2024ApJ...977..215S

We present the spectral and timing study of the bright neutron star low-mass X-ray binary GX 5-1 using AstroSat/Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope observations conducted in 2018. During the observation, the source traces out the complete horizontal branch (HB) and normal branch (NB) of the Z-track in the hardness-intensity diagram (HID). Understanding the spectral and temporal evolution of the source along the Z-track can probe the accretion process in the vicinity of a neutron star. Spectral analysis was performed in the 0.7-20 keV energy range for different segments in the HID using a multitemperature disk blackbody with an average temperature, kT_in ∼ 0.47, and a thermal Comptonization model. It is found that the optical depth of the corona drops from ∼7.07 in HB to ∼2.61 in NB. The timing analysis using the LAXPC instrument indicates the presence of quasiperiodic oscillations in HB and NB of the Z-track. The observed QPO frequencies are similar to the characteristic frequencies of horizontal branch oscillations (HBOs) and normal branch oscillations (NBOs). The HBO frequency increases from ∼12 to 40 Hz toward the hard apex. NBOs are observed at ∼5 Hz. The timing studies conducted in soft and hard bands indicate the association of HBO and NBO origin with the nonthermal component. Further research could explore the implications of this relationship for understanding the dynamics of accretion onto neutron stars.