Evolution of Time-Lags of Swift J1727.8-1613 during the Rising Phase of Its Discovery Outburst

Abstract

We investigate the accretion dynamics of the black hole X-ray binary Swift J1727.8-1613 during its $2023-2024$ discovery outburst that lasted for $\sim10$ months. Insight-HXMT monitored the rising phase of the outburst of Swift J1727.8-1613 roughly continuously from 2023 Aug 25 to 2023 Oct 05. Strong signatures of type-C Quasi-Periodic Oscillations (QPOs) are observed during this phase of the outburst. In our recent paper, nature of the QPOs are studied with the propagating oscillatory shock (POS) model. In this paper, we report on the observation of both positive (or hard) and negative (or soft) time-lags in the $4-10$ keV (LE), $10-30$ keV (ME), and $30 -150$ keV (HE) bands, computed with respect to the $2-4$ keV reference band. We detect a clear transition from hard to soft lags as the outburst evolves. We show the evolution of QPOs and associated time-lags between different X-ray energy bands, correlated with changes in the QPO frequency, spectral state, and the size of the Comptonizing region. Our analysis reveals strong anti-correlations between the time-lags and both QPO frequency and photon index, and a strong positive correlation with the shock location. These evolving lag characteristics and their correlations provide crucial insights into the changing accretion geometry and the interplay of radiative processes, further supporting dynamic models like the POS in explaining the coupled spectro-temporal evolution in black hole X-ray binaries.

Figures (4)

• Figure 1: Evolutions of (a) 2-10 keV MAXI/GSC flux and 15-50 keV Swift/BAT flux in units of mCrab (b) HR1, i.e., the ratio of 15-50 keV Swift/BAT flux to 2-10 keV MAXI/GSC flux (c) MAXI/GSC flux in 2-4 keV and 4-10 keV range in units of mCrab (d) HR2, i.e., the ratio of 4-10 keV to 2-4 keV MAXI/GSC flux are shown with time (MJD). The yellow shaded region indicates the time when LFQPOs are present in the HXMT lightcurves.
• Figure 2: Evolutions of (a) LFQPO centroid frequency in Insight-HXMT 10-30 keV energy band lightcurves, (b) Fractional RMS of the LFQPO, (c) QPO time-lag of 4-10 keV, 10-30 keV and 30-150 keV enrgy bands with respect to 2-4 keV band, (d) Photon index obtained from the spectral fit with the pexrav model, (e) Shock location from the POS model fit with time. Different color highlights indicate the six different phases of evolution of LFQPO frequency as mentioned in Paper-II.
• Figure 3: Variation of QPO time-lags (TLag) in the LE ($4$-$10$ keV), ME ($10$-$30$ keV), and HE ($30$-$150$ keV) bands (with respect to the $2$-$4$ keV band) with QPO frequency, pexrav model fitted photon index ($\Gamma$), POS model fitted shock location ($X_s$) is shown. Spearman rank correlation coefficients and their p-values are mentioned insets. Vertical dashed orange line marks the point where the time lag crosses zero, i.e. transitions from hard lag to soft lag. The zero-crossing frequencies ($\nu_{TLag,0}$), zero-crossing photon indices ($\Gamma_{TLag,0}$) and zero-crossing shock locations ($X_{TLag,0}$) have been mentioned in the plots. The blue dotted lines are the cubic spline functions which are employed here to determine the zero-crossing point.
• Figure 4: Energy dependence of QPO time-lag (w.r.t. 2-4 keV) for different QPO frequencies. Here the label 'LC' refers to the centroid frequency of the best fit Lorentzian component to the QPO peak. (centroid frequency of the QPO).