Zi-Gao Dai

X-ray flares from postmerger millisecond pulsars

Recent observations support the suggestion that short-duration gamma-ray bursts are produced by compact star mergers. The x-ray flares discovered in two short gamma-ray bursts last much longer than the previously proposed postmerger energy-release time scales. Here, we show that they can be produced by differentially rotating, millisecond pulsars after the mergers of binary neutron stars. The differential rotation leads to windup of interior poloidal magnetic fields and the resulting toroidal fields are strong enough to float up and break through the stellar surface. Magnetic reconnection–driven explosive events then occur, leading to multiple x-ray flares minutes after the original gamma-ray burst.

Rebrightening of XRF 030723: Further Evidence for a Two-Component Jet in a Gamma-Ray Burst

We numerically investigate optical afterglows from two-component jets under various configurations. Generally, the light curve is characterized by a rapid rebrightening when the observer is off-axis with respect to the narrow component, with the amplitude and peak time depending on detailed parameters. We further show that the optical afterglow of XRF 030723, especially its notable and rapid rebrightening, can be well explained by a typical two-component jet. This X-ray flash, together with GRB 030329, strongly hints toward the two-component jet model as a unified picture for X-ray flashes and gamma-ray bursts. With a narrow but ultra-relativistic inner outflow and a wide but less energetic outer ejecta, a two-component jet will be observed as a typical gamma-ray burst if our line of sight is within the angular scope of the narrow outflow. Otherwise, if the line of sight is within or slightly beyond the cone of the wide component, an X-ray flash will be detected.

The Inverse Compton Emission Spectra in the Very Early Afterglows of Gamma-Ray Bursts

We calculate the spectra of inverse Compton (IC) emissions in gamma-ray burst (GRB) shocks produced when relativistic ejecta encounters the external interstellar medium, assuming a broken power-law approximation to the synchrotron seed spectrum. Four IC processes, including the synchrotron self-Compton (SSC) processes in GRB forward and reverse shocks, and two combined-IC processes (i.e., scattering of reverse shock photons on the electrons in forward shocks and forward shock photons on the electrons in reverse shocks), are considered. We find that the SSC emission from reverse shocks dominates over other emission processes in energy bands from tens of MeV to tens of GeV, for a wide range of shock parameters. This mechanism may be responsible for the prompt high-energy gamma rays detected by EGRET. At TeV energy bands, however, the combined-IC emissions and/or the SSC emission from the forward shocks become increasingly dominant for a moderately steep distribution of shocked electrons.

ICECUBE NONDETECTION OF GAMMA-RAY BURSTS: CONSTRAINTS ON THE FIREBALL PROPERTIES

The increasingly deep limit on the neutrino emission from gamma-ray bursts (GRBs) with IceCube observations has reached a level that could place useful constraints on the fireball properties. We first present a revised analytic calculation of the neutrino flux that predicts a flux of one order of magnitude lower than that obtained by the IceCube Collaboration. For the benchmark model parameters (e.g., the bulk Lorentz factor is Γ = 102.5, the observed variability time for the long GRBs is t ob v = 0.01 s, and the ratio between the energy in the accelerated protons and in the radiation is η p = 10 for every burst) in the standard internal shock scenario, the predicted neutrino flux from 215 bursts during the period of the 40- and 59-string configurations is a factor of ~3 below the IceCube sensitivity. However, if we accept the recently found inherent relation between the bulk Lorentz factor and the burst energy, then the expected neutrino flux significantly increases and the spectral peak shifts to a lower energy. In this case, the nondetection implies that the baryon-loading ratio should be η p 10 if the variability time of the long GRBs is fixed to t ob v = 0.01 s. Instead, if we relax the standard internal-shock scenario but still assume η p = 10, then the nondetection constrains the dissipation radius, R 4 × 1012 cm, assuming the same dissipation radius for every burst and benchmark parameters for the fireballs. We also calculate the diffuse neutrino flux from the GRBs for different luminosity functions from the literature. The expected flux exceeds the current IceCube limit for some of the luminosity functions, and, thus, the nondetection constrains η p 10 when the variability time of the long GRBs is fixed at t ob v = 0.01 s.

High-energy cosmic rays and neutrinos from semirelativistic hypernovae

The origin of the ultrahigh-energy (UHE) cosmic rays (CRs) from the second knee (

Optical flashes and very early afterglows in wind environments

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Bright Broadband Afterglows of Gravitational Wave Bursts from Mergers of Binary Neutron Stars

) above in the CR spectrum is still unknown. Recently, there has been growing evidence that a peculiar type of supernovae, called hypernovae, are associated with subenergetic gamma-ray bursts, such as SN1998bw/GRB980425 and SN2003lw/GRB031203. Such hypernovae appear to have high (up to mildly relativistic) velocity ejecta, which may be linked to the subenergetic gamma-ray bursts. Assuming a continuous distribution of the kinetic energy of the hypernova ejecta as a function of its velocity

A COMPREHENSIVE STUDY OF GAMMA-RAY BURST OPTICAL EMISSION. II. AFTERGLOW ONSET AND LATE RE-BRIGHTENING COMPONENTS

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GRB 080916C: On the Radiation Origin of the Prompt Emission from keV/MeV TO GeV

with

The updated luminosity correlations of gamma-ray bursts and cosmological implications

\ensuremath{\alpha}\ensuremath{\sim}2