Correlations between Kilohertz Quasiperiodic Oscillations and Spectral Parameters from Time-resolved Spectrotemporal Analysis of 4U 1728-34

Abstract We present a time-resolved analysis of the persistent emission in 4U 1728-34 using AstroSat observations from 2016 to 2019. We detect kilohertz quasiperiodic oscillations (kHz QPOs) during all epochs, with centroid frequencies ranging from ∼350 to 1180 Hz although some detections are of lower significance (<3 σ ). We model the simultaneous spectra from the Soft X-ray Telescope and the Large Area X-ray Proportional Counter using a combination of an absorbed disk component ( diskbb ), a blackbody component ( bbodyrad ), a thermal Comptonization model ( thcomp ), and a broad Gaussian line. From diskbb parameters, we estimate the accretion rate and find that all the observations fall into two accretion regimes, namely AR1 and AR2, with accretion rates of ∼3 × 10 16 and 7 × 10 16 g s −1 , respectively. Interestingly, we find that for AR1, the lower kHz QPO frequency ( ν L ) is always <500 Hz, while for AR2, it is ≳500 Hz. We found that the spectral index showed no clear correlation with ν L . For AR1, the coronal electron temperature ( kT e ) and optical depth ( τ ) are ∼10 and ∼5 keV, respectively. In contrast, for AR2, kT e decreases to ∼3 keV and τ increases to ∼12, showing correlations with ν L , with Spearman’s rank correlation coefficients of −0.78 and 0.71, respectively. The transition of the spectral parameters at ν L ∼ 500 Hz indicates the existence of a critical QPO frequency that is governed or influenced by the source’s accretion state.