Correlations Between kHz QPOs and Spectral Parameters from Time-Resolved Spectro-Temporal 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 quasi-periodic oscillations (kHz QPOs) during all epochs, with centroid frequencies ranging from $\sim 350$ to $1180~\mathrm{Hz}$, although some detections are of lower significance ($< 3σ$). 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 the diskbb parameters, we estimate the accretion rate and find that all observations fall into two accretion regimes, namely AR1 and AR2, with accretion rates of $\sim 3 \times 10^{16}~\mathrm{g\,s^{-1}}$ and $\sim 7 \times 10^{16}~\mathrm{g\,s^{-1}}$, respectively. Interestingly, we find that for AR1, the lower kHz QPO frequency ($ν_{\mathrm{L}}$) is always $< 500~\mathrm{Hz}$, while for AR2 it is $\gtrsim 500~\mathrm{Hz}$. We found that the spectral index showed no clear correlation with $ν_{\mathrm{L}}$. For AR1, the coronal electron temperature ($kT_{\mathrm{e}}$) and optical depth ($τ$) are $\sim 10~\mathrm{keV}$ and $\sim 5$, respectively. In contrast, for AR2, $kT_{\mathrm{e}}$ decreases to $\sim 3~\mathrm{keV}$ and $τ$ increases to $\sim 12$, showing correlations with $ν_{\mathrm{L}}$, with Spearman's rank correlation coefficients of $-0.78$ and $0.71$, respectively. The transition in spectral parameters at $ν_{\mathrm{L}} \sim 500~\mathrm{Hz}$ indicates the existence of a critical QPO frequency governed or influenced by the accretion state of the source.

Figures (8)

• Figure 1: Figure shows the hardness-intensity diagram of 4U 1728--34 using LAXPC20 data from all eight AstroSat observations described in this work.
• Figure 2: The figure shows the 3–30 keV power density spectra of the persistent emission in the 10–1500 Hz frequency range for all individual observations. Panels (a)–(h) correspond to Obs-1 through Obs-8, respectively.
• Figure 3: The 0.7–20.0 keV SXT+LAXPC20 spectra of Seg-1 of Obs-1 (left) and Obs-3 (right). The spectra have been fitted with the model constant*tbabs*(thcomp*diskbb + bbodyrad + gaussian). The red data points represent the SXT spectra, while the black points are the LAXPC ones.
• Figure 4: The figure shows the comparsion of Chi-square between Model A: tbabs*(thcomp*diskbb + gaussian) and Model B: tbabs*(thcomp*bbodyrad + gaussian). The y-axis shows Chi-square values and lower and upper x-axes show segment number and corresponding degrees of freedom, respectively.
• Figure 5: The 0.7--20.0 keV SXT+LAXPC20 spectra for two representative segments are shown. The red data points correspond to the SXT spectra, while the black points represent the LAXPC spectra. The upper and lower panels show the spectra and the corresponding fit residuals for Seg-2 of Obs-2 and Seg-5 of Obs-8, respectively, fitted with Model A (left) and Model B (right).