arXiv subjects

Bing Zhang

Publications and source records attributed to Bing Zhang.

At least 505 records · Page 28Linked to original sources

arXiv2010

The Internal-Collision-Induced Magnetic Reconnection and Turbulence (ICMART) Model of Gamma-Ray Bursts

The recent Fermi observation of GRB 080916C shows that the bright photosphere emission associated with a putative fireball is missing, which suggests a Poynting-flux-dominated outflow. We propose a model of gamma-ray burst (GRB) prompt emission in the Poynting-flux-dominated regime, namely, the Internal-Collision-induced MAgnetic Reconnection and Turbulence (ICMART) model. It is envisaged that the GRB central engine launches an intermittent, magnetically-dominated wind, and that in the GRB emission region, the ejecta is still moderately magnetized. Similar to the internal shock (IS) model, the mini-shells interact internally at the traditional internal shock radius. Most of these early collision have little energy dissipation, but serve to distort the ordered magnetic field lines. At a certain point, the distortion of magnetic field configuration reaches the critical condition to allow fast reconnection seeds to occur, which induce relativistic MHD turbulence in the interaction regions. The turbulence further distorts field lines easing additional magnetic reconnections, resulting in a runway release of the stored magnetic field energy (an ICMART event). Particles accelerated in the ICMART region radiate synchrotron photons that power the observed gamma-rays. Each ICMART event corresponds to a broad pulse in the GRB lightcurve, and a GRB is composed of multiple ICMART events. This model retains the merits of the IS and other models, but may overcome several difficulties/issues faced by the IS model (e.g. low efficiency, fast cooling, electron number excess, Amati/Yonetoku relation inconsistency, and missing bright photosphere). It predicts two-component variability time scales, and a decreasing Ep and polarization degree during each ICMART event. The model may be applied to most Fermi LAT GRBs that have time-resolved, featureless Band-function spectra (abridged).

astro-ph.HE↗

arXiv2010

Are all short-hard gamma-ray bursts produced from mergers of compact stellar objects?

The origin and progenitors of short-hard gamma-ray bursts remain a puzzle and a highly debated topic. Recent Swift observations suggest that these GRBs may be related to catastrophic explosions in degenerate compact stars, denoted as "Type I" GRBs. The most popular models include the merger of two compact stellar objects (NS-NS or NS-BH). We utilize a Monte Carlo approach to determine whether a merger progenitor model can self-consistently account for all the observations of short-hard GRBs, including a sample with redshift measurements in the Swift era (z-known sample) and the CGRO/BATSE sample. We apply various merger time delay distributions invoked in compact star merger models to derive the redshift distributions of these Type I GRBs, and then constrain the unknown luminosity function of Type I GRBs using the observed luminosity-redshift (L - z) distributions of the z-known sample. The best luminosity function model, together with the adopted merger delay model, are then applied to confront the peak flux distribution (log N - log P distribution) of the BATSE and Swift samples. We find that for all the merger models invoking a range of merger delay time scales (including those invoking a large fraction of "prompt mergers"), it is difficult to reconcile the models with all the data. The data are instead statistically consistent with the following two possible scenarios. First, that short/hard GRBs are a superposition of compact-star-merger origin (Type I) GRBs and a population of GRBs that track the star formation history, which are probably related to the deaths of massive stars (Type II GRBs). Sec- ond, the entire short/hard GRB population is consistent with a typical delay of 2 Gyr with respect to the star formation history with modest scatter. This may point towards a different Type I progenitor than the traditional compact star merger models.

astro-ph.HE↗

arXiv2010

Luminosity Distribution of Gamma-Ray Burst Host Galaxies at redshift z=1 in Cosmological Smoothed Particle Hydrodynamic Simulations: Implications for the Metallicity Dependence of GRBs

We study the relationship between the metallicity of gamma-ray burst (GRB) progenitors and the probability distribution function (PDF) of GRB host galaxies as a function of luminosity using cosmological hydrodynamic simulations of galaxy formation. We impose a maximum limit to the gas metallicity in which GRBs can occur, and examine how the predicted luminosity PDF of GRB host galaxies changes in the simulation. We perform the Kolmogorov-Smirnov test, and show that the result from our simulation agrees with the observed luminosity PDF of core-collapse supernovae (SNe) host galaxies when we assume that the core-collapse SNe trace star formation. When we assume that GRBs occur only in a low-metallicity environment with $Z\lesssim 0.1 \Zsun$, GRBs occur in lower luminosity galaxies, and the simulated luminosity PDF becomes quantitatively consistent with the observed luminosity PDF. The observational bias against the host galaxies of optically dark GRBs owing to dust extinction may be another reason for the lower luminosities of GRB host galaxies, but the observed luminosity PDF of GRB host galaxies cannot be reproduced solely by the dust bias in our simulation.

astro-ph.CO↗

arXiv2010

Magnetic Field Amplification by Turbulence in A Relativistic Shock Propagating through An Inhomogeneous Medium

We perform two-dimensional relativistic magnetohydrodynamic simulations of a mildly relativistic shock propagating through an inhomogeneous medium. We show that the postshock region becomes turbulent owing to preshock density inhomogeneity, and the magnetic field is strongly amplified due to the stretching and folding of field lines in the turbulent velocity field. The amplified magnetic field evolves into a filamentary structure in two-dimensional simulations. The magnetic energy spectrum is flatter than the Kolmogorov spectrum and indicates that a so-called small-scale dynamo is occurring in the postshock region. We also find that the amount of magnetic-field amplification depends on the direction of the mean preshock magnetic field, and the time scale of magnetic-field growth depends on the shock strength.

astro-ph.HE↗

arXiv2010

XRF 100316D/SN 2010bh: clue to the diverse origin of nearby supernova-associated GRBs

X-ray Flash (XRF) 100316D, a nearby super-long under-luminous burst with a peak energy E_p \sim 20 keV, was detected by Swift and was found to be associated with an energetic supernova SN 2010bh. Both the spectral and the temporal behavior of this burst are rather similar to that of XRF 060218, except that the latter was associated with a "less energetic" SN 2006aj and had a prominent soft thermal emission component in the spectrum. We analyze the spectral and temporal properties of this burst, and interpret the prompt gamma-ray emission and the early X-ray plateau emission as synchrotron emission from a dissipating Poynting-flux-dominated outflow, probably powered by a magnetar with a spin period of $P \sim 10$ ms and the polar cap magnetic field $B_{\rm p} \sim 3\times 10^{15}$ G. The energetic supernova SN 2010bh associated with this burst is, however, difficult to interpret within the slow magnetar model, which implies that the nascent magnetar may spin much faster with an initial rotation period $\sim 1$ ms, and thus suggests a delay between the core collapse and the emergence of the relativistic magnetar wind from the star. The diverse behaviors of low-luminosity GRBs and their associated SNe may be understood within a unified picture that invokes different initial powers of the central engine and different delay times between the core collapse and the emergence of the relativistic jet from the star.

astro-ph.HE↗

arXiv2010

A New Classification Method for Gamma-Ray Bursts

Recent Swift observations suggest that the traditional long vs. short GRB classification scheme does not always associate GRBs to the two physically motivated model types, i.e. Type II (massive star origin) vs. Type I (compact star origin). We propose a new phenomenological classification method of GRBs by introducing a new parameter epsilon=E_{gamma, iso,52}/E^{5/3}_{p,z,2}, where E_{γ,iso} is the isotropic gamma-ray energy (in units of 10^{52} erg), and E_{p,z} is the cosmic rest frame spectral peak energy (in units of 100 keV). For those short GRBs with "extended emission", both quantities are defined for the short/hard spike only. With the current complete sample of GRBs with redshift and E_p measurements, the epsilon parameter shows a clear bimodal distribution with a separation at epsilon ~ 0.03. The high-epsilon region encloses the typical long GRBs with high-luminosity, some high-z "rest-frame-short" GRBs (such as GRB 090423 and GRB 080913), as well as some high-z short GRBs (such as GRB 090426). All these GRBs have been claimed to be of the Type II origin based on other observational properties in the literature. All the GRBs that are argued to be of the Type I origin are found to be clustered in the low-epsilon region. They can be separated from some nearby low-luminosity long GRBs (in 3sigma) by an additional T_{90} criterion, i.e. T_{90,z}<~ 5 s in the Swift/BAT band. We suggest that this new classification scheme can better match the physically-motivated Type II/I classification scheme.

astro-ph.HE↗

arXiv2010

$e^\pm$ Excesses in the Cosmic Ray Spectrum and Possible Interpretations

The data collected by ATIC, PPB-BETS, FERMI-LAT and HESS all indicate that there is an electron/positron excess in the cosmic ray energy spectrum above $\sim$ 100 GeV, although different instrumental teams do not agree on the detailed spectral shape. PAMELA also reported a clear excess feature of the positron fraction above several GeV, but no excess in anti-protons. Here we review the observational status and theoretical models of this interesting observational feature. We pay special attention to various physical interpretations proposed in the literature, including modified supernova remnant models for the $e^\pm$ background, new astrophysical sources, and new physics (the dark matter models). We suggest that although most models can make a case to interpret the data, with the current observational constraints the dark matter interpretations, especially those invoking annihilation, require much more exotic assumptions than some astrophysical interpretations. Future observations may present some ``smoking-gun'' observational tests to differentiate among different models and to identify the correct interpretation to the phenomenon.

astro-ph.HE↗

arXiv2010

Probing the Nature of High-z Short GRB 090426 with Its Early Optical and X-ray Afterglows

GRB 090426 is a short duration burst detected by Swift ($T_{90}\sim 1.28$ s in the observer frame, and $T_{90}\sim 0.33$ s in the burst frame at $z=2.609$). Its host galaxy properties and some $γ$-ray related correlations are analogous to those seen in long duration GRBs, which are believed to be of a massive-star origin (so-called Type II GRBs). We present the results of its early optical observations with the 0.8-m TNT telescope at Xinglong observatory, and the 1-m LOAO telescope at Mt. Lemmon Optical Astronomy Observatory in Arizona. Our well-sampled optical afterglow lightcurve covers from $\sim 90$ seconds to $\sim 10^4$ seconds post the GRB trigger. It shows two shallow decay episodes that are likely due to energy injection, which end at $\sim 230$ seconds and $\sim 7100$ seconds, respectively. The decay slopes post the injection phases are consistent with each other ($α\simeq 1.22$). The X-ray afterglow lightcurve appears to trace the optical, although the second energy injection phase was missed due to visibility constraints introduced by the {\em Swift} orbit. The X-ray spectral index is $β_X\sim 1.0$ without temporal evolution. Its decay slope is consistent with the prediction of the forward shock model. Both X-ray and optical emission is consistent with being in the same spectral regime above the cooling frequency ($ν_c$). The fact that $ν_c$ is below the optical band from the very early epoch of the observation provides a constraint on the burst environment, which is similar to that seen in classical long duration GRBs. We therefore suggest that death of a massive star is the possible progenitor of this short burst.

astro-ph.HE↗

arXiv2010

Effect of Resonant Neutrino Oscillation on TeV Neutrino Flavor Ratio from Choked GRBs

In the collapsar scenario of the long duration Gamma-Ray Bursts (GRBs), multi-TeV neutrino emission is predicted as the jet makes its way through the stellar envelope. Such a neutrino signal is also expected for more general ``failed'' GRBs in which a putative jet is ``choked'' by a heavy envelope. If the ν_e \rightarrow ν_μneutrino oscillation parameters are in the atmospheric neutrino oscillation range, we show that the resonant oscillation of ν_e\leftrightarrowν_{μ,τ} can take place within the inner high density region of the choked jet progenitor with a heavy envelope, altering the neutrino flavor ratio on its surface to Φ^s_{ν_e}:Φ^s_{ν_μ}:Φ^s_{ν_τ}=5:11:2. Considering vacuum oscillation of these neutrinos on their way to Earth, the final flavor ratio detected on Earth is further modified to either $1:1.095:1.095$ for the large mixing angle solution to the solar neutrino data, or 1:1.3:1.3 for maximal mixing among the muon and tau neutrinos in vacuum.

hep-ph↗

arXiv2010

Very High Energy Cosmic Rays from Centaurus A

Centaurus A is the nearest radio-loud AGN and is detected from radio to very high energy gamma-rays. Its nuclear spectral energy distribution shows two peaks, one in the far-infrared band and another at about 150 keV. By assuming the second peak is due to the electron synchrotron emission and the power index for the differential spectrum of the very high energy cosmic ray proton to be 2.7 we show that only pp interaction is responsible for the observed GeV-TeV emission from Centaurus A. We also found that indeed many very high energy cosmic ray protons from Centaurus A can arrive on Earth thus supporting the recent observation of two events by Pierre Auger Observatory.

hep-ph↗

arXiv2010

GRB 090417B and its Host Galaxy: A Step Towards an Understanding of Optically-Dark Gamma-Ray Bursts

GRB 090417B was an unusually long burst with a T_90 duration of at least 2130 s and a multi-peaked light curve at energies of 15-150 keV. It was optically dark and has been associated with a bright star-forming galaxy at a redshift of 0.345 that is broadly similar to the Milky Way. This is one of the few cases where a host galaxy has been clearly identified for a dark gamma-ray burst and thus an ideal candidate for studying the origin of dark bursts. We find that the dark nature of GRB 090417B cannot be explained by high redshift, incomplete observations, or unusual physics in the production of the afterglow. Assuming the standard relativistic fireball model for the afterglow we find that the optical flux is at least 2.5 mag fainter than predicted by the X-ray flux. The Swift/XRT X -ray data are consistent with the afterglow being obscured by a dense, localized sheet of dust approximately 30-80 pc from the burst along the line of sight. Our results suggest that this dust sheet imparts an extinction of A_V >~ 12 mag, which is sufficient to explain the missing optical flux. GRB 090417B is an example of a gamma-ray burst that is dark due to the localized dust structure in its host galaxy.

astro-ph.CO↗

arXiv2010

Late-time detections of the X-ray afterglow of GRB 060729 with Chandra - the latest detections ever of an X-ray afterglow

We report on 5 Chandra observations of the X-ray afterglow of the Gamma-Ray Burst GRB 060729 performed between 2007 March and 2008 May. In all five observations the afterglow is clearly detected. The last Chandra pointing was performed on 2008-May-04, 642 days after the burst - the latest detection of a GRB X-ray afterglow ever. A reanalysis of the Swift XRT light curve together with the three detections by Chandra in 2007 reveals a break at about 1.0 Ms after the burst with a slight steepening of the decay slope from alpha = 1.32 to 1.61. This break coincides with a significant hardening of the X-ray spectrum, consistent with a cooling break in the wind medium scenario, in which the cooling frequency of the afterglow crosses the X-ray band. The last two Chandra observations in 2007 December and 2008 May provide evidence for another break at about one year after the burst. If interpreted as a jet break, this late-time break implies a jet half opening angle of about 14 degrees for a wind medium. Alternatively, this final break may have a spectral origin, in which case no jet break has been observed and the half-opening angle of the jet of GRB 060729 must be larger than about 15 degrees for a wind medium. We compare the X-ray afterglow of GRB 060729 in a wind environment with other bright X-ray afterglows, in particular GRBs 061121 and 080319B, and discuss why the X-ray afterglow of GRB 060729 is such an exceptionally long-lasting event.

astro-ph.CO↗

arXiv2009

Constraining GRB Initial Lorentz Factor with the Afterglow Onset Feature and Discovery of a Tight Gamma_0-E_iso Correlation

The onset of GRB afterglow is characterized by a smooth bump in the early afterglow lightcurve. We make an extensive search for such a feature. Twenty optically selected GRBs and 12 X-ray selected GRBs are found, among which 17 optically selected GRBs and 2 X-ray-selected GRBs have redshift measurements. We fit the lightcurves with a smooth broken power-law and measure the temporal characteristic timescales of the bumps at FWHM. Strong mutual correlations among these timescales are found, and a dimmer and broader bump tends to peak at a later peak time. The ratio of rising to decaying timescales is almost universal among bursts, but the ratio of the rising time to the peak time varies from 0.3~1. The E_iso is tightly correlated with the peak luminosity and the peak time of the bump in the burst frame. Assuming that the bumps signal the deceleration of the GRB fireballs in a constant density medium, we calculate the initial Lorentz factor (Gamma_0) and the deceleration radius (R_dec) of the GRBs in the optical-selected sample. It is found that Gamma_0 are typically a few hundreds, and the typical deceleration radius is R_dec~10^{17} cm. More intriguingly, a tight correlation between the Gamma_0 and E_iso is found, namely Gamma_0 ~ 195 E_iso, 52}^{0.27} (satisfied for both the optical and X-ray z-known samples). It is helpful to understand GRB physics, and may serve as an indicator of Gamma_0. We find that the early bright X-rays are usually dominated by a different component from the external shock emission, but occasionally (for one case) an achromatic deceleration feature is observed. Components in X-rays would contribute to the diversity of the observed X-ray lightcurves (abridge).

astro-ph.HE↗

arXiv2009

Left-right asymmetry for pion and kaon production in the semi-inclusive deep inelastic scattering process

We analyze the left-right asymmetry in the semi-inclusive deep inelastic scattering (SIDIS) process without introducing any weighting functions. With the current theoretical understanding, we find that the Sivers effect plays a key role in our analysis. We use the latest parametrization of the Sivers and fragmentation functions to reanalyze the $π^\pm$ production process and find that the results are sensitive to the parametrization. We also extend our calculation on the $K^{\pm}$ production, which can help us know more about the Sivers distribution of the sea quarks and the unfavored fragmentation processes. HERMES kinematics with a proton target, COMPASS kinematics with a proton, deuteron, and neutron target (the information on the neutron target can be effectively extracted from the $^3$He target), and JLab kinematics (both 6 GeV and 12 GeV) with a proton and neutron target are considered in our paper.

hep-ph↗

arXiv2009

Modeling Gamma-Ray Burst X-Ray Flares within the Internal Shock Model

X-ray afterglow light curves have been collected for over 400 Swift gamma-ray bursts with nearly half of them having X-ray flares superimposed on the regular afterglow decay. Evidence suggests that gamma-ray prompt emission and X-ray flares share a common origin and that at least some flares can only be explained by long-lasting central engine activity. We have developed a shell model code to address the question of how X-ray flares are produced within the framework of the internal shock model. The shell model creates randomized GRB explosions from a central engine with multiple shells and follows those shells as they collide, merge and spread, producing prompt emission and X-ray flares. We pay special attention to the time history of central engine activity, internal shocks, and observed flares, but do not calculate the shock dynamics and radiation processes in detail. Using the empirical E_p - E_iso (Amati) relation with an assumed Band function spectrum for each collision and an empirical flare temporal profile, we calculate the gamma-ray (Swift/BAT band) and X-ray (Swift/XRT band) lightcurves for arbitrary central engine activity and compare the model results with the observational data. We show that the observed X-ray flare phenomenology can be explained within the internal shock model. The number, width and occurring time of flares are then used to diagnose the central engine activity, putting constraints on the energy, ejection time, width and number of ejected shells. We find that the observed X-ray flare time history generally reflects the time history of the central engine, which reactivates multiple times after the prompt emission phase with progressively reduced energy...

astro-ph.HE↗

arXiv2009

Astrophysics: Most distant cosmic blast seen

The most distant -ray burst yet sighted is the earliest astronomical object ever observed in cosmic history. This ancient beacon offers a glimpse of the little-known cosmic dark ages.

astro-ph.HE↗

arXiv2009

A Comprehensive Analysis of Swift/XRT Data: IV. Single Power-Law Decaying Lightcurves vs. Canonical Lightcurves and Implications for a Unified Origin of X-rays

By analyzing the Swift/XRT lightcurves detected before 2009 July, we find 19 cases that monotonously decay as a single power law (SPL) with an index of 1 ~ 1.7 from tens (or hundreds) to ~ 10^5 seconds post the GRB trigger, apparently different from the canonical lightcurves characterized by a shallow-to-normal decay transition. No statistical difference is found in their prompt gamma-rays, and the X-ray properties of the two samples are also similar, although the SPL sample tend to have a slightly lower NH value of the host galaxies and larger energy release compared with the canonical sample. The SPL XRT lightcurves in the burst frame gradually merge into a conflux.The normal decay segment for the canonical sample has the same feature. Similar to the normal decay segment, the SPL lightcurves satisfy the closure relations of external shock models. If the X-rays are the afterglow of the GRB fireball, our results indicate that the shallow decay would be due to energy injection and the total energy budget after injection for both samples of GRBs is comparable. More intriguing, we find that a prior X-ray emission model proposed by Yamazaki is more straightforward to interpret the data. We show that the zero times (T0) of the X-rays atisfy a log-normal distribution, and the negligible T0's of the SPL sample are consistent with being the tail of T0 distributions at low end. Referenced to T0, the canonical XRT lightcurves well trace the SPL lightcurves. The T0's of the canonical lightcurves in our analysis are usually much larger than the offsets of the known precursors from the main GRBs, indicating the X-rays would be an emission component from external shocks prior to GRB trigger. The lack of detection of a jet-like break in most XRT lightcurves implies that the opening angle of its jet would be usually large.

astro-ph.HE↗

arXiv2009

Discerning the physical origins of cosmological Gamma-ray bursts based on multiple observational criteria: the cases of z=6.7 GRB 080913, z=8.3 GRB 090423, and some short/hard GRBs

(Abridged) The two high-redshift gamma-ray bursts, GRB 080913 at z=6.7 and GRB 090423 at z=8.3, recently detected by Swift appear as intrinsically short, hard GRBs. They could have been recognized by BATSE as short/hard GRBs should they have occurred at z <= 1. We perform a more thorough investigation on two physically distinct types (Type I/II) of cosmological GRBs and their observational characteristics. We reiterate the definitions of Type I/II GRBs and review the observational criteria and their physical motivations. Contrary to the traditional approach of assigning the physical category based on the gamma-ray properties (duration, hardness, and spectral lag), we take an alternative approach to define the Type I and Type II Gold Samples using several criteria that are more directly related to the GRB progenitors, and study the properties of the two Gold Samples and compare them with the traditional long/soft and short/hard samples. We find that the Type II Gold Sample reasonably tracks the long/soft population, although it includes several intrinsically short (shorter than 1s in the rest frame) GRBs. The Type I Gold Sample only has 5 GRBs, 4 of which are not strictly short but have extended emission. Other short/hard GRBs detected in the Swift era represent the BATSE short/hard sample well, but it is unclear whether all of them belong to Type I. We suggest that some (probably even most) high-luminosity short/hard GRBs instead belong to Type II. We suggest that GRB 080913 and GRB 090423 are more likely Type II events. We re-emphasize the importance of invoking multiple observational criteria, and cautiously propose an operational procedure to infer the physical origin of a given GRB with available multiple observational criteria, with various caveats laid out.

astro-ph.HE↗