arXiv subjects

Xiaohong Zhao

Publications and source records attributed to Xiaohong Zhao.

8 recordsLinked to original sources

arXiv2025

Synchrotron Circular Polarization in Gamma-Ray Burst Prompt Optical Emission: Relativistic Thermal Electron Contribution

Synchrotron circular polarization of a non-thermal power-law electron distribution in gamma-ray bursts (GRBs) has been studied. However, some numerical simulations have shown that the resulting distribution of electrons is a combination of a thermal component and a non-thermal power-law component. In this paper, we investigate synchrotron circular polarization using such a hybrid energy distribution of relativistic thermal and nonthermal electrons within a globally toroidal magnetic field in GRB prompt optical emission. Our results show that compared to the solely nonthermal electron model, the synchrotron circular polarization degree (PD) in the hybrid electron model can vary widely in the optical band, depending on different parameters. The lower the electron temperature, the higher the circular PD. The time-averaged circular PD in the hybrid electron model can be higher than $\sim 1\%$ when the electron temperature is as low as $\sim 10^{10}$ K, while in the solely nonthermal electron model is usually lower than $\sim 1\%$. We further calculate the radiative transfer of the circular and linear polarization in the optical band. Our results show that both of the circular and linear PDs decrease with the increase of optical depth, but the linear PDs decline faster than the circular PDs. To further examine the physical mechanisms of both radiation and particle acceleration, we expect that instruments will be capable of measuring the circular polarization of GRB prompt optical emission in the future.

astro-ph.HE↗

arXiv2024

Synchrotron polarization of a hybrid distribution of relativistic thermal and nonthermal electrons in GRB prompt emission

Synchrotron polarization of relativistic nonthermal electrons in gamma-ray bursts (GRBs) has been widely studied. However, recent numerical simulations of relativistic shocks and magnetic reconnection have found that a more realistic electron distribution consists of a power-law component plus a thermal component, which requires observational validation. In this paper, we investigate synchrotron polarization using a hybrid energy distribution of relativistic thermal and nonthermal electrons within a globally toroidal magnetic field in GRB prompt emission. Our results show that, compared to the case of solely non-thermal electrons, the synchrotron polarization degrees (PDs) in these hybrid electrons can vary widely depending on different parameters and that the PD decreases progressively with frequency in the $γ$-ray, X-ray, and optical bands. The time-averaged PD spectrum displays a significant bump in the $γ$-ray and X-ray bands with the PDs higher than $\sim60\%$ if the thermal peak energy of electrons is much smaller than the conjunctive energy of electrons between the thermal and non-thermal distribution. The high synchrotron PD ($\gtrsim 60\%$) in the $γ$-ray and X-ray bands, which generally can not be produced by solely non-thermal electrons with typical power-law slopes, can be achieved by the hybrid electrons and primarily originates from the exponential decay part of the thermal component. Moreover, this model can roughly explain the PDs and spectral properties of some GRBs, where GRB 110301A with a high PD ($70_{-22}^{+22} \%$) may be potential evidence for the existence of relativistic thermal electrons.

astro-ph.HE↗

arXiv2024

The polarization-angle flip in GRB prompt emission

Aims. We aim to provide an explanation for the PA rotation in GRBs and find the physical conditions that lead to the rotation by 90 degrees in the toroidal magnetic-field (MF) model. Moreover, we present some observable polarization properties in the MF model that can be tested in the future. Results. We find that the PA rotation in the toroidal MF is primarily related to three critical factors: the viewing angle, the jet opening angle, and the jet Lorentz factor. Additionally, the PA can experience flips of 90 degrees twice. The conditions for the flips are $q \gtrsim 0.5$ (except for $q\simeq 1$) and $y_j =(Γθ_j)^2 \gtrsim 4$. However, the two flips in the PA might not be concurrently observable due to the constraint of flux. Taking these conditions into account and assuming a random orientation between the jet axis and the line of sight (LOS), we obtain a theoretical upper limit (without any constraints) for the observed rate of GRBs in the X-ray or $γ$-ray band displaying the flips in PA as $R_{ch} \lesssim 80\%$. We further constrain the observed rate as $R_{ch} \sim 16\%$ according to the maximal post-flip polarized flux level, where the observed rate of single and double flips each account for $\sim 8\%$. It should be noted that the observed rates are different in various wavebands. The observed rate of the second PA flip in the optical bands should be higher than that in the X-ray or $γ$-ray band since the flux in the optical band declines much slower than that in the X-ray or $γ$-ray band. Moreover, when the LOS is close to the jet edge ($q\to 1$), it is the easiest case in which to observe the 90-degree PA flip due to the relatively high post-flip polarized flux level.

astro-ph.HE↗

arXiv2020

Secondary-electron radiation accompanying hadronic GeV-TeV gamma-rays from supernova remnants

The synchrotron radiation from secondary electrons and positrons (SEPs) generated by hadronic interactions in the shock of supernova remnant (SNR) could be a distinct evidence of cosmic ray (CR) production in SNR shocks. Here we provide a method where the observed gamma-ray flux from SNRs, created by pion decays, is directly used to derive the SEP distribution and hence the synchrotron spectrum. We apply the method to three gamma-ray bright SNRs. In the young SNR RX J1713.7-3946, if the observed GeV-TeV gamma-rays are of hadronic origin and the magnetic field in the SNR shock is $B\gtrsim 0.5$mG, the SEPs may produce a spectral bump at $10^{-5}-10^{-2}$eV, exceeding the predicted synchrotron component of the leptonic model, and a soft spectral tail at $\gtrsim 100$keV, distinct from the hard spectral slope in the leptonic model. In the middle-aged SNRs IC443 and W44, if the observed gamma-rays are of hadronic origin, the SEP synchrotron radiation with $B\sim 400 - 500 μ$G can well account for the observed radio flux and spectral slopes, supporting the hadronic origin of gamma-rays. Future microwave to far-infrared and hard X-ray (>100keV) observations are encouraged to constraining the SEP radiation and the gamma-ray origin in SNRs.

astro-ph.HE↗

arXiv2019

Security analysis of a self-embedding fragile image watermark scheme

Recently, a self-embedding fragile watermark scheme based on reference-bits interleaving and adaptive selection of embedding mode was proposed. Reference bits are derived from the scrambled MSB bits of a cover image, and then are combined with authentication bits to form the watermark bits for LSB embedding. We find this algorithm has a feature of block independence of embedding watermark such that it is vulnerable to a collage attack. In addition, because the generation of authentication bits via hash function operations is not related to secret keys, we analyze this algorithm by a multiple stego-image attack. We find that the cost of obtaining all the permutation relations of $l\cdot b^2$ watermark bits of each block (i.e., equivalent permutation keys) is about $(l\cdot b^2)!$ for the embedding mode $(m, l)$, where $m$ MSB layers of a cover image are used for generating reference bits and $l$ LSB layers for embedding watermark, and $b\times b$ is the size of image block. The simulation results and the statistical results demonstrate our analysis is effective.

cs.CR↗

arXiv2018

Differential cryptanalysis of image cipher using block-based scrambling and image filtering

Recently, an image encryption algorithm using block-based scrambling and image filtering has been proposed by Hua et al. In this paper, we analyze the security problems of the encryption algorithm in detail and break the encryption by a codebook attack. We construct an linear relation between plain-images and cipher-images by differential cryptanalysis. With this linear relation, we build a codebook containing $(M \times N + 1)$ pairs of plain-images and cipher-images, where $M\times N$ is the size of images. The proposed codebook attack indicates that the encryption scheme is insecure. To resist the codebook attack, an improved algorithm is proposed. Experimental results show that the improved algorithm not only inherits the merits of the original scheme, but also has stronger security against the differential cryptanalysis.

cs.CR↗

arXiv2010

The bulk Lorentz factors of Fermi-LAT GRBs

The Lorentz factor (LF) of gamma-ray burst (GRB) ejecta may be constrained by observations of high-energy (HE) spectral attenuation. The recent Fermi-LAT observations of prompt GeV emission from several bright GRBs have leaded to conclusions of unexpectedly large LFs, $Γ>10^3$. Here we revisit this problem with two main concerns. (1) With one-zone assumption where all photons are assumed to be generated in the same region (radius) and time, we {\em self-consistently} calculate the $γγ$ optical depth by adopting a target photon spectrum with HE cutoff. We find that this might be important when the GRB LF is below a few hundreds. (2) Recent Fermi-LAT observations suggest that the bulk MeV-range and HE ($\ga100$ MeV) emission may arise from different regions. We then consider a two-zone case where HE emission is generated in much larger radii than that of the MeV-range emission. We find that the HE emission may be mainly attenuated by MeV-range emission and that the attenuated HE spectrum does not show an exponential spectral cutoff but a slight steepening. This suggests that there may be no abrupt cutoff due to $γγ$ attenuation if relaxing the one-zone assumption. By studying the spectra of three bright Fermi-LAT GRBs 080916C, 090510 and 090902B, we show that a bulk LF of\textbf{ $Γ\sim600$ }can be consistent with observations in the two-zone case. Even lower LFs can be obtained in the multi-zone case.

astro-ph.HE↗

arXiv2006

Jet Sideways Expansion Effect on Estimating the Gamma-Ray Burst Efficiency

The high efficiency of converting kinetic energy into gamma-rays estimated with late-time afterglows in Gamma-Ray Burst (GRB) phenomenon challenges the commonly accepted internal-shock model. However, the efficiency is still highly uncertain because it is sensitive to many effects. In this Letter we study the sideways expansion effect of jets on estimating the efficiency. We find that this effect is considerable, reducing the efficiency by a factor of $\sim0.5$ for typical parameters, when the afterglow data $\sim 10$ hr after the GRB trigger are used to derive the kinetic energy. For a more dense circumburst medium, this effect is more significant. As samples, taking this effect into account, we specifically calculate the efficiency of two bursts whose parameters were well constrained. Almost the same results are derived. This suggests that the sideways expansion effect should be considered when the GRB efficiency is estimated with the late afterglow data.

astro-ph↗