arXiv subjects

Fa-Yin Wang

Publications and source records attributed to Fa-Yin Wang.

At least 19 recordsLinked to original sources

arXiv2026

Second-scale periodicity in FRB 20201124A: coincident with critical RM evolution epochs and its possible implications

Second-timescale periodicity ($\sim$1.7\,s) has been reported in the repeating fast radio burst FRB\,20201124A during two epochs, MJD\,59310 and MJD\,59347. We find that these two epochs coincide with the days on which the Faraday rotation measure (RM) of the source passes through the transition between the quasi-steady and the strongly varying phases that are predicted by the magnetar/Be-star binary model. To examine whether this coincidence could arise by chance, we re-fitted the secular RM evolution within the same binary framework and estimated the probability of chance coincidence, $P_{\rm CC}\approx2.5\%$. Independently, and in order to cross-validate the reported periodicity, we carried out a coherent joint period search on the bursts of the two epochs, assuming a simple linear evolution of the spin period; we obtain $P_0=1.70603$\,s and $\dot{P}=6.1\times10^{-10}$\,s\,s$^{-1}$ at the reference epoch, with a Gaussian significance of $\sim3.9σ$, consistent with the previous measurement. We propose that the two periodic epochs correspond to passages of the magnetar through the decretion disk of the Be-star companion. The captured disk material suppresses low-latitude magnetospheric emission, so that the polar-cap emission dominates and the spin period becomes detectable; alternatively, because the disk material carries substantial angular momentum, accretion increases the magnetic obliquity and thereby amplifies the periodic modulation of the polar-cap emission. The synchronization between the burst periodicity and the magneto-ionic transition points to a link between burst activity and the dynamical evolution of the source environment. The details of the binary-model fit and of the period search are given in Appendices A and B.

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arXiv2026

A Unified Mass-Loading Model for ASKAP J1745-5051: Linking Long-Period Radio Transients to Magnetic Cataclysmic Variables

Long-period radio transients (LPTs) in white dwarf binaries require sufficiently tenuous plasma to sustain coherent radio emission, posing a challenge in systems with dense accretion flows. ASKAP J1745-5051 (J1745) combines LPT-like radio pulses recurring approximately every 82 min with magnetic accretion and X-ray modulation at a consistent period. We investigate this coexistence using a unified mass-loading framework for Roche-lobe overflow and a tenuous donor wind in the binary magnetic field. The dense stream supplies the accretion curtain and near-white-dwarf X-ray sources, while the tenuous wind feeds flux tubes of $(1-4)\times10^7\,{\rm cm^{-3}}$, satisfying the local density requirement for electron-cyclotron maser emission. A fiducial geometry places the X-ray maximum near orbital phase $0.886$ and produces a radio-visibility window at phases $0.22-0.33$, consistent with the primary observed pulse window. Geometrical modulation and curtain absorption produce stronger soft-band than hard-band X-ray variability. Long-term phase-connected radio monitoring and simultaneous X-ray observations can test this interpretation through recurring radio phase changes and correlated changes in X-ray modulation. These results extend the proposed connection between detached binary LPTs and magnetic cataclysmic variables (mCVs). J1745 may represent an evolved mCV near the orbital-period minimum, suggesting that LPT-like activity can occur at a late accreting stage.

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arXiv2026

EP251129a: A Fast X-ray Transient with Two X-Ray Plateaus and Its Off-axis Afterglow

Extragalactic fast X-ray transients (EFXTs) detected by the Einstein Probe are extending the parameter space of relativistic stellar explosions toward softer and longer-lived transients. Here we report EP251129a/GRB 251129A at z=0.505, which exhibits two X-ray plateaus: an early plateau lasting $\sim 10^{4}\,\rm s$ followed by a rapid decay with spectral softening, and a second plateau extending to 60 days, placing it at the extreme long-duration end of Gamma Ray Burst (GRB) plateaus. The early optical emission evolves independently of the X-ray, indicating a different origin; whereas the unusual radio afterglow shows a rapid rise of $F_ν\propto t^{1.9-2.5}$ and a delayed peak at $\sim36-40$ days, providing strong evidence for an off-axis geometry. We find that the early X-ray behavior resembles those of ultra-long GRBs (ULGRBs) and can be naturally explained by the internal dissipation from a newborn millisecond magnetar undergoing gravitational-wave-dominated spin-down before collapsing into a black hole. The early optical, radio and late X-ray emission are well described by an off-axis two-component jet afterglow with additional energy injection, possibly powered by fallback accretion onto black hole. EP251129a therefore connects the emerging EFXT population with ULGRB-like central-engine activity and off-axis afterglow, offering a rare opportunity to probe both the evolution of central engine and jet structure in a single event.

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arXiv2026

Propagation Diagnostics of Supernova Remnant Environments around Young Repeating Fast Radio Bursts. I. Hydrodynamic Evolution of the Source-local Dispersion Measure

Repeating fast radio bursts may reside in young supernova remnant (SNR) environments whose evolving plasma contributes to the observed dispersion measure (DM). We use two-dimensional axisymmetric hydrodynamic simulations to study the interaction between a continuous anisotropic wind from a young neutron star and homologously expanding supernova ejecta. We follow the evolution to approximately 160 yr and calculate the source-local DM along different viewing directions, using a passive tracer to separate wind and nonwind contributions. In the fiducial model, the strongly polar-focused wind inflates a low-density cavity, while the swept-up shell remains broadly rounded and the DM shows moderate angular variation. The DM is dominated by ejecta and swept-up nonwind material. The solid-angle-averaged ambient-subtracted excess DM declines throughout the evolution, approximately following $t_{\rm age}^{-2}$ during the first several tens of years and becoming modestly steeper later. Variations in wind and ejecta parameters modify the normalization, early evolution, and viewing-angle dependence, but the angle-averaged DM declines in all models, while different bipolar wind profiles produce similar long-term evolution. For FRB 20190520B, the fiducial model reaches a decline rate comparable to the source-frame value inferred from observations at approximately 20 yr, when the mean excess DM is approximately $1.8\times10^2~{\rm pc,cm^{-3}}$. Thus, such a young environment can retain a substantial electron column while producing a rapid secular decrease. Repeater diversity suggests that SNR-driven expansion may coexist with additional time-dependent plasma structures or ionization changes.

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arXiv2026

Constraining gamma-ray burst viewing angles with Swift/XRT afterglow light curves

Gamma-ray bursts (GRBs) are among the most energetic phenomena in the universe, and their afterglow light curves encode information about jet geometry and viewing angle. To constrain GRB viewing angles, we analyzed jet break features in Swift X-Ray Telescope afterglow light curves using two top-hat jet models: a simplified geometric model without high-latitude emission (model 1) and a comprehensive model including it (model 2). Both models were applied to a sample of 20 GRBs in an interstellar medium (ISM) and 20 in a wind medium, selected so that jet breaks are attributed to the edge effect with sufficient data coverage, and fitted with Markov Chain Monte Carlo methods. We examined viewing angles and off-axis ratios q = $θ_{\rm obs}/θ_{\rm jet}$ under both density profiles, evaluating the impact of high-latitude emission. Based on reduced chi-squared and Bayesian information criterion comparisons, model 1 fits all GRBs better. Most GRBs have small off-axis ratios (mean q = 0.1851 for model 1), indicating viewing angles generally close to the jet axis; the log-space viewing-angle distribution is approximately Gaussian. A Kolmogorov-Smirnov test shows no significant difference in off-axis ratios between ISM and wind media, nor between bursts with and without an X-ray plateau. While viewing angles decrease significantly with redshift, the off-axis ratio shows no significant evolution, consistent with off-axis alignment being independent of cosmic epoch.

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arXiv2026

Multi-scale Memory and Regime Shift in the Hyperactive Repeating FRB 20240114A

We present a statistical analysis of FRB~20240114A, a hyperactive repeating fast radio burst, based on 11,553 bursts detected by FAST over 214 days. Our main findings are fourfold. (1) On the most active day (MJD~60381, 3,197 bursts in 4.38 hr), event-rate coherence analysis reveals persistent correlated activity extending up to 3600~s, the longest reported for any repeating FRB, showing memory persists even in intense bursting epochs. (2) The waiting-time distribution on this day is well described by three exponentials, whereas the full 214-day sample develops a threshold power-law tail, indicating burst statistics depend on the observational baseline, with long-range correlations emerging only over longer timescales, a hallmark of self-organized criticality. (3) Rescaled range (R/S) analysis of waiting times reveals a broken power law, with Hurst exponents $H_1=0.63\pm0.02$ (short-lag weak memory) and $H_2=1.04\pm0.02$ (long-lag non-stationary drift). The break corresponds to $\sim$1 hour, consistent with the 3600~s coherence limit. R/S analysis of energies similarly exhibits a break ($H_1=0.60\pm0.01$, $H_2=1.10\pm0.05$) at a different lag, reinforcing that non-stationarity affects both temporal and energetic properties. (4) Energy distributions exhibit waiting-time-dependent slopes that are consistent with the full and daily samples, and the high-energy cutoff remains constant across waiting-time groups, suggesting that the maximum energy scale is an intrinsic source property. Together, these results establish a multi-scale memory framework: the source behaves stochastically on short timescales but exhibits systemic non-stationarity over months, providing benchmarks for burst models and highlighting the need for long-term, high-cadence monitoring to capture temporal complexity.

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arXiv2026

Evidence of self-organized criticality in the prompt emission of a bright gamma-ray burst

Gamma-ray bursts (GRBs) are the most energetic explosive events in the Universe, yet the physical mechanism of their prompt emission remains a mystery. Especially, it is unclear whether the energy dissipation mechanism in the GRB jet is dominated by kinetic energy or magnetic energy. Here, we studied the pulses in the prompt emission of the second brightest GRB to date, GRB 230307A, which was accurately measured by the Gravitational wave high-energy electromagnetic counterpart all-sky monitor (GECAM), with focus on the cumulative distributions of peak counts and duration of pulses as well as the waiting time between pulses. We find that these cumulative distributions show scale-invariant behavior, well consistent with the prediction of the self-organized criticality (SOC) theory. This is the first robust evidence of an SOC feature in the prompt emission of a single GRB. Moreover, the statistical properties of pulses in the prompt emission of GRB 230307A are very similar to those of solar flares. Our findings suggest that the prompt emission of GRB is powered by the dissipation of magnetic energy in the ultra-relativistic jet, supporting the Poynting-flux-dominated prompt models.

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arXiv2026

Constraining Circum-burst Environments of GRBs with Jet Break Features in X-ray Afterglows

The nature of the circum-burst medium serves as a key diagnostic for probing the progenitor systems and the physics of relativistic jet propagation in gamma-ray bursts (GRBs). In this work, we systematically infer the density profile index $k$ (where $n \propto r^{-k}$) from the change in the temporal decay index at the jet break ($Δα$). Within the framework of the uniform jet model, the two quantities are linked by the relation $Δα= (3 - k)/(4 - k)$. We apply this diagnostic to a substantial and uniformly selected sample of 170 GRBs with clear jet breaks, identified from over 1,400 Swift/XRT X-ray afterglows observed from 2004 to 2024. By fitting the light curves with a broken power-law model, we obtain $Δα$ for each burst and subsequently derive the corresponding $k$ value. We then use the derived $k$ values to classify the circum-burst environment of each GRB. Our results reveal a near-even split: 82 bursts ($\sim48\%$) are consistent with a constant-density interstellar medium (ISM, $k \approx 0$), while 88 bursts ($\sim52\%$) favor a wind environment ($k \approx 2$). For the 35 bursts with optical data, our X-ray-based classifications are generally consistent with independent multi-band analyses. Additionally, we derive jet opening angles and true beaming-corrected energies for bursts with known redshifts.

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arXiv2026

A Falsifiable Timing Test for the Double-White-Dwarf Model of Long-Period Transients

Long-period transients (LPTs) are a newly identified class of radio sources with burst recurrence times from minutes to hours, and their diversity suggests multiple physical origins. CHIME/ILT J1634+44, with a short period of 841 s, a long-period modulation of 4206 s, and a significant negative period derivative, strongly suggests a binary origin. For such a short-period source, Roche-lobe constraints strongly favor an ultra-compact companion, motivating a double-white-dwarf (WD--WD) interpretation. In this Letter, we show that the WD--WD channel makes a sharp timing prediction: if the burst period is the orbital clock and the long-period modulation is a spin-orbit beat, then the modulation period is not a free timescale. Instead it must evolve jointly with the orbital clock and the spin clock through gravitational-wave losses, magnetic dissipation, and tidal interaction. For CHIME/ILT J1634+44-like parameters, we find that the beat clock drift $|\dot P_b|\sim 10^{-10} \text{ s s}^{-1}$, implying an observed-minus-calculated drift of tens of seconds in one year. Joint measurements of the burst period, modulation period, and their derivatives provide a minimal and falsifiable timing test of an ultra-compact binary origin.

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arXiv2026

A Review on Resolving the Hubble Tension via Late-Universe Physics

The $Λ$CDM cosmological model has been successful in explaining many astronomical observations. However, recent observations increasingly point to deviations from the standard $Λ$CDM framework. Among these, one of the most significant discrepancies is the \textit{Hubble tension}, which refers to the difference in values obtained for the Hubble constant $H_0$ from high-redshift measurement and local observation. To address this issue, numerous cosmological models and methodological approaches have been proposed. This review offers a concise overview of recent progress in resolving the Hubble tension. The combination of Dark Energy Spectroscopic Instrument (DESI) Baryon Acoustic Oscillations (BAO) and uncalibrated Type Ia supernovae data yields a value for $H_0$ that is significantly higher than the $Λ$CDM predication based on early-universe probes, even without incorporating local distance ladder constraints. This result indicates that the origin of the Hubble tension lies in new physics at low redshifts. Our findings suggest that although many unresolved systematics persist in current observations, they are insufficient to account for the magnitude of the current Hubble tension. This implies the likely existence of new physical mechanisms that have yet to be discovered.

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arXiv2026

Magnetar-powered long gamma-ray bursts and connection to superluminous supernovae and fast radio bursts

Based on X-ray afterglow observations from the Swift satellite, we construct a sample of 169 long gamma-ray bursts (LGRBs) exhibiting the canonical magnetar plateau signature, i.e., a plateau followed by a $t^{-2}$ decay. We derive the plateau luminosity $L_0$ and break time $t_b$ for each burst by performing Markov Chain Monte Carlo (MCMC) fits to the light curves, and estimate pseudo-redshifts for bursts lacking known redshifts via the Amati relation. The fundamental magnetar parameters are subsequently inferred: the surface polar magnetic field strength $B_p \in [0.39,\ 23.08] \times 10^{15}$G and the initial spin period $P_0 \in [0.95,\ 13.79]$ms. Statistical analysis shows that both the known-redshift subsample and the full sample follow the Dainotti correlation between $L_0$ and $t_b$ with a slope close to $-1$, supporting a constant energy injection rate during the plateau phase. Furthermore, we identify a significant correlation between $B_p$ and $P_0$: $B_p \propto P_0^{0.83 \pm 0.09}$ for the full sample and $B_p \propto P_0^{0.80 \pm 0.16}$ for the known-redshift subsample, with both slopes consistent within uncertainties. Compared to magnetars powering superluminous supernovae (SLSNe), GRB magnetars possess systematically stronger magnetic fields (by approximately one order of magnitude), suggesting fundamental differences in their progenitor systems or collapse conditions; while their magnetic field strengths show no significant difference from those powering fast radio bursts (FRBs), suggesting a possible common evolutionary pathway. This study provides a physics-motivated, model-consistent sample of magnetar-candidate GRBs, offering a robust foundation for statistical investigations within the magnetar central engine model and placing new observational constraints on the birth properties of these extreme compact objects.

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arXiv2026

Periodic Emission Frequency Modulation in a Hyperactive Fast Radio Burst

Fast radio bursts (FRBs) are intense, short-duration radio transients of mysterious origin. They have been detected across a wide range of frequencies from 110 MHz to 8 GHz. Their spectral properties, remaining poorly understood, are essential for understanding the intrinsic radiation mechanism and propagation effects. Here, we report the discovery of a periodic modulation in the central emission frequency of FRB 20240114A, based on more than one thousand bursts collected by an ultra-wideband receiving system. The burst central frequencies reveals a significant modulation with a period of $\sim 112$ days. The statistical significance of this detected periodicity exceeds $6σ$ for both the Lomb-Scargle and phase-folding methods. Within a single period, the central emission frequency exhibits a systematic drift from lower to higher values. We evaluate several physical mechanisms for this unique spectral evolution. The free-free absorption together with cyclotron resonant absorption in a binary system or free precession models could potentially explain such behavior. The discovery of this periodic frequency modulation unveils a new layer of complexity in the underlying radiation mechanism and propagation effect of FRBs.

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arXiv2026

FAST Polarization Catalog of FRB 20240114A

Polarization measurements of fast radio bursts (FRBs) probe the magnetized plasma surrounding their central engines. FRB~20240114A is an exceptionally active repeating source, with 17,356 bursts detected between 2024 January 28 and 2025 May 30 by FAST, enabling time-resolved polarimetric studies. In this work, we present a polarimetric catalog of 6,131 bright bursts (with a signal-to-noise ratio S/N $\geq$ 20, 35.3% of the total sample), including arrival time (MJD$_{\text{topo}}$), dispersion measure (DM), burst width (W$_{\text{eff}}$), bandwidth, Faraday rotation measure (RM), linear and circular polarization degrees (DOL, DOC), and intrinsic polarization angle (PA$_0$). We detect a clear temporal evolution of RM: after an initial stable phase, it decreases linearly by $\sim$200 $\rm rad\ m^{-2}$ over 200 days, forming a bimodal distribution, whereas DM remains stable at 528.9 $\rm pc\ cm^{-3}$. The linear polarization fraction is generally high, with the 3$σ$ lower bound around 76%, while circular polarization is low, with 1,157 of 17,356 bursts (6.67%) having DOC $\geq$10%. We perform a power-law fit between $|\textrm{V}|$/I and $|\textrm{RM}|$, which yields an index of $-2.98 \pm 0.80$. It is found that the combined 2D distribution of L/I versus V/I remains stable, implying that the emission mechanism is largely invariant. Our PA$_0$ measurements show a broad, non-uniform distribution, implying a complex emission geometry. These results suggest that FRB~20240114A resides in a dynamically evolving magneto-ionic environment. This catalog provides a foundation for studies of repeating FRB progenitors and their environments.

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arXiv2026

Spectral Hardness as the Primary Discriminator: Unveiling the Collapsar--Merger Boundary with a Gold-Standard Gamma-Ray Burst Sample

In this Letter, we establish a robust, physically motivated classification method using a Support Vector Machine (SVM) trained on a "gold-standard" sample of 24 GRBs with spectroscopically confirmed progenitors (associated SNe or KNe). By isolating the prompt main spike to excise contamination from extended emission, we derive a quantitative classification index, I_SVM = 5.01 log_10 E_p,i - 1.25 log_10 E_iso - 0.34 log_10 T_90,z - 12.90 (units: keV, 10^52 erg, s). Events with I_SVM > 0 are classified as mergers. Analysis of the standardized classification weights reveals that the discriminative power of E_p,i is approximately 5 times that of T_90,z, while E_iso contributes a weight comparable to E_p,i. This quantitatively demonstrates that spectral hardness and energetics, rather than duration, are the primary physical signatures distinguishing mergers from collapsars. The derived boundary implies a stringent hardness ceiling for collapsars, while mergers are identified as outliers with excessive hardness relative to their energy budget. The classifier successfully identifies the nature of historic test cases, including the ultra-long GRB 111209A (collapsar) and the short GRB 050709 (merger), independent of instrumental eras. This tool paves the way for cleaning archival and future high-redshift GRB samples for precision cosmology.

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arXiv2026

Tidal capture and repeating partial tidal disruption events of giant stars

When an object is scattered near a supermassive black hole (SMBH), tidal oscillations excited within it reduce its orbital energy, leading to capture by the SMBH. This process, called tidal capture, can also occur when the object approaches even closer to the SMBH, resulting in a partial tidal disruption event (pTDE). Previous studies on pTDEs of main-sequence stars have shown that as the disruption intensifies, dynamical effects dominate over tidal oscillations, causing the remnant material to acquire a kick velocity instead of being captured by the SMBH. In this work, we performed hydrodynamic numerical simulations of pTDEs involving giant stars. We found that for weaker disruptions, the dynamical behavior of the remnant material resembles that of main-sequence stars. However, as the disruptions deepen, the remnant material transitions from gaining energy to losing energy, leading to capture by the SMBH. This behavior markedly differs from that of main-sequence stars, demonstrating that the presence of a compact core significantly influences the dynamical processes in pTDEs. Our simulations reveal that the energy change of the remnant material strongly correlates with asymmetric mass -- lossspecifically, the difference in mass outflow between the Lagrange points L1 and L2. This suggests that the energy change stems from asymmetric mass loss, consistent with conclusions from previous studies on main-sequence stars. However, quantitative analysis contradicts earlier models, indicating that the dynamical model of pTDEs requires further refinement. Finally, we discuss the characteristics of repeating pTDEs produced by this process and their potential observability, as well as the implications for the long-term orbital evolution of high eccentricity extreme mass ratio inspiral systems.

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arXiv2026

The disk precession in a Be star-magnetar binary and its application to the rotation measure of FRB 20201124A

Fast radio bursts (FRBs) are bright, millisecond-duration radio bursts with poorly known origins. Most FRB sources are detected only once, while some are repeaters. Variation patterns observed in the rotation measure (RM) of some repeaters -- indicate that the local magneto-ionic environments of these FRB sources are highly dynamic. It has been suggested that a Be star-magnetar binary system is a possible origin for such variation. FRB 20201124A is notable among these sources since it is the most active one and exhibits substantial temporal variations of RM measured by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The physics behind this long-term behavior is poorly understood. Here we propose that, within the framework of the Be star-magnetar binary scenario, the observed variation of RM is attributed to a combination of orbital motion and the precession of the circumstellar disk of the Be star. While a ~785-day precession of the disk contributes to the observed decrease in the amplitude of the variation, our model predicts that the amplitude oscillates with this period.

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arXiv2026

Exploring the central engines of gamma-ray bursts from prompt light curves

Hyperaccreting stellar-mass black hole systems are leading candidates for the central engines of gamma-ray bursts (GRBs). Their jets are thought to be powered by either the Blandford-Znajek (BZ) process or neutrino-dominated accretion flows (NDAFs), but discriminating between these mechanisms remains challenging. To address this, we propose using the luminosity decay slope (parameter d) of GRB light curves to distinguish between the BZ and NDAF mechanisms, thereby linking the light-curve morphology to the central engine physics. By analysing 85 single-peaked GRBs with fast-rise, exponential-decay (FRED) profiles observed by Swift/BAT using 64 ms background-subtracted light curves, we fit the decay slope (parameter d) with the empirical Kocevski-Ryde-Liang (KRL) function and compare the results with theoretical predictions for the BZ (d approximately 1.67) and the NDAF (d approximately 3.7 to 7.8) mechanisms. We find that the decay slope (parameter d) can differentiate central engine mechanisms, with 15 GRBs consistent with the BZ mechanism and 22 supporting the NDAF mechanism. However, most events exhibit slopes within the range between 2 and 4, suggesting a hybrid of mechanisms, with NDAF being dominant.

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arXiv2025

Hubble constant measurement from QPEs as electromagnetic counterparts to extreme mass ratio inspirals

Gravitational waves (GWs) accompanied by electromagnetic (EM) counterparts provide a novel methodology to measure the Hubble constant ($H_0$), known as bright sirens. However, the rarity of such multi-messenger events limits the precision of the $H_0$ constraint. Recently, the newly-discovered nuclear transient, quasi-periodic eruptions (QPEs) show intriguing evidence of a stellar-mass companion captured by a supermassive black hole (SMBH) in an extreme/intermediate mass-ratio inspiral (EMRI/IMRI), which is the most promising sources of the space-based GW detectors, such as LISA. Here, we model the secular orbital evolution of known QPE systems using two frameworks: a stripping scenario in which periodic mass transfer at periapsis drives the evolution; and an orbiter-disk collision scenario in which the companion interacts with a misaligned accretion disk, modulated by coupled orbiter-disk precession. For each framework, we assess detectability by LISA, together with the resulting constraints on $H_0$. Our principal findings are: (i) in the stripping scenario, no currently known QPE reaches detectability within a four-year LISA mission. (ii) in the orbiter-disk scenario, two sources-eRO-QPE2 and eRO-QPE4-are detectable with signal-to-noise ratios $\simeq 8.5-28.8$ and constrain $H_0$ with fractional uncertainty of 6.7-14.9\%. QPE systems remain uncertain on the decade-long secular evolution. Therefore, they motivate continued time-domain monitoring of QPE candidates.

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