Spectral and temporal features of GX 13+1 as revealed by AstroSat

Abstract

GX 13+1, a neutron star low mass X-ray binary that exhibits the properties of both atoll and Z sources, was studied using data from Soft X-ray Telescope and Large Area X-ray Proportional Counter (LAXPC) onboard AstroSat. The source traced a ν shaped track in its hardness-intensity diagram (HID). Spectral modelling of the data in the 0.7−30.0 keV energy range, with the model - $\mathtt {constant}$ × $\mathtt {tbabs}$ × $\mathtt {thcomp}$ × $\mathtt {bbodyrad}$ + $\mathtt {relxillNS}$, yielded orbital inclination angle (θ) of 77$\substack{+10\\-8}$○. Flux resolved spectral analysis revealed the ν shaped pattern in the plots of spectral parameters kTe, kTbb and Γ versus Fbol, closely resembling the pattern traced in LAXPC HID. This indicates changes in the spectral properties of the corona and the boundary layer/accretion disc. Assuming that the accretion disc was truncated at the Alfvén radius, the upper limit of the magnetic field strength (B) at the poles of neutron star in GX 13+1 was calculated to be 2.55× 108 G (for kA = 1 and η = 0.1), which is close to that of atoll sources. Furthermore, thickness of the boundary layer was estimated to be 5.70 km, which resulted in the neutron star radius value of ≲14.50 km. Quasi-periodic oscillations at 56±4 and 54±4 Hz were detected in Regions D and E of HID, respectively. Their frequencies are similar to the characteristic frequency of horizontal branch oscillation and they do not exhibit a positive correlation with mass accretion rate.