implies a distance of ~10 kpc, instead of the usually quoted 5 kpc.