arXiv subjects

Sai-En Xu

Publications and source records attributed to Sai-En Xu.

6 recordsLinked to original sources

arXiv2026

QPEs from Warped Disk Collisions with EMRIs: Brightness-Recurrence Diagram and Gravitational-Wave Follow-up

Quasi-Periodic Eruptions (QPEs) display correlated long/short and strong/weak patterns that remain unexplained by existing flat-disk collision models. We propose that these features arise from an extreme-mass-ratio inspiral (EMRI) colliding with a warped accretion disk, likely formed after a tidal disruption event. The warp modulates both recurrence time and burst energy, encoding the disk geometry -- and thus the spin of the central supermassive black hole (SMBH) -- into the X-ray light curve. We introduce the Brightness-Recurrence Diagram (BRD) to visualize this correlation, where QPE bursts trace an elliptical trajectory driven by the EMRI's apsidal precession; the tilt of this ellipse encodes whether the EMRI is prograde or retrograde relative to the SMBH spin. Applying this model to the prototypical QPE source GSN 069 successfully reproduces the observed patterns. The data are consistent with either a prograde stellar secondary or a retrograde stellar-mass black hole. In the stellar-mass black hole scenario, ongoing orbital decay could render the EMRI detectable by LISA within a few decades, facilitating gravitational-wave follow-up and independent multimessenger constraints on the system.

astro-ph.HE↗

arXiv2026

The disk luminosity deficit as a tracer of receding disk during Soft-to-Hard transitions in Black Hole X-ray Binaries

Tracing the evolution of the thin accretion disk during the soft-to-hard state transition in black hole X-ray binaries (BHXRBs) remains difficult because conventional spectral estimates of the disk inner radius become highly model-dependent once the thermal component weakens. We present evidence that the thin disk recedes during this transition, obtained from a systematic study of RXTE/PCA observations of 26 BHXRBs. In 24 outbursts, the disk luminosity decays exponentially in the soft state, then drops significantly below the extrapolated baseline. This thermal luminosity deficit is considered a signature of reduced accretion efficiency, caused by the outward receding of the optically thick disk. Under this framework, we found that the estimated characteristic truncation radius increases rapidly as the systems evolve through the soft-to-hard transition. This interpretation is supported by timing analysis: in observations with well-constrained power density spectra, the characteristic frequencies of broadband noise and low-frequency QPOs generally decrease as the inferred truncation radius increases, consistent with the expansion of a hot inner flow. The onset and rapidity of recession vary substantially across different sources and outbursts. Our results demonstrate that luminosity deficits provide a practical empirical tracer of thin disk receding during soft-to-hard transitions, when direct spectral radius measurements become unreliable.

astro-ph.HE↗

arXiv2026

Dynamic disk-corona coupling during the state transition of Swift J1727.8-1613

State transitions during outbursts of black hole X-ray binaries exhibit complex, rapidly evolving disk-corona coupling. Understanding this dynamic phase is essential for deciphering accretion physics and the mechanisms that drive outbursts, yet it remains poorly understood because of the scarcity of high-quality, high-cadence observations. Here, we present an analysis of observations from the Hard X-ray Modulation Telescope (HXMT) during the 2023 outburst of the newly discovered low-mass black hole X-ray binary Swift J1727.8-1613. Follow-up, high-cadence monitoring reveals pronounced variability in disk emission, attributable to fluctuations in the accretion rate. These disk fluctuations exhibit damped amplitudes and shortened flare periods. This evolving disk emission modulates the supply of soft seed photons to the corona, producing a dynamically changing positive correlation between the photon index $Γ$ and the Comptonization luminosity $L_{\rm Comp}$. As the transition proceeds, the correlation shifts toward higher $Γ$ and a narrower range of $L_{\rm Comp}$. We further suggest that the damped disk variability arises from fluctuations generated at large disk radii and propagating inward, possibly linked to the thermal-viscous disk instability.

astro-ph.HE↗

arXiv2026

Reverberation lags viewed in hard X-rays from an accreting stellar-mass black hole

Accreting black holes are thought to swallow matter in the form of a disk and a hot cloud of plasma that glows brightly in X-rays, known as the corona. The X-ray emitting region is far too small to be directly imaged, but rapid variability of the X-ray signal can be used to infer the geometry by measuring time lags caused by material propagating towards the black hole and by coronal X-rays reflecting off the disk to imprint a reverberation lag. Reverberation lags can be recognized by characteristic spectral features, including an iron emission line at $\sim 6.4$ keV and a broad Compton hump peaking at $\sim 30$ keV. These reverberation features have both previously been detected for a few supermassive black holes in active galactic nuclei (AGNs). However, it is much more challenging to detect reverberation lags from stellar-mass black holes because they are more than a million times smaller. Previous reverberation lag measurements for stellar-mass black holes in X-ray binary systems have thus been limited to energies below 10 keV. Here we report on the first detection of the Compton hump reverberation feature from an X-ray binary, achieved by measuring lags in the broad energy range of $\sim 1-150$ keV. The accompanying detection of an iron line feature confirms the scenario of X-ray reverberation and provides strong evidence that the accretion flows in AGNs and X-ray binaries are governed by an ubiquitous process. Reverberation lags are prominent only in the most rapid variability, whereas lags in the slower variability are commonly attributed to propagating mass accretion rate perturbations. Our lag measurements up to the highest energy to date reveal that this lag in the slower variability evolves dramatically on timescales of days.

astro-ph.HE↗

arXiv2026

Covariance spectrum of MAXI J1820+070: On the nature of the Comptonizing flow

We present an analysis of the covariance spectrum of the black hole X-ray binary MAXI J1820+070 during its hard state. For the first time, we extend coherence and covariance studies into the hard X-ray band up to 150 keV. We detect a clear drop in coherence above 30 keV on both short- and long-timescales relative to the 2-10 keV reference band. To investigate the origin of the coherent variability, we simultaneously fit the short- and long-timescale covariances and the time-averaged spectra with a Comptonization model. Surprisingly, the electron temperature associated with long-timescale variability is significantly higher than that on short timescales. Moreover, the temperature on long timescales remains relatively constant throughout the hard state, whereas the short-timescale temperature evolves with X-ray luminosity. We attribute the drop in coherence to multiple sources of seed photons, i.e., the blackbody and synchrotron photons. The independence between these two photon fields leads to the drop in coherence. To explain the lower electron temperature on short timescales, we propose a two-Comptonization framework in which short-timescale variability arises from a vertically extended central region, while long-timescale variability originates at larger radii. The elevated geometry of the inner region leads to illumination primarily by cooler outer-disk photons, yielding a lower electron temperature. In this case, the evolution of the height of the elevated region could explain the evolution of the electron temperature associated with the coherent variability throughout the hard state.

astro-ph.HE↗

arXiv2025

Temporal evolution of quasi-periodic oscillations in an accreting black hole Swift J1727.8-1613: coevolution of the disk-corona during the state transition

Low-frequency quasi-periodic oscillations (QPOs) are commonly observed in black hole X-ray binaries, and their frequency has been found to correlate with various spectral properties. In this work, we present a detailed timing analysis of Swift J1727.8-1613, revealing a novel two-branch correlation between the QPO frequency and the observed disk emission, which differs from previous findings of a single correlation. Specifically, at QPO frequencies below 3 Hz, the QPO frequency is negatively correlated with the observed disk emission. This negative relation transitions to a positive one, as the QPO frequency exceeds approximately 3 Hz. The correlation between QPO frequency and Compton flux exhibits an opposite trend, with a positive correlation at lower frequencies and a negative correlation at higher ones. We interpret these behaviors as signatures of an evolving disk-corona geometry, within the framework of a Lense-Thirring precessing hot flow. Additionally, we find that during the flare state, the QPO fractional root-mean-square (rms) remains nearly constant above 15 keV, but increases with energy below this threshold. The slope of the rms-energy relation increases as the energy spectrum softens.

astro-ph.HE↗