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

Abstract

ABSTRACT GX 13+1, a neutron star low-mass X-ray binary that exhibits the properties of both atoll and Z sources, is studied using data from Soft X-ray Telescope and Large Area X-ray Proportional Counter (LAXPC) onboard AstroSat. The source traces 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 – $\tt {constant}$ × $\tt {tbabs}$ × $\tt {thcomp}$ × $\tt {bbodyrad}$ + $\tt {relxillNS}$, yields orbital inclination angle (θ) of 77$\substack{+10\\-8}$°. Flux resolved spectral analysis reveals 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 truncates at the Alfvén radius, the upper limit of the magnetic field strength (B) at the poles of neutron star in GX 13+1 is calculated to be 5.10 × 108 G (for kA = 1 and η = 0.1), which is close to that of atoll sources. Furthermore, thickness of the boundary layer is estimated to be 5.70 km, which results in the neutron star radius value of ≲14.50 km. Quasi-periodic oscillations (QPOs) at 56 ± 4 and 54 ± 4 Hz are detected in Regions D and E of HID, respectively. The frequencies of these QPOs are similar to the characteristic frequency of horizontal branch oscillation and these do not exhibit a positive correlation with mass accretion rate.