Fu-Wen Zhang

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.

HIGH-ENERGY EMISSION OF GRB 130427A: EVIDENCE FOR INVERSE COMPTON RADIATION

A nearby superluminous burst GRB 130427A was simultaneously detected by six gamma-ray space telescopes (Swift, the Fermi GLAST Burst Monitor (GBM)/Large Area Telescope, Konus-Wind, SPI-ACS/INTEGRAL, AGILE, and RHESSI) and by three RAPTOR full-sky persistent monitors. The isotropic gamma-ray energy release is similar to 10(54) erg, rendering it the most powerful explosion among gamma-ray bursts (GRBs) with a redshift z 1 TeV neutrinos from GRB 130427A by IceCube are discussed.

The Photospheric Radiation Model for the Prompt Emission of Gamma-Ray Bursts: Interpreting Four Observed Correlations

We show that the empirical E-p-L, Gamma-L, E-p-Gamma, and (eta) over bar (gamma)-E-p correlations (where L is the time-averaged luminosity of the prompt emission, E-p is the spectral peak energy, Gamma is the bulk Lorentz factor, and (eta) over bar (gamma) is the emission efficiency of gamma-ray bursts, GRBs) are well consistent with the relations between the analogous parameters predicted in the photospheric radiation model of the prompt emission of GRBs. The time-resolved thermal radiation of GRB 090902B does follow the E-p-L and Gamma-L correlations. A reliable interpretation of the four correlations in alternative models is still lacking. These may point toward a photospheric origin of prompt emission of some GRBs.

GRB 120422A: A LOW-LUMINOSITY GAMMA-RAY BURST DRIVEN BY A CENTRAL ENGINE

GRB 120422A is a low-luminosity gamma-ray burst (GRB) associated with a bright supernova, which distinguishes itself by its relatively short T-90 (similar to 5 s) and an energetic and steep-decaying X-ray tail. We analyze the Swift Burst Alert Telescope and X-ray Telescope data and discuss the physical implications. We show that the steep decline early in the X-ray light curve can be interpreted as the curvature tail of a late emission episode around 58-86 s, with a curved instantaneous spectrum at the end of the emission episode. Together with the main activity in the first similar to 20 s and the weak emission from 40 s to 60 s, the prompt emission is variable, which points to a central engine origin in contrast to a shock-breakout origin, which is used to interpret some other nearby low-luminosity supernova GRBs. Both the curvature effect model and interpreting the early shallow decay as the coasting external forward shock emission in a wind medium provide a constraint on the bulk Lorentz factor Gamma to be around several. Comparing the properties of GRB 120422A and other supernova GRBs, we find that the main criterion to distinguish engine-driven GRBs from shock-breakout GRBs is the time-averaged gamma-ray luminosity. Engine-driven GRBs likely have a luminosity above similar to 10(48) erg s(-1).

Dependence of temporal properties on energy in long-lag, wide-pulse gamma-ray bursts

We employed a sample by Norris et al. (2005, ApJ, 625, 324) to study the dependence of the pulse temporal properties on energy in long-lag, wide-pulse gamma-ray bursts. Our analysis shows that the pulse peak time, rise time scale, and decay time scale are power-law functions of energy, which is a preliminary report on the relationships between the three quantities and energy. The power-law indexes associated with the pulse width, rise time scale, and decay time scale are correlated, and the correlation between the indexes associated with the pulse width and the decay time scale is more obvious. In addition, we have found that the pulse peak lag is strongly correlated with the CCF lag, but the centroid lag is less correlated with the peak lag and the CCF lag. Based on these results and some previous investigations, we tend to believe that all energy-dependent pulse temporal properties may come from the joint contribution of both the hydrodynamic processes of the outflows and the curvature effect, where the energy-dependent spectral lag may be mainly dominated by the dynamic process, and the energy-dependent pulse width may be mainly determined by the curvature effect.

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.

Broadband light curve characteristics of GRBs 980425 and 060218 and comparison with long-lag, wide-pulse GRBs

It has been recently argued that low-luminosity gamma-ray bursts (LL-GRBs) are likely a unique GRB population. Here, we present systematic analysis of the light-curve characteristics from X-ray to gamma-ray energy bands for the two prototypical LL-GRBs, 980425 and 060218. It is found that both the pulse width (w) and the ratio of the rising width to the decaying width (r/d) of theses two bursts are energy-dependent over a broad energy band. There exists a significant trend that the pulses tend to be narrower and more symmetrical at higher energy bands for the two events. Both the X-rays and the gamma-rays follow the same w-E and r/d-E relations. These facts may indicate that the X-ray emission tracks the gamma-ray emission, and both are likely to originate from the same physical mechanism. Their light curves show significant spectral lags. We calculate the three types of lags with the pulse peaking time (t(peak)), the pulse centroid time (t(cen)), and the cross-correlation function (CCF). The derived t(peak) and t(cen) are power-law functions of energy. The lag calculated by CCF is strongly correlated with that derived from tpeak. However, the lag derived from tcen correlates less with that derived from tpeak and CCF. The energy dependence of the lags is shallower at higher energy bands. These characteristics are consistent with that observed in typical long-lag, wide-pulse GRBs, suggesting that GRBs 980425 and 060218 may share a similar radiation physics with them.

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.

Gamma-ray bursts: the isotropic-equivalent-energy function and the cosmic formation rate

Gamma-ray bursts (GRBs) are brief but intense emissions of soft gamma-rays, mostly lasting from a few to a few thousand seconds. For these types of high-energy transients, the isotropic-equivalent-energy (Eiso) function may be more scientifically meaningful than the GRB isotropic-equivalent-luminosity (Liso) function, as the traditional luminosity function refers to steady emission much longer than a few thousand seconds. In this work we construct, for the first time, the isotropic-equivalent-energy function for a sample of 95 bursts with measured redshifts (z), and find an excess of high-z GRBs. Assuming that the excess is caused by a GRB luminosity function evolution in a power-law form, we find a cosmic evolution of , which is comparable to that between Liso and z, namely (both 1s). The evolution-removed isotropic-equivalent-energy function can be reasonably fitted by a broken power law, in which the dim and bright segments are and , respectively (1s). The cosmic GRB formation rate increases quickly in the region of , remains approximately constant for , and finally decreases with a power index of -3.80 +/- 2.16 for z?4, in good agreement with the cosmic star formation rate observed to date.

Relationship between pulse width and energy in GRB 060124: from X-ray to gamma-ray bands

GRB 060124 is the first event that both prompt and afterglow emission were observed simultaneously by the three Swift instruments. Its main peak also triggered Konus-Wind and HETE-II. Therefore, investigation on both the temporal and spectral properties of the prompt emission can be extended to X-ray bands. We perform a detailed analysis on the two well identified pulses of this burst, and find that the pulses are narrower at higher energies, and both X-rays and gamma-rays follow the same w-E relation for an individual pulse. However, there is no a universal power-law index of the w-E relation among pulses. We find also that the rise-to-decay ratio r/d seems not to evolve with E and the r/d values are well consistent with that observed in typical GRBs. The broadband spectral energy distribution also suggests that the X-rays are consistent with the spectral behavior of the gamma-rays. These results indicate that the X-ray emission tracks the gamma-ray emission and the emissions in the two energy bands are likely to be originated from the same physical mechanism. (c) 2008 Elsevier B.V. All rights reserved.