arXiv subjects

Bing Zhang

Publications and source records attributed to Bing Zhang.

At least 595 records · Page 33Linked to original sources

arXiv2004

On the Kinetic Energy and Radiative Efficiency of Gamma-Ray Bursts

Using measured X-ray luminosities to 17 Gamma-Ray Bursts (GRBs) during the afterglow phase and accounting for radiative losses, we calculate the kinetic energy of these bursts and investigate its relation to other GRB properties. We then use the observed radiated energy during the prompt phase to determine the radiative efficiency of these bursts, and explore how the efficiency relates to other GRB observables. We find that the kinetic energy in the afterglow phase is directly correlated with the radiated energy, total energy as well as possibly the jet opening angle and spectral peak energy. More importantly, we find the intriguing fact that the efficiency is correlated with the radiated energy, and mildly with the total energy, jet opening angle and spectral peak energy. XRF020903 also seems to follow the trends we find for our GRB sample. We discuss the implications of these results for the GRB radiation and jet models.

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arXiv2003

Planetary Collisions: Electromagnetic Signals

We investigate the electromagnetic signals accompanied with planetary collisions and their event rate, and explore the possibility of directly detecting such events. A typical Earth--Jupiter collision would give rise to a prompt EUV-soft-X-ray flash lasting for hours and a bright IR afterglow lasting for thousands of years. With the current and forthcoming observational technology and facilities, some of these collisional flashes or the post-collision remnants could be discovered.

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arXiv2003

Early afterglow, magnetized central engine, and a quasi-universal jet configuration for long GRBs

Two separate topics are discussed. (1) We describe the classifications of the long GRB early afterglow lightcurves within the framework of the fireball shock model, focusing on the interplay between the reverse and forward shock emission components. We will also provide evidence that the central engine of at least two bursts are entrained with strong magnetic fields, and discuss the implications of this result for our understanding of the GRB phenomenon; (2) We argue that the current gamma-ray burst (GRB) and X-ray flash (XRF) data are consistent with a picture that all GRB-XRF jets are structured and quasi-universal, with a typical Gaussian-like jet structure.

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arXiv2003

Quasi-universal Gaussian jets: a unified picture for gamma-ray bursts and X-ray flashes

An observed correlation Ep \propto (E_{iso})^{1/2} extending from Gamma-ray Bursts (GRBs) to X-ray flashes (XRFs) poses problems both for a power-law universal jet model where the energy per solid angle decreases as the inverse square of the angle respect to the jet axis, and for a conical jet model with a uniform energy density within the jet beam and a sharp energy cut-off at the jet edge. Here we show that the current GRB-XRF prompt emission/afterglow data can be understood in terms of a picture in which the GRB-XRF jets are quasi-universal and structured, with a Gaussian-like or similar structure, i.e., one where the jet has a characteristic angle, with a mild variation of energy inside and a rapid (e.g. exponential) decrease of energy outside of it. A Monte Carlo simulation shows that the current data is compatible with such a quasi-universal Gaussian jet with a typical opening angle of 5.7(+3.4)(-2.1) degrees, and with a standard jet energy of about log (E_j/1 erg)= 51.1+-0.3. According to this model, the true-to-observed number ratio of the whole GRB-XRF population is about 14 with the current instrumental sensitivity.

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arXiv2003

A Characteristic Dense Environment or Wind Signature in Prompt GRB Afterglows

We discuss the effects of synchrotron self-absorption in the prompt emission from the reverse shock of GRB afterglows occurring in a dense environment, such as the wind of a massive stellar progenitor or a dense ISM in early galaxies. We point out that, when synchrotron losses dominate over inverse Compton losses, the higher self-absorption frequency in a dense environment implies a bump in the reverse shock emission spectrum, which can result in a more complex optical/IR light curve than previously thought. This bump is prominent especially if the burst ejecta is highly magnetized. In the opposite case of low magnetization, inverse Compton losses lead to a prompt X-ray flare. These effects give a possible new diagnostic for the magnetic energy density in the fireball, and for the presence of a dense environment.

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arXiv2003

Gamma-Ray Bursts: Progress, Problems & Prospects

The cosmological gamma-ray burst (GRB) phenomenon is reviewed. The broad observational facts and empirical phenomenological relations of the GRB prompt emission and afterglow are outlined. A well-tested, successful fireball shock model is introduced in a pedagogical manner. Several important uncertainties in the current understanding of the phenomenon are reviewed, and prospects of how future experiments and extensive observational and theoretical efforts may address these problems are discussed.

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arXiv2003

On the Structure of Quasi-Universal Jets for Gamma-Ray Bursts

The idea that GRBs originate from uniform jets has been used to explain numerous observations of breaks in the GRB afterglow lightcurves. We explore the possibility that GRBs instead originate from a structured jet that may be quasi-universal, where the variation in the observed properties of GRBs is due to the variation in the observer viewing angle. We test how various models reproduce the jet data of Bloom, Frail, & Kulkarni (2003), which show a negative correlation between the isotropic energy output and the inferred jet opening angle (in a uniform jet configuration). We find, consistent with previous studies, that a power-law structure for the jet energy as a function of angle gives a good description. However, a Gaussian jet structure can also reproduce the data well, particularly if the parameters of the Gaussian are allowed some scatter. We place limits on the scatter of the parameters in both the Gaussian and power-law models needed to reproduce the data, and discuss how future observations will better distinguish between these models for the GRB jet structure. In particular, the Gaussian model predicts a turnover at small opening angles and in some cases a sharp cutoff at large angles, the former of which may already have been observed. We also discuss the predictions each model makes for the observed luminosity function of GRBs and compare these predictions with the existing data.

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arXiv2003

Electromagnetic signals from planetary collisions

Planet-planet collisions are expected during the early stages of the formation of extra-solar planets, and are also possible in mature planetary systems through secular planet-planet perturbations. We investigate the electromagnetic signals accompanied with these planetary collisions and their event rate, and explore the possibility of directly detecting such events. A typical Earth-Jupiter collision would give rise to a prompt EUV-soft-X-ray flash lasting for hours and a bright IR afterglow lasting for thousands of years. It is suggested that with the current and forthcoming observational technology and facilities, some of these collisional flashes or the post-collision remnants could be discovered.

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arXiv2003

Early Optical Afterglows from Wind-Type Gamma-Ray Bursts

We study prompt optical emission from reverse shocks in the wind-type gamma-ray bursts. The emission is evaluated in both the thick and thin shell regimes. We discuss the angular time delay effect and the post-shock evolution of the fireball ejecta, which determine the decay index of the prompt optical emission and the duration of the radio flare. We discuss distinct emission signatures of the wind environment compared with the constant interstellar medium environment. We also present two recipes for directly constraining the initial or early time Lorentz factor of the fireball using the reverse and forward shock optical afterglow data for the wind case.

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arXiv2003

Gamma-ray burst early optical afterglows: implications for the initial Lorentz factor and the central engine

Early optical afterglows have been observed from GRB 990123, GRB 021004, and GRB 021211, which reveal rich emission features attributed to reverse shocks. It is expected that Swift will discover many more early afterglows. Here we investigate in a unified manner both the forward and the reverse external shock emission components, and introduce a straightforward recipe for directly constraining the initial Lorentz factor of the fireball using early optical afterglow data. The scheme is largely independent of the shock microphysics. We identify two types of combinations of the reverse and forward shock emission, and explore their parameter regimes. We also discuss a possible diagnostic for magnetized ejecta. There is evidence that the central engine of GRB 990123 is strongly magnetized.

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arXiv2003

High energy neutrinos from magnetars

Magnetars can accelerate cosmic rays to high energies through the unipolar effect, and are also copious soft photon emitters. We show that young, fast-rotating magnetars whose spin and magnetic moment point in opposite directions emit high energy neutrinos from their polar caps through photomeson interactions. We identify a neutrino cut-off band in the magnetar period-magnetic field strength phase diagram, corresponding to the photomeson interaction threshold. Within uncertainties, we point out four possible neutrino emission candidates among the currently known magnetars, the brightest of which may be detectable for a chance on-beam alignment. Young magnetars in the universe would also contribute to a weak diffuse neutrino background, whose detectability is marginal, depending on the typical neutrino energy.

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arXiv2002

GRB 021004: Reverse Shock Emission

We show that the re-brightening in the GRB 021004 optical afterglow light curve around ~ 0.1 day can be explained within the framework of the standard fireball model. The superposition of a forward and reverse shock emissions result in the peculiar behavior. With the standard values of parameters obtained in other afterglow observations, we can construct an example case in which theoretical estimates reasonably fit the broadband observations. Therefore, the peculiarity of the light curve might be a common feature in early gamma-ray burst afterglows.

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arXiv2002

Some recent developments in gamma-ray burst afterglow and prompt emission models

Extensive observational campaigns of afterglow hunting have greatly enriched our understanding of the gamma-ray burst (GRB) phenomenon. Efforts have been made recently to explore some afterglow properties or signatures that will be tested by the on-going or the future observational campaigns yet come. These include the properties of GRB early afterglows in the temporal domain; the GeV-TeV afterglow signatures in the spectral domain; as well as a global view about the GRB universal structured jet configuration. These recent efforts are reviewed. Within the standard cosmological fireball model, the very model(s) responsible for the GRB prompt emission is (are) not identified. These models are critically reviewed and confronted with the current data.

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arXiv2002

Magnetars and pulsars: a missing link

There is growing evidence that soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are isolated neutron stars with superstrong magnetic fields, i.e., magnetars, marking them a distinguished species from the conventional species of spindown-powered isolated neutron stars, i.e., radio pulsars. The current arguments in favor of the magnetar interpretation of SGR/AXP phenomenology will be outlined, and the two energy sources in magnetars, i.e. a magnetic dissipation energy and a spindown energy, will be reviewed. I will then discuss a missing link between magnetars and pulsars, i.e., lack of the observational evidence of the spindown-powered behaviors in known magnetars. Some recent theoretical efforts in studying such behaviors will be reviewed along with some predictions testable in the near future.

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arXiv2002

Radio pulsar death

Pulsar radio emission is believed to be originated from the electron-positron pairs streaming out from the polar cap region. Pair formation, an essential condition for pulsar radio emission, is believed to be sustained in active pulsars via one photon process from either the curvature radiation (CR) or the inverse Compton scattering (ICS) seed photons, or sometimes via two photon process. In pulsars with super-critical magnetic fields, some more exotic processes, such as magnetic photon splitting and bound pair formation, will also play noticeable roles. All these effects should be synthesized to discuss radio pulsar death both in the conventional long-period regime due to the turnoff of the active gap, and in the high magnetic field regime due to the possible suppression of the free pair formation. Here I briefly review some recent progress in understanding radio pulsar death.

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arXiv2002

An analysis of gamma-ray burst spectral break models

Typical gamma-ray burst spectra are characterized by a spectral break, Ep, which for bright BATSE bursts is found to be narrowly clustered around 300 keV. Recently identified X-ray flashes, which may account for a significant portion of the whole GRB population, seem to extend the Ep distribution to a broader range below 40 keV. Positive correlations among Ep and some other observed parameters have been noticed. On the other hand, within the cosmological fireball model, the issues concerning the dominant energy ingredient of the fireball as well as the location of the GRB emission site are still unsettled, leading to several variants of the fireball model. Here we analyze these models within a unified framework, and critically reexamine the Ep predictions in the various model variants. Attention is focused on the predictions of the narrowness of the Ep distribution in different models, and the correlations among Ep and some other measurable observables. These model properties may be tested against the current and upcoming GRB data, through which the nature of the fireball as well as the mechanism and site of the GRB emission will be identified. In view of the current data, various models are appraised through a simple Monte-Carlo simulation, and a tentative discussion about the possible nature of X-ray flashes is presented.

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arXiv2002

Regimes of Pulsar Pair Formation and Particle Energetics

We investigate the conditions required for the production of electron-positron pairs above a pulsar polar cap (PC) and the influence of pair production on the energetics of the primary particle acceleration. Assuming space-charge limited flow acceleration including the inertial frame-dragging effect, we allow both one-photon and two-photon pair production by either curvature radiation (CR) photons or photons resulting from inverse-Compton scattering of thermal photons from the PC by primary electrons. We find that, while only the younger pulsars can produce pairs through CR, nearly all known radio pulsars are capable of producing pairs through non-resonant inverse-Compton scatterings. The effect of the neutron star equations of state on the pair death lines is explored. We show that pair production is facilitated in more compact stars and more massive stars. Therefore accretion of mass by pulsars in binary systems may allow pair production in most of the millisecond pulsar population. We also find that two-photon pair production may be important in millisecond pulsars if their surface temperatures are above about three million degrees K. Pulsars that produce pairs through CR will have their primary acceleration limited by the effect of screening of the electric field. In this regime, the high-energy luminosity should follow a $L_{HE} \propto \dot E_{rot}^{1/2}$ dependence. The acceleration voltage drop in pulsars that produce pairs only through inverse-Compton emission will not be limited by electric field screening. In this regime, the high-energy luminosity should follow a $L_{HE} \propto \dot E_{rot}$ dependence. Thus, older pulsars will have significantly lower gamma-ray luminosity.

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arXiv2002

The Dynamical Evolution of Substructure

The evolution of substructure embedded in non-dissipative dark halos is studied through N-body simulations of isolated systems, both in and out of initial equilibrium, complementing cosmological simulations of the growth of structure. We determine by both analytic calculations and direct analysis of the N-body simulations the relative importance of various dynamical processes acting on the clumps, such as the removal of material by global tides, clump-clump heating, clump-clump merging and dynamical friction. Our comparison between merging and disruption processes implies that spiral galaxies cannot be formed in a proto-system that contains a few large clumps, but can be formed through the accretion of many small clumps; elliptical galaxies form in a more clumpy environment than do spiral galaxies. Our results support the idea that the central cusp in the density profiles of dark halos is the consequence of self-limiting merging of small, dense halos. This implies that the collapse of a system of clumps/substructure is not sufficient to form a cD galaxy, with an extended envelope; plausibly subsequent accretion of large galaxies is required. Persistent streams of material from disrupted clumps can be found in the outer regions of the final system, and at an overdensity of around 0.75, can cover 10% to 30% of the sky.

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