arXiv subjects

Bing Zhang

Publications and source records attributed to Bing Zhang.

At least 541 records · Page 30Linked to original sources

arXiv2007

Prompt Emission of High Energy Photons from Gamma Ray Bursts

Within the internal shock scenario we consider different mechanisms of high energy ($>1$ MeV) photon production inside a Gamma Ray Burst (GRB) fireball and derive the expected high energy photon spectra from individual GRBs during the prompt phase. The photon spectra of leptonic and hadronic origins are compared within different sets of parameter regimes. Our results suggest that the high energy emission is dominated by the leptonic component if fraction of shock energy carried by electrons is not very small (e.g. $ε_e > 10^{-3}$). For very small values of $ε_e$ the hadronic emission component could be comparable to or even exceed the leptonic component in the GeV-TeV regime. However, in this case a much larger energy budget of the fireball is required to account for the same level of the observed sub-MeV spectrum. The fireballs are therefore extremely inefficient in radiation. For a canonical fireball bulk Lorentz factor (e.g. $Γ=400$), emissions above $\sim 10$ GeV are attenuated by two-photon pair production processes. For a fireball with an even higher Lorentz factor, the cutoff energy is higher, and emissions of 10 TeV - PeV due to $π^0$-decay can also escape from the internal shocks. The flux level is however too low to be detected by current TeV detectors, and these photons also suffer attenuation by external soft photons.

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arXiv2007

Constraining Galactic $pγ$ Interactions with Cosmic Ray Electron and Positron Spectra

High energy protons produced by various sources of cosmic rays, {\it e.g.} supernova remnants, pulsar wind nebulae, active galactic nuclei and gamma-ray bursts participate in $pγ$ and $pp$ interactions. Although $pp$ interactions may be the dominant mechanism in our Galaxy, it is unclear how important $pγ$ process is. We show that the upper bound on the fraction of protons participating in $pγ$ interactions inside all Galactic astrophysical sources of cosmic rays is 10%.

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arXiv2007

Swift and XMM-Newton Observations of the Extraordinary GRB 060729: An afterglow with a more than 100 days X-ray light curve

We report the results of the Swift and XMM observations of the Swift-discovered long Gamma-Ray Burst GRB 060729 ($T_{90}$=115s). The afterglow of this burst was exceptionally bright in X-rays as well as at UV/Optical wavelengths showing an unusually long slow decay phase ($α$=0.14\plm0.02) suggesting a larger energy injection phase at early times than in other bursts. The X-ray light curve displays a break at about 60 ks after the burst. The X-ray decay slope after the break is $α$=1.29\plm0.03. Up to 125 days after the burst we do not detect a jet break, suggesting that the jet opening angle is larger than 28 degrees. In the first 2 minutes after the burst (rest frame) the X-ray spectrum of the burst changed dramatically from a hard X-ray spectrum to a very soft one. We find that the X-ray spectra at this early phase can all be fitted by an absorbed single power law model or alternatively by a blackbody plus power law model. The power law fits show that the X-ray spectrum becomes steeper while the absorption column density decreases. In Swift's UV/Optical telescope the afterglow was clearly detected up to 9 days after the burst in all 6 filters and even longer in some of the UV filters with the latest detection in the UVW1 31 days after the burst. A break at about 50 ks is clearly detected in all 6 UVOT filters from a shallow decay slope of about 0.3 and a steeper decay slope of 1.3. In addition to the \swift observations we also present and discuss the results from a 61 ks ToO observation by XMM. (Abriviated)

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arXiv2007

Gamma-Ray Bursts in the Swift Era

Since the successful launch of NASA's dedicated gamma-ray burst (GRB) mission, Swift, the study of cosmological GRBs has entered a new era. Here I review the rapid observational and theoretical progress in this dynamical research field during the first two-year of the Swift mission, focusing on how observational breakthroughs have revolutionized our understanding of the physical origins of GRBs. Besides summarizing how Swift helps to solve some pre-Swift mysteries, I also list some outstanding problems raised by the Swift observations. An outlook of GRB science in the future, especially in the GLAST era, is briefly discussed.

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arXiv2007

Gamma-ray burst afterglows

Extended, fading emissions in multi-wavelength are observed following Gamma-ray bursts (GRBs). Recent broad-band observational campaigns led by the Swift Observatory reveal rich features of these GRB afterglows. Here we review the latest observational progress and discuss the theoretical implications for understanding the central engine, composition, and geometric configuration of GRB jets, as well as their interactions with the ambient medium.

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arXiv2007

Neutrino Spectra from Low and High Luminosity Populations of Gamma Ray Bursts

The detection of GRB 060218 at z=0.033 by {\em Swift} within 1.5 years of operation, together with the detection of GRB 980425 at $z=0.0085$ by BeppoSAX, suggest that these low-luminosity (LL) GRBs have a much higher event rate than the canonical high-luminosity (HL) GRBs, and they form a distinct new component in the GRB luminosity function. We explore the contribution of this previously neglected GRB population to the diffuse neutrino background within the internal shock model and compare it with that of the canonical HL population. By considering a wide range of distributions of various parameters (e.g. luminosity, spectral break energy, duration, variability time, Lorentz factor, redshift) for both populations, we find that although it is difficult to detect neutrinos from the individual LL GRBs, the contribution of the LL population to the diffuse neutrino background is more than the HL population above about $10^{8}$ GeV.

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arXiv2006

Thermal radiation from hot polar cap in pulsars

Thermal radiation from hot polar caps is examined in radio pulsars with drifting subpulses. It is argued that if these subpulses correspond to sparking discharges of the inner acceleration region right above the polar cap surface then a simple relationship between the observed subpulse drift rate in radio and thermal X-ray luminosity from the polar cap heated by sparks should exist. This relationship is derived and tested in pulsars for which an appropriate good quality data is available.

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arXiv2006

Astrophysics: A burst of new ideas

Gigantic cosmological gamma-ray bursts have fallen into a dichotomy of long and short bursts, each with a very different origin. The discovery of an oddball burst calls for a rethink of that classification.

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arXiv2006

Making a Short Gamma-Ray Burst from a Long one: Implications for the Nature of GRB 060614

The absence of a supernova accompanying the nearby long GRB 060614 poses a great puzzle about the progenitor of this event and challenges the current GRB classification scheme. This burst displays a short-hard emission episode followed by extended soft emission with strong spectral evolution. Noticing that this burst has an isotropic gamma-ray energy only ~8 times that of GRB 050724, a good candidate of merger-type short GRBs, we generate a ``pseudo'' burst that is ~8 times less energetic than GRB 060614 based on the spectral properties of GRB 060614 and the Ep ~ Eiso^{1/2} (Amati) relation. We find that this pseudo-burst would have been detected by BATSE as a marginal short-duration GRB, and would have properties in the Swift BAT and XRT bands similar to GRB 050724. This suggests that GRB 060614 is likely a more intense event in the traditional short-hard GRB category as would be detected by BATSE. Events like GRB 060614 that seem to defy the traditional short vs. long classification of GRBs may require modification of our classification terminology for GRBs. By analogy with supernova classifications, we suggest that GRBs be classified into Type I (typically short and associated with old populations) and Type II (typically long and associated with young populations). We propose that GRB 060614 belongs to Type I, and predict that similar events will be detected in elliptical galaxies.

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arXiv2006

Contribution of GRB Emission to the GeV Extragalactic Diffuse Gamma-Ray Flux

TeV gamma rays emitted by GRBs are converted into electron-positron pairs via interactions with the extragalactic infrared radiation fields. In turn the pairs produced, whose trajectories are randomized by magnetic fields, will inverse Compton scatter off the cosmic microwave background photons. The beamed TeV gamma ray flux from GRBs is thus transformed into a GeV isotropic gamma ray flux, which contributes to the total extragalactic gamma-ray background emission. Assuming a model for the extragalactic radiation fields, for the GRB redshift distribution and for the GRB luminosity function, we evaluate the contribution of the GRB prompt and scattered emissions to the measured extragalactic gamma-ray flux. To estimate this contribution we optimistically require that the energy flux at TeV energies is about 10 times stronger than the energy flux at MeV energies. The resulting gamma-ray diffuse background is only a small fraction of what is observed, allowing blazars and other sources to give the dominant contribution.

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arXiv2006

Temporal Profiles and Spectral Lags of XRF 060218

The spectral and temporal properties of the non-thermal emission ofthe nearby XRF 060218 in 0.3-150 keV band are studied. We show that both the spectral energy distribution and the light curve properties suggest the same origin of the non-thermal emission detected by {\em Swift} BAT and XRT. This event has the longest pulse duration and spectral lag observed to date among the known GRBs. The pulse structure and its energy dependence are analogous to typical GRBs. By extrapolating the observed spectral lag to the {\em CGRO/BATSE} bands we find that the hypothesis that this event complies with the same luminosity-lag relation with bright GRBs cannot be ruled out at $2σ$ significance level. These intriguing facts, along with its compliance with the Amati-relation, indicate that XRF 060218 shares the similar radiation physics as typical GRBs.

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arXiv2006

On the origins of part-time radio pulsars

Growing evidence suggests that some radio pulsars only act sporadically. These "part-time'' pulsars include long-term nulls, quasi-periodic radio flares in PSR B1931+24, as well as the so-called Rotating RAdio Transients (RRATs). Based on the assumption that these objects are isolated neutron stars similar to conventional radio pulsars, we discuss two possible interpretations to the phenomenon. The first interpretation suggests that these objects are pulsars slightly below the radio emission "death line'', which become active occasionally only when the conditions for pair production and coherent emission are satisfied. The second interpretation invokes a radio emission direction reversal in conventional pulsars, as has been introduced to interpret the peculiar mode changing phenomenon in PSR B1822-09. In this picture, our line of sight misses the main radio emission beam of the pulsar but happens to sweep the emission beam when the radio emission direction is reversed. These part-time pulsars are therefore the other half of "nulling'' pulsars. We suggest that X-ray observations may provide clues to differentiate between these two possibilities.

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arXiv2006

Inverse Compton X-ray Flare from GRB Reverse Shock

We study synchrotron self-inverse Compton radiation from a reverse-shocked fireball. If the inverse Compton process dominates the cooling of shocked electrons, an X-ray flare produced by the first order Compton scattering would emerge in the very early afterglow phase, with the bulk of the shock energy radiated in the second order scattering component at 10-100 MeV. The dominance of inverse Compton cooling leads to the lack of prompt optical flashes. We show that for plausible parameters this scattering process can produce an X-ray flare with a relative amplitude change by a factor of several. Flares with a larger amplitude and multiple X-ray flares in a single event are likely to be produced by another mechanism (e.g. internal shocks).

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arXiv2006

The Onset of Gamma-Ray Burst Afterglow

We discuss the reference time t_0 of afterglow light curves in the context of the standard internal-external shock model. The decay index of early afterglow is very sensitive to the reference time one chooses. In order to understand the nature of early afterglow, it is essential to take a correct reference time. Our simple analytic model provides a framework to understand special relativistic effects involved in early afterglow phase. We evaluate light curves of reverse shock emission as well as those of forward shock emission, based on full hydrodynamic calculations. We show that the reference time does not shift significantly even in the thick shell case. For external shock emission components, measuring times from the beginning of the prompt emission is a good approximation and it does not cause an early steep decay. In the thin shell case, the energy transfer time from fireball ejecta to ambient medium typically extends to thousands of seconds. This might be related to the shallow decay phases observed in early X-ray afterglow at least for some bursts.

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arXiv2006

GRB Radiative Efficiencies Derived from the Swift Data: GRBs vs. XRFs, Long vs. Short

We systematically analyze the prompt emission and the early afterglow data of a sample of 31 GRBs detected by {\em Swift} before September 2005, and estimate the GRB radiative efficiency. BAT's narrow band inhibits a precise determination of the GRB spectral parameters, and we have developed a method to estimate these parameters with the hardness ratio information. The shallow decay component commonly existing in early X-ray afterglows, if interpreted as continuous energy injection in the external shock, suggests that the GRB efficiency previously derived from the late-time X-ray data were not reliable. We calculate two radiative efficiencies using the afterglow kinetic energy E_K derived at the putative deceleration time t_{dec}) and at the break time (t_b) when the energy injection phase ends, respectively. At t_b XRFs appear to be less efficient than normal GRBs. However, when we analyze the data at t_{dec} XRFs are found to be as efficient as GRBs. Short GRBs have similar radiative efficiencies to long GRBs despite of their different progenitors. Twenty-two bursts in the sample are identified to have the afterglow cooling frequency below the X-ray band. Assuming ε_e = 0.1, we find η_γ(t_b) usually <10% and η_γ(t_{dec}) varying from a few percents to > 90%. Nine GRBs in the sample have the afterglow cooling frequency above the X-ray band for a very long time. This suggests a very small ε_B and/or a very low ambient density n.

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arXiv2006

Swift XRT Observations of the Afterglow of XRF 050416A

Swift discovered XRF 050416A with the BAT and began observing it with its narrow field instruments only 64.5 s after the burst onset. Its very soft spectrum classifies this event as an X-ray flash. The afterglow X-ray emission was monitored up to 74 days after the burst. The X-ray light curve initially decays very fast, subsequently flattens and eventually steepens again, similar to many X-ray afterglows. The first and second phases end about 172 and 1450 s after the burst onset, respectively. We find evidence of spectral evolution from a softer emission with photon index Gamma ~ 3.0 during the initial steep decay, to a harder emission with Gamma ~ 2.0 during the following evolutionary phases. The spectra show intrinsic absorption in the host galaxy. The consistency of the initial photon index with the high energy BAT photon index suggests that the initial phase of the X-ray light curve may be the low-energy tail of the prompt emission. The lack of jet break signatures in the X-ray afterglow light curve is not consistent with empirical relations between the source rest-frame peak energy and the collimation-corrected energy of the burst. The standard uniform jet model can give a possible description of the XRF 050416A X-ray afterglow for an opening angle larger than a few tens of degrees, although numerical simulations show that the late time decay is slightly flatter than expected from on-axis viewing of a uniform jet. A structured Gaussian-type jet model with uniform Lorentz factor distribution and viewing angle outside the Gaussian core is another possibility, although a full agreement with data is not achieved with the numerical models explored.

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arXiv2006

Jet Breaks in Short Gamma-Ray Bursts. II: The Collimated Afterglow of GRB 051221A

We report the best evidence to date of a jet break in a short Gamma-Ray Burst (GRB) afterglow, using Chandra and Swift XRT observations of the X-ray afterglow of GRB 051221A. The combined X-ray light curve, which has three breaks, is similar to those commonly observed in Swift observations of long GRBs. A flat segment of the light curve at ~0.1 days after the burst represents the first clear case of strong energy injection in the external shock of a short GRB afterglow. The last break occurs at ~4 days post-burst and breaks to a power-law decay index of ~2. We interpret this as a jet break, with important implications for models of short GRBs, since it requires collimation of the afterglow into a jet with an initial opening angle ~4-8 degrees and implies a total jet kinetic energy of E_jet ~(1-5) x 10^{49} erg. Combined with the lack of a jet break in GRB 050724, this suggests a wide range in jet collimation in short GRBs, with at least some having collimation similar to that found in long GRBs, though with significantly lower jet energies.

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arXiv2006

Synchrotron emission in small scale magnetic field as possible explanation for prompt emission spectra of gamma-ray bursts

Synchrotron emission is believed to be a major radiation mechanism during gamma-ray bursts' (GRBs) prompt emission phase. A significant drawback of this assumption is that the theoretical predicted spectrum, calculated within the framework of the ``internal shocks'' scenario using the standard assumption that the magnetic field maintains a steady value throughout the shocked region, leads to a slope F_ν\propto ν^{-1/2} below 100 keV, which is in contradiction to the much harder spectra observed. This is due to the electrons cooling time being much shorter than the dynamical time. In order to overcome this problem, we propose here that the magnetic field created by the internal shocks decays on a length scale much shorter than the comoving width of the plasma. We show that under this assumption synchrotron radiation can reproduce the observed prompt emission spectra of the majority of the bursts. We calculate the required decay length of the magnetic field, and find it to be \~10^4 - 10^5 cm (equivalent to 10^5 - 10^6 skin depths), much shorter than the characteristic comoving width of the plasma, ~3*10^{9} cm. We implement our model to the case of GRB050820A, where a break at <~ 4 keV was observed, and show that this break can be explained by synchrotron self absorption. We discuss the consequences of the small scale magnetic field scenario on current models of magnetic field generation in shock waves.

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