Evolution of the Inner Accretion Flow in Swift J1727.8$-$1613 across Intermediate States: Insights from Broadband Spectral and Timing Analysis

TL;DR

This study analyzes Swift J1727.8--1613's 2023 outburst with broadband data up to $\sim200$ keV to map the inner accretion flow across intermediate states. The HIMS requires two thermal Comptonizing regions and truncated disk geometry, with a weak reflection component, while the SIMS is described by a single Comptonizing region and a disk extending close to the ISCO, accompanied by a higher disk temperature. A high-energy tail above $\sim100$ keV is best described by a hybrid (thermal+non-thermal) electron distribution, indicating non-thermal particle acceleration in the corona during the transition. Joint disk continuum and reflection modeling yields $a\approx0.79$, $M_{BH}\approx10.5\,M_\odot$, $i\approx46^{\circ}$, and $D\approx3.5$ kpc, supporting a consistent physical picture of a transitioning inner flow and enabling cross-checks with independent measurements. Overall, the results show an evolution from a radially extended, warm corona over a truncated disk in the HIMS to a compact, near-ISCO disk with enhanced non-thermal coronal activity in the SIMS, highlighting the dynamic coupling between disk structure and coronal physics in BHXRB outbursts.

Abstract

We present a comprehensive broadband spectral and variability study of the newly detected black hole X-ray binary Swift~J1727.8--1613 in the intermediate states during its 2023 outburst, using multi-mission observations from NICER, NuSTAR, AstroSat, and Insight-HXMT. The spectral data up to $78$ keV in the hard-intermediate state (HIMS) requires models with two Comptonizing regions. In contrast, models with a single Comptonizing region adequately describe the soft-intermediate states (SIMS), implying a significant evolution in the disk-corona geometry between the states. The hard X-ray tail above $100$ keV in the HIMS, detected with both AstroSat/CZTI and Insight-HXMT/HE, indicates that the electron population in the corona is not purely thermal but rather hybrid, with a power-law distribution above the thermal cutoff. While both the reflection modeling and disk continuum fitting favor a truncated disk geometry in the HIMS, the disk substantially moves close to the innermost stable circular orbit in the SIMS, accompanied by a significant rise in the disk temperature. This interpretation is further supported by the increase in the QPO frequency from $\sim1.3$ to $\sim6.6$ Hz. From joint modeling of the disk continuum and reflection component, we estimate a black hole mass of $10.5^{+7.7}_{-3.0}$, spin of $0.79^{+0.03}_{-0.13}$, and disk inclination angle of $42^\circ$-$50^\circ$, which match well with the previously reported spectro-polarimetric measurements. The inferred source distance of $\sim3.5$ kpc is consistent with the recent estimate based on optical spectroscopy. We find a weakly variable or stable disk and a highly variable Comptonized component.

Figures (8)

• Figure 1: MAXI long-term lightcurve in the $2-20$ keV band during the outburst of Swift J1727.8--1613 in 2023. The vertical lines indicate the positions of the two sets of observations considered in this work. The shaded region is the period that has been used to derive the HID in Figure \ref{['fig:hid_maxi']}.
• Figure 2: HID derived using MAXI observations (shaded region of Figure \ref{['fig:maxi_ltcrv']}) of the 2023 outburst of Swift J1727.8--1613. The squares mark the positions of the NICER observations used in this work, overplotted on the HID.
• Figure 3: Upper panel: NuSTAR/FPMA spectral data in the HIMS (blue), the SIMS (red), and the fitted model tbabs*(thcomp*diskbb) (black solid line). Middle panel: data residuals with respect to the fitted model. Lower panel: data-to-model ratio. The presence of a prominent Fe $K_\alpha$ line and a weak reflection hump is seen.
• Figure 4: Joint NICER (red), NUSTAR/FPMA (black), and FPMB (blue) spectral data in the HIMS along with the best-fit models M1: plabs*tbabs(thcomp*diskbb+mbknpo*relxillCp) (left panel), and M2: plabs*tbabs*relconv*xilconv(thcomp*diskbb+compps[thermal]) (middle panel). The right panel shows the joint NICER, NuSTAR/FPMA, FPMB, HE (cyan), and CZTI (olive) spectral data along with the best-fit model M3: plabs*tbabs*relconv*xilconv*(thcomp*diskbb+compps[hybrid]). The black dotted line indicates the thcomp convolved with diskbb component, whereas the magenta dotted line represents the relxillCp for M1, and relconv and xilconv components convolved with compps components for M2 and M3.
• Figure 5: Upper panel: NICER (red), NUSTAR/FPMA (black), FPMB (blue), HE (cyan), and CZTI (olive) spectral data in the HIMS along with the fitted model M2, which considers only a Maxwellian distribution of the electrons in the corona. Middle and lower panels: deviations of jointly fitted NICER (red), NuSTAR/FPMA (black) and FPMB (blue), HE (cyan), and CZTI (olive) spectral data from the model M2. The black and magenta dotted lines have the same meaning as in Figure \ref{['fig:hardstate_Obs1']} for model M2. A prominent excess above $100$ keV is observed. The data is rebinned for plotting purposes only.