Comprehensive investigations on spectral and temporal features of GX 5−1 using AstroSat observations

Abstract

Comprehensive spectro-temporal analyses of the Z-type neutron star low mass X-ray binary GX 5−1 were performed using 10 broad-band observations from AstroSat/Soft X-ray Telescope and Large Area X-ray Proportional Counter (LAXPC) instruments. The LAXPC-20 hardness−intensity diagram showed horizontal and normal branches (HBs and NBs) of the Z track which exhibited secular motion. The time-averaged spectra in the energy range 0.7 − 25.0 keV could be fitted with the model combination - $\mathtt {constant}$ × $\mathtt {tbabs}$ × $\mathtt {edge}$ × $\mathtt {edge}$ × $\mathtt {thcomp}$ × $\mathtt {diskbb}$. This yielded Γ ∼ 2, kTe ∼ 3.3 keV and Fdisc/Ftotal ∼ 0.8 indicating the soft/intermediate spectral state of the source during the observations. Flux resolved spectral analysis revealed a positive correlation between kTin and Fbol. However, a negative correlation was observed between them in one of the NBs. Time-averaged temporal analysis revealed multiple HBOs and NBOs, and peaked noise components in the ∼5 − 50 Hz range. Furthermore, flux resolved temporal analysis showed that the frequency of the HBOs correlates positively whereas the strength of HBOs correlates negatively with Fbol, indicating their probable origin from the accretion disc. In contrast, the frequency and strength of NBOs remain fairly constant with Fbol, suggesting that they originate from a different region in the system. Using the relativistic precession model along with highest frequency of the HBO, the upper limits of the magnetic dipole moment (μ) and field strength (B) at the poles of the neutron star in the system were found to be 25.60×1025 G cm3 and 3.64×108 G, respectively, for kA = 1.