arXiv subjects

Hao-Xuan Gao

Publications and source records attributed to Hao-Xuan Gao.

15 recordsLinked to original sources

arXiv2026

Extreme superluminal expansion traces an ultra-narrow GRB jet across relativistic regimes

The long-term evolution of gamma-ray burst (GRB) jets from ultra-relativistic launch to a slowly expanding radio source remains a fundamental problem in high-energy astrophysics, rarely resolved directly by imaging. We use very long baseline interferometry (VLBI) to trace the radio angular scale of GRB 221009A, the brightest GRB ever detected, from 5 to 299 days after the burst, and constrain the jet's geometry and dynamics. The Very Long Baseline Array data at 8.4 and 15.2 GHz are calibrated and imaged with AIPS and Difmap, and the source size is characterized by a circular-Gaussian model fitted to the complex visibilities. The jet dynamics, multi-wavelength emission, and source size are modeled within a two-shell collision scenario. A hydrodynamic simulation confirms that such an ultra-narrow jet undergoes lateral spreading. The observations reveal apparent superluminal expansion, with the expansion speed declining from about 90c at 5 days to about 6c at 300 days, tracing the jet's deceleration from the ultra-relativistic to the mildly relativistic regime and placing GRB 221009A far beyond the luminosity--velocity envelope of active galactic nuclei and microquasar jets. Combined with the multi-wavelength afterglow, they establish an ultra-narrow relativistic jet with a half-opening angle of about 0.01--0.03 radians, propagating through a wind-like medium at small radii before entering an interstellar-medium-like profile at larger radii. Joint modeling favors a two-phase size evolution with tentative evidence for lateral spreading. These results provide a direct geometric view of a GRB outflow and establish GRB 221009A as a benchmark for relativistic jet physics.

astro-ph.HE↗

arXiv2026

Are Repeaters Prevalent Among the Known Fast Radio Burst Sources?

Fast radio bursts (FRBs) are millisecond radio pulses of unknown origin. Despite extensive follow-up observations, only $\sim3\%$ of FRBs have been confirmed as repeaters. It remains unclear whether the rest are truly one-off bursts, or essentially repeating sources that have only been detected once due to limited monitoring time. Using the second CHIME/FRB catalog, we test this debate by comparing non-repeaters with two repeater-based subsamples: the first-detected bursts of repeaters and their highest-fluence bursts. A non-parametric method that accounts for selection effects is employed to derive the energy functions and event rates of these samples. All samples are well described by broken power-law energy distributions with comparable break energies ($\sim 5\times10^{38}$ erg), but with significantly different slopes between repeating and non-repeating populations. Their event-rate evolution also differs significantly. Assuming $ρ(z) \propto (1+z)^B$, we have $B = -5.57^{+0.15}_{-0.15}$ for non-repeaters and $B = -8.63^{+0.46}_{-0.41}$ and $-9.10^{+0.55}_{-0.56}$ for the two repeater samples. Size-matched resampling shows that the repeater event-rate indices lie far outside the 5$σ$ range expected from non-repeater subsamples, ruling out sample size as the reason for the observed difference. These results indicate that at least a subset of one-off FRBs are intrinsically non-repeating, implying that repeating sources may represent a distinct and possibly less common population.

astro-ph.HE↗

arXiv2026

A second-scale periodicity in an active repeating fast radio burst source

Fast radio bursts (FRBs) are fierce radio flashes from the deep sky. Abundant observations have indicated that highly magnetized neutron stars might be involved in these energetic bursts, but the underlying trigger mechanism is still enigmatic. Especially, the widely expected periodicity connected to the spin of the central engine has never been discovered, which leads to further debates on the nature of FRBs. Here we report the first discovery of a $\sim$ 1.7 s period in the repeating source of FRB 20201124A. This is an active repeater, from which more than 2800 bursts have been detected over a total of 49 days. The phase-folding method is adopted to analyze the bursts on each day separately. While no significant periodic signal is found in most days, a clear periodicity does appear on two specific days: a period of 1.706024(13) s on MJD 59310, and a slightly larger period of 1.707968(9) s on MJD 59347. A global Monte Carlo analysis based on all single-day datasets yields a significance level of $5.5 σ$ for the periodicity. A period derivative of $6.11(5)\times10^{-10}$ s s$^{-1}$ can be derived from these two periods, which further implies a surface magnetic field strength of $1.03\times10^{15}$ G and a spin-down age of $44$ years for the central engine. It is concluded that FRB 20201124A should be associated with a young magnetar.

astro-ph.HE↗

arXiv2026

Modeling the Multi-Wavelength Afterglow of Short Gamma-Ray Bursts with a Plateau Phase

Short gamma-ray bursts (GRBs) exhibiting a plateau phase provide valuable insights into the post-merger activity of their central engines. Although the physical origin of the plateau remains uncertain, the magnetar energy injection model offers a compelling explanation that reproduces the observed temporal and luminosity features. However, previous studies relying solely on X-ray data have suffered from strong parameter degeneracies when constraining the magnetar parameters. Here we perform broadband afterglow modeling on seven short GRBs with plateau features by combining X-ray, optical, and radio observations within the framework of the magnetar energy injection model. Key model parameters are derived by using the Markov Chain Monte Carlo method. It is found that the energy injection substantially modifies the afterglow dynamics in most events. Compared with X-ray-only analyses, our broadband modeling yields systematically a lower magnetic field strength and a shorter spin period for the central magnetar, corresponding to a higher injection luminosity. The study clearly shows that incorporating multi-wavelength data effectively alleviates the degeneracy between the magnetar parameters and X-ray radiative efficiency. In addition, the distribution of our short GRBs differs markedly from long GRBs when they are plotted on the initial Lorentz factor versus gamma-ray energy plane. This offset, consistent with the observed harder spectrum of short GRBs, may serve as a useful diagnostic for investigating the progenitor as larger samples are available.

astro-ph.HE↗

arXiv2025

Softness Ratio of SWIFT Gamma-ray Bursts and Relevant Correlations

The properties of X-ray flashes (XRFs) and X-ray rich gamma-ray bursts (XRRs) as compared with classical gamma-ray bursts (C-GRBs) have been widely discussed during the \emph{Swift} era. It has been proposed that XRFs and XRRs are low-energy extensions of the GRB population so that they should follow similar correlations. To further examine this idea, we collect a sample of $303$ GRBs detected by \emph{Swift} over the past two decades, all of which have reliable redshifts and spectral parameters. The bursts are classified into XRFs, XRRs, and C-GRBs based on their softness ratio (SR), which is calculated by dividing the $25-50$ keV fluence with the $50-100$ keV fluence. A strong correlation is found among the isotropic energy $E_{\mathrm{iso}}$, peak luminosity $L_{\mathrm{p}}$, and rest frame burst duration $T_{90, \mathrm{rest}}$, i.e., $E_{\mathrm{iso}} \propto L_{\mathrm{p}}^{0.88\pm0.02} T_{90, \mathrm{rest}}^{0.58\pm0.02}$. Additionally, two tight three-parameter correlations involving SR and the rest-frame peak energy $E_{\mathrm{p}}$ are also derived, i.e. $E_{\mathrm{p}} \propto E_{\mathrm{iso}}^{0.20\pm0.02} \mathrm{SR}^{-2.27\pm0.15}$ and $E_{\mathrm{p}} \propto L_{\mathrm{p}}^{0.17\pm0.02} \mathrm{SR}^{-2.33\pm0.14}$. It is interesting to note that XRFs, XRRs, and C-GRBs all follow the above correlations. The implications of these correlations and their potential application in cosmology are discussed.

astro-ph.HE↗

arXiv2025

GRB Timing: Decoding the Hidden Slow Jets in GRB 060729

Gamma-ray bursts (GRBs) are luminous stellar explosions characterized by the ejection of relativistic jets. This work proposes a novel paradigm to study these GRB jets. By analyzing the timing information of prompt pulses and X-ray flares, in conjunction with the multi-wavelength afterglow observations, we identify three distinct jets in the extraordinary GRB 060729, with initial bulk Lorentz factors ranging from approximately 20 to 80, smaller than typical values of $> 100$. These three jets undergo two successive collisions, producing the observed pair of X-ray flares. Following these interactions, the system evolves into a fast, narrow jet and a slower, hollow jet that continues to propagate in the circumburst medium, evidenced by the notable twin bumps observed in the X-ray and optical afterglow of GRB 060729. Our findings demonstrate that the timing of the early emission enables us to measure the velocities of the GRB jets. The proposed paradigm enhances our understanding of jet dynamics and shock interactions and serves as a powerful tool for probing the physics of the central engine with the expanded sample in the current golden era of GRB research.

astro-ph.HE↗

arXiv2025

Quasi-thermal Photosphere Emission from Structured Jets of Gamma-Ray Bursts

The prompt emission of gamma-ray bursts (GRBs) is supposed to be released from the relativistic jet launched from the central engine. Apart from the non-thermal nature of the spectra in a majority of GRBs, there is evidence for the presence of quasi-thermal components in the prompt emission of a few GRBs according to observations by Fermi satellite. On the other hand, the GRB jet has been revealed as structured in recent research. The theoretical observed spectra of photosphere emissions by an off-axis observer and the dependence of the spectra on the viewing angle under the assumption of structured jets remain unexplored. In this paper, we numerically calculate the instantaneous photosphere spectra by different viewing angles from a structured jet, from which relevant temporal and spectral characteristics are derived. Moreover, we address the necessity of proper treatment of the outflow boundary in the photosphere emission scenario. Furthermore, our calculations suggest that the Einstein Probe and Space-based multi-band astronomical Variable Object Monitor will have the capability to detect the short GRBs similar to GRB 170817A up to a luminosity distance of 200Mpc if the off-axis viewing angle is less than 10 degrees.

astro-ph.HE↗

arXiv2025

The Soft X-ray Aspect of Gamma-ray Bursts in the Einstein Probe Era

The Einstein Probe (EP) satellite, dedicated at time-domain high-energy astrophysics and multi-messenger astronomy, was recently launched and successfully put into operation. The wide-field X-ray telescope (WXT, 0.5-4 keV) onboard has identified multiple gamma-ray burst (GRB) events, with an average duration of several hundred seconds. This duration is several times longer than the average duration of long gamma-ray bursts (LGRBs) detected by the Neil Gehrels Swift Observatory, which typically stands at several tens of seconds. Additionally, EP has detected some unknown X-ray transients whose connection to GRBs is uncertain, due to the absence of gamma-ray counterparts and efficient follow-up observation at multi-wavelengths. Several main factors could account for the longer time, including the Doppler effect of off-axis viewing, the spectral lag effect of the synchrotron spectrum of cooling electrons, and some unknown prolonged intrinsic X-ray activities. Our studies indicate that EP GRBs may primarily consist of off-axis viewed bursts, forming a unique population among the GRB zoo, yet the intrinsic origin for the specific bursts could not be excluded. By analyzing the statistical properties of the historical LGRB samples, we explored observable properties of on-axis and off-axis LGRBs in the soft X-ray band. The predicted characteristics of off-axis viewed GRBs, including the duration, energy fluence, low-energy spectral index, and the slopes of Amati and Yonetoku relations, could be tested with a larger sample of GRB events detected by EP in the future.

astro-ph.HE↗

arXiv2025

Bounding the photon mass with gravitationally lensed fast radio bursts

The gravitational time delays of macro-lenses can be used to constrain the rest mass of the photon with high accuracy. Assuming a point-mass $+$ external shear lens model, we prove that an upper limit of the photon mass can be derived directly from two observables--the time delay $Δt$ and the leading-to-trailing flux ratio $R$ of strongly lensed fast radio bursts (FRBs). Using the observed values of $Δt$ and $R$ of a lensed FRB candidate, i.e., FRB 20190308C, as a reference, we obtain a strict upper limit of the photon mass between $m_γ< 5.3 \times {10}^{-42}\,\rm kg$, for a given external shear strength of $γ' = 0.01$, and $m_γ < 2.1 \times 10^{-41}-2.4 \times 10^{-42}\,\text{kg}$, within the external shear range of $0<γ'<1$. This provides the most stringent limit to date on the photon mass through gravitational lensing time delays, improving by 1 to 2 orders of magnitude the previous results obtained from lensed active galactic nuclei.

astro-ph.HE↗

arXiv2024

GRB 240529A: A Tale of Two Shocks

Thanks to the rapidly increasing time-domain facilities, we are entering a golden era of research on gamma-ray bursts (GRBs). In this Letter, we report our observations of GRB 240529A with the Burst Optical Observer and Transient Exploring System, the 1.5-meter telescope at Observatorio Sierra Nevada, the 2.5-meter Wide Field Survey Telescope of China, the Large Binocular Telescope, and the Telescopio Nazionale Galileo. The prompt emission of GRB 240529A shows two comparable energetic episodes separated by a quiescence time of roughly 400 s. Combining all available data on the GRB Coordinates Network, we reveal the simultaneous apparent X-ray plateau and optical re-brightening around $10^3-10^4$ s after the burst. Rather than the energy injection from the magnetar as widely invoked for similar GRBs, the multi-wavelength emissions could be better explained as two shocks launched from the central engine separately. The optical peak time and our numerical modeling suggest that the initial bulk Lorentz factor of the later shock is roughly 50, which indicates that the later jet should be accretion-driven and have a higher mass loading than a typical one. The quiescence time between the two prompt emission episodes may be caused by the transition between different accretion states of a central magnetar or black hole, or the fall-back accretion process. A sample of similar bursts with multiple emission episodes in the prompt phase and sufficient follow-up could help to probe the underlying physics of GRB central engines.

astro-ph.HE↗

arXiv2024

Probing Thermal Electrons in GRB Afterglows

Particle-in-cell simulations have unveiled that shock-accelerated electrons do not follow a pure power-law distribution, but have an additional low-energy "thermal" part, which owns a considerable portion of the total energy of electrons. Investigating the effects of these thermal electrons on gamma-ray burst (GRB) afterglows may provide valuable insights into the particle acceleration mechanisms. We solve the continuity equation of electrons in the energy space, from which multi-wavelength afterglows are derived by incorporating processes including synchrotron radiation, synchrotron self-absorption, synchrotron self-Compton scattering, and gamma-gamma annihilation. First, there is an underlying positive correlation between temporal and spectral indices due to the cooling of electrons. Moreover, thermal electrons would result in the simultaneous non-monotonic variation in both spectral and temporal indices at multi-wavelength, which could be individually recorded by the 2.5-meter Wide Field Survey Telescope and Vera Rubin Observatory Legacy Survey of Space and Time (LSST). The thermal electrons could also be diagnosed from afterglow spectra by synergy observation in the optical (with LSST) and X-ray bands (with the Microchannel X-ray Telescope on board the Space Variable Objects Monitor). Finally, we use Monte Carlo simulations to obtain the distribution of peak flux ratio ($R_{\rm X}$) between soft and hard X-rays, and of the time delay ($Δt$) between peak times of soft X-ray and optical light curves. The thermal electrons significantly raise the upper limits of both $R_{\rm X}$ and $Δt$. Thus the distribution of GRB afterglows with thermal electrons is more dispersive in the $R_{\rm X} - Δt$ plane.

astro-ph.HE↗

arXiv2024

Gravitational Wave Emission from Close-in Strange Quark Planets Around Strange Stars with Magnetic Interactions

According to the strange quark matter hypothesis, strange planets may exist, which are planetary mass objects composed of almost equal numbers of up, down and strange quarks. A strange planet can revolve around its host strange star in a very close-in orbit. When it finally merges with the host, strong gravitational wave emissions will be generated. Here the gravitational waveforms are derived for the merging process, taking into account the effects of the strange star's magnetic field on the dynamics. Effects of the inclination angle are also considered. Templates of the gravitational waveforms are derived. It is found that the magnetic interactions significantly speed up the merging process. Coalescence events of such strange planetary systems occurring in our Galaxy as well as in local galaxies can be effectively detected by current and future gravitational experiments, which may hopefully provide a new method to test the strange quark matter hypothesis and probe the magnetic field of compact stars.

astro-ph.HE↗

arXiv2022

Gravitationally Lensed Orphan Afterglows of Gamma-Ray Bursts

The cosmological nature of gamma-ray bursts (GRBs) implies that a small portion of them could be gravitationally lensed by foreground objects during their propagation. The gravitational lensing effect on the GRB prompt emission and on-axis afterglows has been discussed, and some candidates have been found in the literature. In this work, considering the high detection rate of GRB orphan afterglows in future wide-field survey era, we investigate the gravitationally lensed orphan afterglows in view of three lens models, i.e., the point-mass model, the singular isothermal sphere model, and the Chang-Refsdal model. The structure of the GRB jet itself is also incorporated in calculating the lensed afterglow light curves. It is found that lensed optical/X-ray orphan afterglows in principle could be diagnosed through their temporal characteristics, and the optical band is the best band to observe the galaxy-lensed orphan afterglows. Moreover, the event rate for galaxy-lensed orphan afterglows is estimated to be $\lesssim$ 1.8 $\text{yr}^{-1}$ for the whole sky. If most orphan afterglows could be identified (from other transients in the survey data), the optimistic detection rates of the 2.5m Wide Field Survey Telescope of China and 8.4m Vera Rubin Observatory Legacy Survey of Space and Time for galaxy-lensed orphan afterglows in the optical band are $\lesssim$ 0.01$\sim$0.02 $\text{yr}^{-1}$ and $\lesssim$ 0.04$\sim$0.08 $\text{yr}^{-1}$, respectively.

astro-ph.HE↗

arXiv2021

Evolution Patterns of the Peak Energy in the GRB Prompt Emission

There are two different evolution patterns of the peak energy ($E_\text{p}$) exhibited during the prompt emission phase of gamma-ray bursts (GRBs), i.e., hard-to-soft and intensity-tracking, of which the physical origin remains unknown. Except for low-energy indices of GRB prompt spectra, the evolution patterns of $E_\text{p}$ may be another crucial indicator to discriminate radiation mechanisms (e.g., synchrotron or photosphere) for GRBs. We explore the parameter space to find conditions that could generate different evolution patterns of the peak energy in the framework of synchrotron radiation. We have developed a code to calculate the synchrotron emission from a simplified shell numerically, considering three cooling processes (synchrotron, synchrotron self-Compton (SSC), and adiabatic) of electrons, the effect of decaying magnetic field, the effect of the bulk acceleration of the emitting shell, and the effect of a variable source function that describes electrons accelerated in the emitting region. After exploring the parameter space of the GRB synchrotron scenario, we find that the intensity-tracking pattern of $E_\text{p}$ could be achieved in two situations. One is that the cooling process of electrons is dominated by adiabatic cooling or SSC+adiabatic cooling at the same time. The other is that the emitting region is under acceleration in addition to the cooling process being dominated by SSC cooling. Otherwise, hard-to-soft patterns of $E_\text{p}$ are normally expected. Moreover, a chromatic intensity-tracking pattern of $E_\text{p}$ could be induced by the effect of a variable source function.

astro-ph.HE↗

arXiv2019

Geometric effects on the electronic structure and the bound states in annular corrugated wires

In the spirit of the thin-layer quantization scheme, we give the effective Hamiltonian describing the noninteracting electrons confined to an annular corrugated surface, and find that the geometrically induced potential is considerably influenced by corrugations. By using numerical calculation, we investigate the eigenenergies and the corresponding eigenstates, and find that the transition energies can be sufficiently improved by adding corrugations. Particularly, the transition energy between the adjacent eigenstates corresponds to energy levels difference based on the wavefunction of annular wire, and the number of the energy levels is equal to the number of corrugations. And the larger magnitude of corrugations is capable of increasing the number of bound states. In addition, the distribution of ground state probability density is reconstructed by the corrugations, and the energy shift is generated.

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