arXiv subjects

Bing Zhang

Publications and source records attributed to Bing Zhang.

At least 343 records · Page 19Linked to original sources

arXiv2018

Toward an understanding of GRB prompt emission mechanism: II. Patterns of peak energy evolution and their connection to spectral lags

The prompt emission phase of gamma-ray bursts (GRBs) exhibits two distinct patterns of the peak-energy ($E_p$) evolution, i.e., time-resolved spectral analyses of $νF_ν$ spectra of broad pulses reveal (1) "hard-to-soft" and (2) "flux-tracking" patterns of $E_p$ evolution in time, the physical origin of which still remains not well understood. We show here that these two patterns can be successfully reproduced within a simple physical model invoking synchrotron radiation in a bulk-accelerating emission region. We show further that the evolution patterns of the peak energy have, in fact, direct connections to the existence of two different (positive or negative) types of spectral lags, seen in the broad pulses. In particular, we predict that (1) only the positive type of spectral lags is possible for the hard-to-soft evolution of the peak energy, (2) both the positive and negative type of spectral lags can occur in the case of flux-tracking pattern of the peak energy, (3) for the flux-tracking pattern, the peak location of the flux light curve slightly lags behind the peak of the $E_p$ evolution with time if the spectral lags are positive, and (4) in the case of flux-tracking pattern, double-peaked broad pulses can appear in the light curves, the shape of which is energy-dependent.

astro-ph.HE↗

arXiv2018

FRB Energetics and detectability from high redshifts

We estimate the upper limit redshifts of known FRBs using the dispersion measure (DM) - redshift ($z$) relation and derive the upper limit peak luminosity $L_p$ and energy $E$ of FRBs within the observational band. The average $z$ upper limits range from 0.17 to 3.10, the average $L_p$ upper limits range from $1.24 \times 10^{42} \rm erg \ s^{-1}$ to $7.80 \times 10^{44} \rm erg \ s^{-1}$, and the average $E$ upper limits range from $6.91 \times 10^{39}$ erg to $1.94 \times 10^{42}$ erg. FRB 160102 with DM $=2596.1 \pm 0.3 \ {\rm pc \ cm^{-3}}$ likely has a redshift greater than 3. Assuming that its intrinsic DM contribution from the host and FRB source is ${\rm DM_{host}+DM_{scr}}\sim 100 \ {\rm pc \ cm^{-3}}$, such an FRB can be detected up to $z \sim 3.6$ by Parkes and by FAST under ideal conditions up to $z \sim 10.4$. Assuming that there exist FRBs detectable at $z\sim 15$ by sensitive telescopes such as FAST, the upper limit DM for FRB searches may be set to $\sim 9000 \ {\rm pc \ cm^{-3}}$. For single-dish telescopes, those with a larger aperture tend to detect more FRBs than those with a smaller aperture if the FRB luminosity function index $α_{\rm L}$ is steeper than 2, and vice versa. In any case, large-aperture telescopes such as FAST are more capable of detecting high-$z$ FRBs, even though most of FRBs detected by them are still from relatively low redshifts.

astro-ph.HE↗

arXiv2018

Strongly lensed repeating Fast Radio Bursts as precision probes of the universe

Fast Radio bursts (FRBs), bright transients with millisecond durations at $\sim$ GHz and typical redshifts probably $>0.8$, are likely to be gravitationally lensed by intervening galaxies. Since the time delay between images of strongly lensed FRB can be measured to extremely high precision because of the large ratio $\sim10^9$ between the typical galaxy-lensing delay time $\sim\mathcal{O}$(10 days) and the width of bursts $\sim\mathcal{O}$(ms), we propose strongly lensed FRBs as precision probes of the universe. We show that, within the flat $Λ$CDM model, the Hubble constant $H_0$ can be constrained with a $\sim0.91\%$ uncertainty from 10 such systems probably observed with the Square Kilometer Array (SKA) in $<$ 30 years. More importantly, the cosmic curvature can be model-independently constrained to a precision of $\sim0.076$. This constraint can directly test the validity of the cosmological principle and break the intractable degeneracy between the cosmic curvature and dark energy.

astro-ph.CO↗

arXiv2018

Bunching Coherent Curvature Radiation in Three-Dimensional Magnetic Field Geometry: Application to Pulsars and Fast Radio Bursts

The extremely high brightness temperatures of pulsars and fast radio bursts (FRBs) require their radiation mechanisms to be coherent. Coherent curvature radiation by bunches has been long discussed as the mechanism for radio pulsars and recently for FRBs. Assuming that bunches are already generated in pulsar magnetospheres, we calculate the spectrum of coherent curvature radiation under a three-dimensional magnetic field geometry. Different from previous works assuming parallel trajectories and a mono-energetic energy distribution of electrons, we consider a bunch characterized by its length, curvature radius of the trajectory family, bunch opening angle, and electron energy distribution. We find that the curvature radiation spectra of the bunches are characterized by a multi-segment broken power law, with the break frequencies depending on bunch properties and trajectory configuration. We also emphasize that in a pulsar magnetosphere only the fluctuation of net charges with respect to the background (Goldreich-Julian) outflow can make a contribution to coherent radiation. We apply this model to constrain the observed spectra of pulsars and FRBs. For a typical pulsar ($B_p=10^{12}~{\rm G}$, $P=0.1~{\rm s}$), a small fluctuation of the net charge $δn_{\rm GJ}\sim 0.1 n_{\rm GJ}$ can provide the observable flux. For FRBs, the fluctuating net charge may be larger due to its abrupt nature. For $δn_{\rm GJ}\sim n_{\rm GJ}$, a neutron star with a strong magnetic field and fast rotation is required to power an FRB in the spindown-powered model. The requirement is less stringent in the cosmic comb model thanks to the larger cross section and compressed charge density of the bunch made by the external astrophysical stream that combs the magnetosphere.

astro-ph.HE↗

arXiv2018

Long-lived Remnants from Binary Neutron Star Mergers

Massive neutron star (NS) with lifetimes of at least several seconds are expected to be the result of a sizable fraction of NS mergers. We study their formation using a large set of numerical relativity simulations. We show that they are initially endowed with angular momentum that significantly exceeds the mass-shedding limit for rigidly-rotating equilibria. We find that gravitational-wave (GW) emission is not able to remove this excess angular momentum within the time over which solid body rotation should be achieved. Instead, we argue that the excess angular momentum could be carried away by massive winds. Long-lived merger remnants are also formed with larger gravitational masses than those of rigidly-rotating NSs having the same number of baryons. The excess mass is likely radiated in the form of neutrinos. The evolution of long-lived remnants on the viscous timescale is thus determined by the interplay of finite-temperature effects, mass ejection, and neutrinos with potentially dramatic consequences for the remnants' properties and stability. We also provide an empirical fit for the spin of the remnant at the end of its viscous evolution as a function of its final mass, and we discuss the implications for the magnetar model of short gamma-ray bursts (SGRBs). Finally, we investigate the possible electromagnetic signatures associated with the viscous ejecta. Massive outflows possibly resulting from the formation of long-lived remnants would power unusually bright, blue kilonova counterparts to GW events and SGRBs whose detection would provide smoking gun evidence for the formation of long-lived remnants.

astro-ph.HE↗

arXiv2018

Diffusion-controlled Alloying of Single-phase Multi-principal Covalent Transition Metal Carbides with Enhanced Damage tolerance and Exceptional Thermal Properties

Multicomponent alloying has displayed extraordinary potential for producing exceptional structural and functional materials. However, the synthesis of single-phase, multi-principal covalent compounds remains a challenge. Here we present a diffusion-controlled alloying strategy for the successful realization of covalent multi-principal transition metal carbides (MPTMCs) with a single face-centered cubic (FCC) phase. The increased interfacial diffusion promoted by the addition of a nonstoichiometric compound leads to rapid formation of the new single phase at much lower sintering temperature. Direct atomic-level observations via scanning transmission electron microscopy demonstrate that MPTMCs are composed of a single phase with a random distribution of all cations, which holds the key to the unique combinations of improved fracture toughness, superior Vickers hardness, and extremely lower thermal diffusivity achieved in MPTMCs. The present discovery provides a promising approach toward the design and synthesis of next-generation high-performance materials.

cond-mat.mtrl-sci↗

arXiv2018

On the normalised FRB luminosity function

Thirty-three fast radio bursts (FRBs) had been detected by March 2018. Although the sample size is still limited, meaningful statistical studies can already be carried out. The normalised luminosity function places important constraints on the intrinsic power output, sheds light on the origin(s) of FRBs, and can guide future observations. In this paper, we measure the normalised luminosity function of FRBs. Using Bayesian statistics, we can naturally account for a variety of factors such as receiver noise temperature, bandwidth, and source selection criteria. We can also include astronomical systematics, such as host galaxy dispersion measure, FRB local dispersion measure, galaxy evolution, geometric projection effects, and Galactic halo contribution. Assuming a Schechter luminosity function, we show that the isotropic luminosities of FRBs have a power-law distribution that covers approximately three orders of magnitude, with a power-law index ranging from $-1.8$ to $-1.2$ and a cut off $\sim 2\times 10^{44}\,\rm erg\,s^{-1}$. By using different galaxy models and well-established Bayesian marginalisation techniques, we show that our conclusions are robust against unknowns, such as the electron densities in the Milky Way halo and the FRB environment, host galaxy morphology, and telescope beam response.

astro-ph.HE↗

arXiv2018

Multi-color blackbody emission in GRB 081221

The radiation mechanism of the prompt emission of gamma-ray bursts (GRBs) remains an open question. Although their spectra are usually well fitted with the empirical Band function, which is widely believed to be fully non-thermal and interpreted as an optically thin synchrotron emission, accumulating evidence shows that a thermal component actually exists. In this paper, a multi-color blackbody (mBB) model is proposed for the time-integrated spectrum of GRB 081221 by assuming a power-law distribution of the thermal luminosities with temperature, which manifests photospheric emissions from a different radius and/or angle. The effects of the minimum temperature $kT_{\min }$, the maximum temperature $kT_{\max }$ and the power law index $m$ of the luminosity distribution of an mBB are discussed. The fitting to the time-integrated spectrum during the bright phase (from 20s to 30s since the trigger) of GRB 081221 by the mBB model yields $kT_{\min }$ = $4.4\pm 0.3$ keV, $kT_{\max }$ = 57.0 $_{-1.4}^{+1.6}$ keV, and $m=-0.46_{-0.06}^{+0.05}$. When the time bin is small enough, the time-resolved spectra of GRB 081221 are well fitted with a series of single-temperature blackbodies. Our results imply the prompt emission of GRB 081221 is dominated by the photosphere emission and its time-integrated spectrum is a superposition of pure blackbody components at different times, indicating that some empirical Band spectra may be interpreted as mBB if the temperature is widely distributed.

astro-ph.HE↗

arXiv2018

Synchrotron radiation from electrons with a pitch-angle distribution

In most astrophysical processes involving synchrotron radiation, the pitch-angle distribution of the electrons is assumed to be isotropic. However, if electrons are accelerated anisotropically, e.g, in a relativistic shock wave with an ordered magnetic field or in magnetic reconnection regions, the electron pitch angles might be anisotropic. In this work, we study synchrotron radiation from electrons with a pitch-angle distribution with respect to a large-scale uniform magnetic field. Assuming that the pitch-angle distribution is normal with a scatter of $σ_p$ and that the viewing direction is where the pitch-angle direction peaks, we find that for electrons with a Lorentz factor $γ$, the observed flux satisfies $F_ν\proptoν^{2/3}$ for $ν\llν_{\rm cr}$ ($ν_{\rm cr}$ is the critical frequency of synchrotron), if $σ_p\lesssim1/γ$ is satisfied. On the other hand, if $σ_p\gg1/γ$, the spectrum below $ν_{\rm cr}$ is a broken power law with a break frequency $ν_{\rm br}\sim2ν_{\rm cr}/σ_p^3γ^3$, e.g., $F_ν\proptoν^{2/3}$ for $ν\llν_{\rm br}$ and $F_ν\proptoν^{1/3}$ for $ν_{\rm br}\llν\llν_{\rm cr}$. Thus the ultimate synchrotron line of death is $F_ν\proptoν^{2/3}$. We discuss the application of this theory to blazars and gamma-ray bursts (GRBs).

astro-ph.HE↗

arXiv2018

A comprehensive analysis of Fermi Gamma-Ray Burst Data: IV. Spectral lag and Its Relation to Ep Evolution

The spectral evolution and spectral lag behavior of 92 bright pulses from 84 gamma-ray bursts (GRBs) observed by the Fermi GBM telescope are studied. These pulses can be classified into hard-to-soft pulses (H2S, 64/92), H2S-dominated-tracking pulses (21/92), and other tracking pulses (7/92). We focus on the relationship between spectral evolution and spectral lags of H2S and H2S-dominated-tracking pulses. %in hard-to-soft pulses (H2S, 64/92) and H2S-dominating-tracking (21/92) pulses. The main trend of spectral evolution (lag behavior) is estimated with $\log E_p\propto k_E\log(t+t_0)$ (${\hatτ} \propto k_{\hatτ}\log E$), where $E_p$ is the peak photon energy in the radiation spectrum, $t+t_0$ is the observer time relative to the beginning of pulse $-t_0$, and ${\hatτ}$ is the spectral lag of photons with energy $E$ with respect to the energy band $8$-$25$ keV. For H2S and H2S-dominated-tracking pulses, a weak correlation between $k_{\hatτ}/W$ and $k_E$ is found, where $W$ is the pulse width. We also study the spectral lag behavior with peak time $t_{\rm p_E}$ of pulses for 30 well-shaped pulses and estimate the main trend of the spectral lag behavior with $\log t_{\rm p_E}\propto k_{t_p}\log E$. It is found that $k_{t_p}$ is correlated with $k_E$. We perform simulations under a phenomenological model of spectral evolution, and find that these correlations are reproduced. We then conclude that spectral lags are closely related to spectral evolution within the pulse. The most natural explanation of these observations is that the emission is from the electrons in the same fluid unit at an emission site moving away from the central engine, as expected in the models invoking magnetic dissipation in a moderately-high-$σ$ outflow.

astro-ph.HE↗

arXiv2018

Learning to score the figure skating sports videos

This paper targets at learning to score the figure skating sports videos. To address this task, we propose a deep architecture that includes two complementary components, i.e., Self-Attentive LSTM and Multi-scale Convolutional Skip LSTM. These two components can efficiently learn the local and global sequential information in each video. Furthermore, we present a large-scale figure skating sports video dataset -- FisV dataset. This dataset includes 500 figure skating videos with the average length of 2 minutes and 50 seconds. Each video is annotated by two scores of nine different referees, i.e., Total Element Score(TES) and Total Program Component Score (PCS). Our proposed model is validated on FisV and MIT-skate datasets. The experimental results show the effectiveness of our models in learning to score the figure skating videos.

cs.MM↗

arXiv2018

On the non-detection of Glashow resonance in IceCube

Electron anti-neutrinos at the Glashow resonance (GR, at $E_{\bar ν_e} \sim 6.3$ PeV) have an enhanced probability to be detected. With three neutrinos detected by IceCube in the (1-2) PeV energy range at present, one would expect that about 1 to 4 GR $\barν_e$ should have been detected. The high-energy $\sim 8.7$ PeV muon neutrino detected by IceCube may not be a GR event. If so, we expect to detect 50 to 70 GR $\barν_e$, then one would have a "missing Glashow-resonance problem". This would suggest (1) that $pγ$ interaction rather than $pp$ interaction is the dominant channel to produce the observed IceCube high-energy neutrinos, (2) that multi-pion $pγ$ interactions are suppressed, and (3) that the magnetic field and photon energy density in the $pγ$ emission region is such that significant $μ^+$ cooling occurs before decaying, yet $π^+$'s essentially do not cool before decaying.

hep-ph↗

arXiv2018

Gamma-Ray Burst/Supernova Associations: Energy partition and the case of a magnetar central engine

The favored progenitor model for Gamma-ray Bursts (GRBs) with Supernova (SN) association is the core collapse of massive stars. One possible outcome of such a collapse is a rapidly spinning, strongly magnetized neutron star ("magnetar"). We systematically analyze the multi-wavelength data of GRB/SN associations detected by several instruments before 2017 June. Twenty GRB/SN systems have been confirmed via direct spectroscopic evidence or a clear light curve bump, as well as some spectroscopic evidence resembling a GRB-SN. We derive/collect the basic physical parameters of the GRBs and the SNe, and look for correlations among these parameters. We find that the peak brightness, $\rm ^{56}Ni$ mass, and explosion energy of SNe associated with GRBs are statistically higher than other Type Ib/c SNe. A statistically significant relation between the peak energy of GRBs and the peak brightness of their associated SNe is confirmed. No significant correlations are found between the GRB energies (either isotropic or beaming-corrected) and the supernova energy. We investigate the energy partition within these systems and find that the beaming-corrected GRB energy of most systems is smaller than the SN energy, with less than 30\% of the total energy distributed in the relativistic jet. The total energy of the systems is typically smaller than the maximum available energy of a millisecond magnetar ($2\times 10^{52}$ erg), especially if aspherical SN explosions are considered. The data are consistent with-though not a proof of-the hypothesis that most, but not all, GRB/SN systems are powered by millisecond magnetars.

astro-ph.HE↗

arXiv2018

What Powered the Optical Transient AT2017gfo Associated with GW170817?

The groundbreaking discovery of the optical transient AT2017gfo associated with GW170817 opens a unique opportunity to study the physics of double neutron star (NS) mergers. We argue that the standard interpretation of AT2017gfo as being powered by radioactive decays of r-process elements faces the challenge of simultaneously accounting for the peak luminosity and peak time of the event, as it is not easy to achieve the required high mass, and especially the low opacity of the ejecta required to fit the data. A plausible solution would be to invoke an additional energy source, which is probably provided by the merger product. We consider energy injection from two types of the merger products: (1) a post-merger black hole powered by fallback accretion; and (2) a long-lived NS remnant. The former case can only account for the early emission of AT2017gfo, with the late emission still powered by radioactive decay. In the latter case, both early- and late-emission components can be well interpreted as due to energy injection from a spinning-down NS, with the required mass and opacity of the ejecta components well consistent with known numerical simulation results. We suggest that there is a strong indication that the merger product of GW170817 is a long-lived (supramassive or even permanently stable), low magnetic field NS. The result provides a stringent constraint on the equations of state of NSs.

astro-ph.HE↗

arXiv2018

The Origin of the Prompt Emission for Short GRB 170817A: Photosphere Emission or Synchrotron Emission?

The first gravitational-wave event from the merger of a binary neutron star system (GW170817) was detected recently. The associated short gamma-ray burst (GRB 170817A) has a low isotropic luminosity ($\sim 10^{47}$ erg s$ ^{-1}$) and a peak energy $E_{p}\sim 145$ keV during the initial main emission between -0.3 and 0.4 s. The origin of this short GRB is still under debate, but a plausible interpretation is that it is due to the off-axis emission from a structured jet. We consider two possibilities. First, since the best-fit spectral model for the main pulse of GRB 170817A is a cutoff power law with a hard low-energy photon index ($α=-0.62_{-0.54}^{+0.49} $), we consider an off-axis photosphere model. We develop a theory of photosphere emission in a structured jet and find that such a model can reproduce a low-energy photon index that is softer than a blackbody through enhancing high-latitude emission. The model can naturally account for the observed spectrum. The best-fit Lorentz factor along the line of sight is $\sim 20$, which demands that there is a significant delay between the merger and jet launching. Alternatively, we consider that the emission is produced via synchrotron radiation in an optically thin region in an expanding jet with decreasing magnetic fields. This model does not require a delay of jet launching but demands a larger bulk Lorentz factor along the line of sight. We perform Markov Chain Monte Carlo fitting to the data within the framework of both models and obtain good fitting results in both cases.

astro-ph.HE↗

arXiv2018

GRB120729A: external shock origin for both the prompt gamma-ray emission and afterglow

Gamma-ray burst (GRB) 120729A was detected by Swift/BAT and Fermi/GBM, and then rapidly observed by Swift/XRT, Swift/UVOT, and ground-based telescopes. It had a single long and smooth γ-ray emission pulse, which extends continuously to the X-rays. We report Lick/KAIT observations of the source, and make temporal and spectral joint fits of the multiwavelength light curves of GRB 120729A. It exhibits achromatic light-curve behavior, consistent with the predictions of the external shock model. The light curves are decomposed into four typical phases: onset bump (Phase I), normal decay (Phase II), shallow decay (Phase III), and post-jet break (Phase IV). The spectral energy distribution (SED) evolves from prompt γ-ray emission to the afterglow with photon index from $Γ_{\rm γ}=1.36$ to $Γ\approx 1.75$. There is no obvious evolution of the SED during the afterglow. The multiwavelength light curves from γ-ray to optical can be well modeled with an external shock by considering energy injection, and a time-dependent microphysics model with $ε_B\propto t^{α_B}$ for the emission at early times, $T < T_{\rm 0} + 157$~s. Therefore, we conclude that both the prompt γ-ray emission and afterglow of GRB 120729A have the same external shock physical origin. Our model indicates that the $ε_B$ evolution can be described as a broken power-law function with $α_{\rm B,1} = 0.18 \pm 0.04$ and $α_{\rm B,2} = 0.84 \pm 0.04$. We also systematically investigate single-pulse GRBs in the Swift era, finding that only a small fraction of GRBs (GRBs 120729A, 051111, and 070318) are likely to originate from an external shock for both the prompt γ-ray emission and afterglow.

astro-ph.HE↗

arXiv2018

Gamma-ray burst jet breaks revisited

Gamma-ray Burst (GRB) collimation has been inferred with the observations of achromatic steepening in GRB light curves, known as jet breaks. Identifying a jet break from a GRB afterglow lightcurve allows a measurement of the jet opening angle and true energetics of GRBs. In this paper, we reinvestigate this problem using a large sample of GRBs that have an optical jet break which is consistent with being achromatic in the X-ray band. Our sample includes 99 GRBs from February 1997 to March 2015 that have optical and, for Swift GRBs, X-ray lightcurves that are consistent with the jet break interpretation. Out of 99 GRBs we have studied, 55 GRBs are found to have temporal and spectral behaviors both before and after the break consistent with the theoretical predictions of the jet break models, respectively. These include 53 long/soft (Type II) and 2 short/hard (Type I) GRBs. Only 1 GRB is classified as the candidate of a jet break with energy injection. Another 41 and 3 GRBs are classified as the candidates with the lower and upper limits of the jet break time, respectively. The typical beaming correction factor $f_b^{-1} \sim 1000$ for Type II GRBs, suggesting an even higher total GRB event rate density in the universe. Both isotropic and jet-corrected energies have a wide span in their distributions. We also investigate several empirical correlations (Amati, Frail, Ghirlanda and Liang-Zhang) previously discussed in the literature. We find that in general most of these relations are less tight than before. The existence of early jet breaks and hence small opening angle jets, which were detected in the {\em Swfit era}, is most likely the source of scatter. If one limits the sample to jet breaks later than $10^4$ s, the Liang-Zhang relation remains tight and the Ghirlanda relation still exists. These relations are derived from Type II GRBs, and Type I GRBs usually deviate from them.

astro-ph.HE↗

arXiv2018

Are there multiple populations of Fast Radio Bursts?

The repeating FRB 121102 (the "repeater") shows {repetitive bursting activities} and was localized in a host galaxy at $z=0.193$. On the other hand, despite dozens of hours of telescope time spent on follow-up observations, no other FRBs have been observed to repeat. Yet, it has been speculated that the repeater is the prototype of FRBs, and that other FRBs should show similar repeating patterns. Using the published data, we compare the repeater with other FRBs in the observed time interval ($Δt$) - flux ratio ($S_i / S_{i+1}$) plane. We find that whereas other FRBs occupy the upper (large $S_i / S_{i+1}$) and right (large $Δt$) regions of the plane due to the non-detections of other bursts, some of the repeater bursts fall into the lower-left region of the plot (short interval and small flux ratio) excluded by the non-detection data of other FRBs. The trend also exists even if one only selects those bursts detectable by the Parkes radio telescope. If other FRBs were similar to the repeater, our simulations suggest that the probability that none of them have been detected to repeat with the current searches would be $\sim (10^{-4}-10^{-3})$. We suggest that the repeater is not representative of the entire FRB population and that there is strong evidence of more than one population of FRBs.

astro-ph.HE↗