Lang Shao

REVISITING THE LONG/SOFT-SHORT/HARD CLASSIFICATION OF GAMMA-RAY BURSTS IN THE FERMI ERA

We perform a statistical analysis of the temporal and spectral properties of the latest Fermi gamma-ray bursts (GRBs) to revisit the classification of GRBs. We find that the bimodalities of duration and the energy ratio (E-peak/Fluence) and the anti-correlation between spectral hardness (hardness ratio (HR), peak energy, and spectral index) and duration (T-90) support the long/soft-short/hard classification scheme for Fermi GRBs. The HR-T-90 anti-correlation strongly depends on the spectral shape of GRBs and energy bands, and the bursts with the curved spectra in the typical BATSE energy bands show a tighter anti-correlation than those with the power-law spectra in the typical BAT energy bands. This might explain why the HR-T-90 correlation is not evident for those GRB samples detected by instruments like Swift with a narrower/softer energy bandpass. We also analyze the intrinsic energy correlation for the GRBs with measured redshifts and well-defined peak energies. The current sample suggests E-p,E-rest = 2455x(E-iso/10(52))(0.59) for short GRBs, significantly different from that for long GRBs. However, both the long and short GRBs comply with the same E-p,(rest)-L-iso correlation.

Sw 1644+57/GRB 110328A: The Physical Origin and the Composition of the Relativistic Outflow

Sw 1644+57/GRB 110328A is a remarkable cosmological X-ray outburst detected by the Swift satellite. Its early-time (t less than or similar to 0.1 days since the trigger) X-ray emission resembles some gamma-ray bursts (GRBs), e. g., GRB 090417B. But the late-time flaring X-ray plateau lasting >40 days renders it unique. We examine the possibilities that the outburst is a super-long GRB powered either by the fallback accretion onto a nascent black hole or by a millisecond pulsar, and find out that these two scenarios can address some but not all of the main observational features. We then focus on the model of tidal disruption of a (giant) star by a massive black hole. The mass of the tidal-disrupted star is estimated to be greater than or similar to a few solar masses. A simple/straightforward argument for a magnetic origin of the relativistic outflow is presented.

IMPLICATIONS OF UNDERSTANDING SHORT GAMMA-RAY BURSTS DETECTED BY SWIFT

In an effort to understand the puzzle of classifying gamma-ray bursts (GRBs), we perform a systematic study of Swift GRBs and investigate several short GRB issues. Though short GRBs have a short (less than or similar to 2 s) prompt duration as monitored by the Burst Alert Telescope, the composite light curves including both the prompt and afterglow emission suggest that most of the short GRBs have a similar radiative feature to long GRBs. Furthermore, some well-studied short GRBs might also have an intrinsically long prompt duration, which renders them as a type of short GRB imposters. Genuine short GRBs detected by Swift might be rare, so determining the observed short GRBs is, not surprisingly, troublesome. In particular, the observational biases in the host identification and redshift measurement of GRBs should be taken with great caution. The redshift distribution, which has been found to be different for long and short GRBs, might have been strongly affected by the measurement methods. We find that the redshifts measured from the presumed host galaxies of long and short GRBs appear to have a similar distribution.

SPECTRAL SOFTENING IN THE X-RAY AFTERGLOW OF GRB 130925A AS PREDICTED BY THE DUST SCATTERING MODEL

Gamma-ray bursts (GRBs) usually occur in a dense star-forming region with a massive circumburst medium. The small-angle scattering of intense prompt X-ray emission off the surrounding dust grains will have observable consequences and sometimes can dominate the X-ray afterglow. In most of the previous studies, only the Rayleigh-Gans (RG) approximation is employed for describing the scattering process, which works accurately for the typical size of grains (with radius of a <= 0.1 mu m) in the diffuse interstellar medium. When the size of the grains may significantly increase, as in a more dense region where GRBs would occur, the RG approximation may not be valid enough for modeling detailed observational data. In order to study the temporal and spectral properties of the scattered X-ray emission more accurately with potentially larger dust grains, we provide a practical approach using the series expansions of anomalous diffraction (AD) approximation based on the complicated Mie theory. We apply our calculations to understand the puzzling X-ray afterglow of recently observed GRB 130925A that showed a significant spectral softening. We find that the X-ray scattering scenarios with either AD or RG approximation adopted could well reproduce both the temporal and spectral profile simultaneously. Given the plateau present in the early X-ray light curve, a typical distribution of smaller grains as in the interstellar medium would be suggested for GRB 130925A.

Underlying Global Features of the X-ray Light Curves of Swift Gamma-ray Bursts

With its rapid response, Swift has revealed plenty of unexpected properties of gamma-ray bursts (GRBs). With an abundance of observations, our current understanding is only limited by the complexity of early X-ray light curves. In this work, based on the public Swift data of 150 well-monitored GRBs with measured redshifts, we find some interesting global features in the rest-frame X-ray light curves. The distinct spectral behaviors between the prompt emission and the afterglow emission imply dissimilar radiation scenarios. Interestingly, an unforeseen plateau is exhibited in the prompt X-ray light curves despite the presence of complex spikes, which might indicate the presence of a steady central engine. In particular, the seemingly continuous evolution with a single power law from the prompt to the afterglow of most GRBs might place strong constraints on the theoretical models.

A New Test of Lorentz Invariance Violation: The Spectral Lag Transition of GRB 160625B

Possible violations of Lorentz invariance (LIV) have been investigated for a long time using the observed spectral lags of gamma-ray bursts (GRBs). However, these generally have relied on using a single photon in the highest energy range. Furthermore, the search for LIV lags has been hindered by our ignorance concerning the intrinsic time lag in different energy bands. GRB 160625B, the only burst so far with a well-defined transition from

On the Late-Time Spectral Softening Found in X-ray Afterglows of Gamma-Ray Bursts

positive

GRB 131231A: IMPLICATIONS OF THE GeV EMISSION

lags to

On the Puzzle of Long and Short Gamma-Ray Bursts

negative

Cosmological Time Dilation in Durations of Swift Long Gamma-Ray Bursts

lags provides a unique opportunity to put new constraints on LIV. Using multi-photon energy bands we consider the contributions to the observed spectral lag from both the intrinsic time lag and the lag by LIV effects, and assuming the intrinsic time lag to have a positive dependence on the photon energy, we obtain robust limits on LIV by directly fitting the spectral lag data of GRB 160625B. Here we show that these robust limits on the quantum gravity energy scales are