arXiv subjects

Zhi-Qiu Huang

Publications and source records attributed to Zhi-Qiu Huang.

9 recordsLinked to original sources

arXiv2026

Late-time X-ray afterglows of GRBs: Implications for particle acceleration at relativistic shocks

Particle-in-cell (PIC) numerical simulations are currently among the most advanced tools to investigate particle acceleration at relativistic shocks. Still, they come with limitations imposed by finite computing power, whose impact is not straightforward to evaluate a priori. Observational features are hence required as verification. energy electrons accelerated at external shocks, provides a testbed for such predictions. Current numerical studies suggest that in GRB afterglows the maximum synchrotron photon energy, which corresponds to the limit of electron acceleration, may fall within the $\sim$ 0.1--10 keV X-ray energy band at late times, $t\gtrsim 10^6 - 10^7$ s. To test this prediction, we analyzed the X-ray spectra of six GRBs with \emph{Swift}/XRT detections beyond $10^7$ s: our analysis reveals no clear evidence of a spectral cutoff. Using a model that accounts for the effect of the finite opening angle of the shock on the observed maximum synchrotron photon energy, we show that these observations are incompatible with PIC simulation predictions, unless one or more physical afterglow parameters attain values at odds with those typically inferred from afterglow modeling (small radiative efficiency, low ambient density, large equipartition fraction $ε_{\rm B}$ of the magnetic field). These findings challenge existing numerical simulation results and imply a more efficient acceleration of electrons to high-energies than seen in PIC simulations, with important implications for our understanding of particle acceleration in relativistic shocks.

astro-ph.HE↗

arXiv2023

Prospects for ultra-high-energy particle acceleration at relativistic shocks

We study the acceleration of charged particles by ultra-relativistic shocks using test-particle Monte-Carlo simulations. Two field configurations are considered: (i) shocks with uniform upstream magnetic field in the plane of the shock, and (ii) shocks in which the upstream magnetic field has a cylindrical geometry. Particles are assumed to diffuse in angle due to frequent non-resonant scattering on small-scale fields. The steady-state distribution of particles' Lorentz factors is shown to approximately satisfy $dN/dγ\propto γ^{-2.2}$ provided the particle motion is scattering dominated on at least one side of the shock. For scattering dominated transport, the acceleration rate scales as $t_{\rm acc}\propto t^{1/2}$, though recovers Bohm scaling $t_{\rm acc}\propto t$ if particles become magnetised on one side of the shock. For uniform field configurations, a limiting energy is reached when particles are magnetised on both sides of the shock. For the cylindrical field configuration, this limit does not apply, and particles of one sign of charge will experience a curvature drift that redirects particles upstream. For the non-resonant scattering model considered, these particles preferentially escape only when they reach the confinement limit determined by the finite system size, and the distribution approaches the escapeless limit $dN/dγ\propto γ^{-1}$. The cylindrical field configuration resembles that expected for jets launched by the Blandford $\&$ Znajek mechanism, the luminous jets of AGN and GRBs thus provide favourable sites for the production of ultra-high energy cosmic rays.

astro-ph.HE↗

arXiv2022

Particle acceleration at ultrarelativistic, perpendicular shock fronts

Using an eigenfunction expansion to solve the transport equation, complemented by Monte-Carlo simulations, we show that ultrarelativistic shocks can be effective particle accelerators even when they fail to produce large amplitude turbulence in the downstream plasma. This finding contradicts the widely held belief that a uniform downstream magnetic field perpendicular to the shock normal inhibits acceleration by the first order Fermi process. In the ultrarelativistic limit, we find a stationary power-law particle spectrum of index s=4.17 for these shocks, close to that predicted for a strictly parallel shock.

physics.plasm-ph↗

arXiv2022

The implications of TeV detected GRB afterglows for acceleration at relativistic shocks

Motivated by the detection of very high energy gamma-rays deep in the afterglow emission of a gamma-ray burst, we revisit predictions of the maximum energy to which electrons can be accelerated at a relativistic blast wave. Acceleration at the weakly-magnetized forward shock of a blast-wave can be limited either by the rapid damping of turbulence generated behind the shock, by the effect of a large-scale ambient magnetic field, or by radiation losses. Within the confines of a standard, single zone, synchrotron-self-Compton (SSC) model, we show that observations of GRB190829A rule out a rapid damping of the downstream turbulence. Furthermore, simultaneous fits to the X-ray and TeV gamma-ray emission of this object are not possible unless the limit on acceleration imposed by the ambient magnetic field is comparable or weaker than that imposed by radiation losses. This requires the dominant length scale of the turbulence behind the shock to be larger than that implied by particle-in-cell simulations. However, even then, Klein-Nishina effects prevent production of the hard VHE gamma-ray spectrum suggested by observations. Thus, TeV observations of GRB afterglows, though still very sparse, are already in tension with the SSC emission scenario.

astro-ph.HE↗

arXiv2020

Examining the secondary product origin of cosmic ray positrons with the latest AMS-02 data

Measurements of cosmic-ray (CR) positron fraction by PAMELA and other experiments have found an excess above 10 GeV relative to the standard predictions for secondary production in the interstellar medium (ISM). Although the excess has been mostly suggested to arise from some primary sources of positrons (such as pulsars and or annihilating dark matter particles), the almost constant flux ratio of $e^{+}/ \bar{p}$ argues for an alternative possibility that the excess positrons and antiprotons up to the highest energies are secondary products generated in hadronic interactions. Recently, Yang \& Aharonian (2019) revisit this possibility by assuming the presence of an additional population of CR nuclei sources. Here we examine this secondary product scenario using the \texttt{DRAGON} code, where the radiative loss of positrons is taken into account consistently. We confirm that the CR proton spectrum and the antiproton data can be explained by assuming the presence of an additional population of CR sources. However, the corresponding positron spectrum deviates from the measured data significantly above 100 GeV due to the strong radiative cooling. This suggests that, although hadronic interactions can explain the antiproton data, the corresponding secondary positron flux is still not enough to account for the AMS data. Hence contribution from some primary positron sources, such as pulsars or dark matter, is non-negligible.

astro-ph.HE↗

arXiv2019

GeV observations of the extended pulsar wind nebulae constrain the pulsar interpretations of the cosmic-ray positron excess

It has long been suggested that nearby pulsars within $\sim 1 \,{\rm kpc}$ are the leading candidate of the 10-500 GeV cosmic-ray positron excess measured by PAMELA and other experiments. The recent measurement of surface brightness profile of TeV nebulae surrounding Geminga and PSR~B0656+14 by the High-Altitude Water Cherenkov Observatory (HAWC) suggests inefficient diffusion of particles from the sources, giving rise to a debate on the pulsar interpretation of the cosmic-ray positron excess. Here we argue that GeV observations provide more direct constraints on the positron density in the TeV nebulae in the energy range of 10-500 GeV and hence on the origin of the observed positron excess. Motivated by this, we search for GeV emission from the TeV nebulae with the \textsl{Fermi} Large Area Telescope (LAT). No spatially-extended GeV emission is detected from these two TeV nebulae in the framework of two-zone diffusion spatial templates, suggesting a relatively low density of GeV electrons/positrons in the TeV nebulae. A joint modelling of the data from HAWC and \textsl{Fermi}-LAT disfavors Geminga and PSR~B0656+14 as the dominant source of the positron excess at $\sim 50-500$ GeV for the usual Kolmogorov-type diffusion, while for an energy-independent diffusion, a dominant part of the positron excess contributed by them cannot be ruled out by the current data.

astro-ph.HE↗

arXiv2018

Kilonova emission from black hole-neutron star mergers: observational signatures of anisotropic mass ejection

The gravitational wave event GW170817 associated with the short gamma-ray burst (GRB) 170817A confirms that binary neutron star (BNS) mergers are one of the origins of short GRBs. The associated kilonova emission, radioactively powered by nucleosynthesized heavy elements, was also detected. Black hole-neutron star (BH-NS) mergers have been argued to be another promising origin candidate of short GRBs and kilonovae. Numerical simulations show that the ejecta in BH-NS mergers is geometrically much more anisotropic than the BNS merger case. In this paper, we investigate observational signatures of kilonova emission from the anisotropic ejecta in BH-NS mergers. We find that a bump appears on the bolometric luminosity light curve due to the inhomogeneous mass distribution in the latitudinal direction. The decay slope of the single-band light curve becomes flatter and the spectrum also deviates from a single-temperature blackbody radiation spectrum due to the gradient in the velocity distribution of the ejecta. Future detection or non-detection of such signatures would be useful to test the mass ejection geometry in BH-NS mergers.

astro-ph.HE↗

arXiv2018

Inefficient cosmic ray diffusion around Vela X : constraints from H.E.S.S. observations of very high-energy electrons

Vela X is a nearby pulsar wind nebula (PWN) powered by a $\sim 10^4$ year old pulsar. Modeling of the spectral energy distribution of the Vela X PWN has shown that accelerated electrons have largely escaped from the confinement, which is likely due to the disruption of the initially confined PWN by the SNR reverse shock. The escaped electrons propagate to the earth and contribute to the measured local cosmic-ray (CR) electron spectrum. We find that the escaped CR electrons from Vela X would hugely exceed the measured flux by HESS at $\sim 10$ TeV if the standard diffusion coefficient for the interstellar medium is used. We propose that the diffusion may be highly inefficient around Vela X and find that a spatially-dependent diffusion can lead to CR flux consistent with the HESS measurement. Using a two-zone model for the diffusion around Vela X, we find that the diffusion coefficient in the inner region of a few tens of parsecs should be $<10^{28}{\rm cm^2 s^{-1}}$ for $\sim10$ TeV CR electrons, which is about two orders of magnitude lower than the standard value for ISM. Such inefficient diffusion around PWN resembles the case of the Geminga and Monogem PWNe, suggesting that inefficient diffusion may be common in the vicinity of PWNe spanning a wide range of ages.

astro-ph.HE↗

arXiv2018

Early soft X-ray to UV emission from double neutron star mergers: implications from the long-term radio and X-ray emissions of GW 170817

Recent long-term radio follow-up observations of GW 170817 reveals a simple power-law rising light curve, with a slope of $t^{0.78}$, up to 93 days after the merger. The latest X-ray detection at 109 days is also consistent with such a temporal slope. Such a shallow rise behavior requires a mildly relativistic outflow with a steep velocity gradient profile, so that slower material with larger energy catches up with the decelerating ejecta and re-energizes it. It has been suggested that this mildly relativistic outflow may represent a cocoon of material. We suggest that the velocity gradient profile may form during the stage that the cocoon is breaking out of the merger ejecta, resulted from shock propagation down a density gradient. The cooling of the hot relativistic cocoon material immediately after it breaks out should have produced soft X-ray to UV radiation at tens of seconds to hours after the merger. The soft X-ray emission has a luminosity of $L_{\rm X}\sim 10^{45}{\rm erg s^{-1}}$ over a period of tens of seconds for a merger event like GW 170817. The UV emission shows a rise initially and peaks at about a few hours with a luminosity of $L_{\rm UV}\sim 10^{42} {\rm erg s^{-1}}$. The soft X-ray transients could be detected by future wide-angle X-ray detectors, such as the Chinese mission Einstein Probe. This soft X-ray/UV emission would serve as one of the earliest electromagnetic counterparts of gravitation waves from double neutron star mergers and could provide the earliest localization of the sources.

astro-ph.HE↗