Spectral properties of Fermi/GBM gamma-ray bursts and the GeV emission detection rate with Fermi/LAT

With a sample of 58 Fermi/GBM GRBs detected before 2009 May, we compare the spectral properties of GBM GRBs with those detected by CGRO/BTASE and HETE-2. Our results show that the spectral index distributions are very consistent with those observed by BATSE. However, the E _p distribution is quite different from that observed with BATSE and HETE-2. The GBM GRBs tend to be softer than the BATSE sample, but harder than the HETE-2 sample. This may be due to the instrumental selection effects and artificial sample effect on the BATSE sample. The distribution of the pseudo redshifts derived from the luminosity indicator based on the Amati-relation shows rough consistency with the spectroscopic redshifts of Swift GRBs. We estimate the detection rate of GBM GRBs with LAT based on the observed spectrum in the GBM band, and the inferred burst ratio of LAT detection with over 5 photons to GBM detection is 6%, yielding a detection rate pf 12 GRBs/yr with over 5 photons in the 1–300 GeV band. This is roughly consistent with the results in the first half year of Fermi operation. The low detection rate compared with theoretical predictions is a key for revealing the radiation mechanisms and particle acceleration of the prompt gamma-rays.     

Correlation between pulse-width and peak luminosity of single-pulse long gamma-ray bursts

With a sample of 21 single-pulse GRBs with redshift measurement and a sample of 65 single-pulse GRBs with pseudo redshift estimated by the luminosity-spectral lag relation, we show that the peak luminosities of GRB pulses are anti-correlated with pulse width, indicating that longer-pulses tend to be dimmer in the gamma-ray band. The anti-correlation is accessible with an internal shock scenario.     

Luminosity-peak energy relation in the decay phases of gamma-ray burst pulses

Using time-resolved spectral data for a sample of 30 pulses in 27 bright GRBs detected with CGRO/BATSE, we investigate the luminosity-peak energy relation (L-E p relation) in the decay phases of these pulses. A tight L-E p relation is found for most of the pulses, but its power law index is various among pulses, which is normally distributed at 1.84±0.60(1σ) for the pulses in our sample, roughly consistent with the L-E p relation within a GRB and the isotropic gamma-ray energy-E p relation among GRBs. The large scatter of the power law index cannot be explained with both the statistical or observational effects and it may be an intrinsic feature, indicating that no universal L-E p relation would be expected among GRBs/pulses. This may strongly weaken the cosmological use of this relation.     

GRB 090423: Marking the death of a massive star at z=8.2

GRB 090423 is the new high-z record holder of Gamma-ray bursts (GRBs) with z～ 8.2. We present a detailed analysis of both the spectral and temporal features of GRB 090423 observed with Swift/BAT and Fermi/GBM. We find that the T ₉₀ observed with BAT in the 15–150 keV band is 13.2 s, corresponding to ～ 1.4 s at z=8.2. It once again gives rise to the issue of whether the progenitors of high-z GRBs are massive stars or mergers since the discovery of GRB 080913 at z=6.7. In comparison with the T ₉₀ distribution in the burst frame of the current redshift-known GRB sample, we find that it is marginally grouped into the long group (Type II GRBs). The spectrum observed with both BAT and GBM is well fitted by a power-law with exponential cutoff, which yields an E _p=(50.4±7.0) keV. The event satisfies the Amati-relation well for Type II GRBs within their 3σ uncertainty range. Our results indicate that this event would be produced by the death of a massive star. Based on the Amati-relation, we derive its distance modulus, which follows the Hubble diagram of the concordance cosmology model at a redshift of ～8.2.     

The flux-E p relation within GRB060218 in comparison with typical GRB pulses

The prompt gamma-ray/X-ray emission of gamma-ray burst (GRB) 060218 was simultaneously observed by the Burst Alert Telescope (BAT) and X-ray Telescope (XRT) onboard Swift. Its peak energy of the joint vf _v spectrum (E _p) clearly evolves with time from tens of keV to ∼1 keV, crossing both the BAT and XRT bands. The best fit yields log E _p=(4.61±0.23)+(−1.29±0.08) log t, with a correlation coefficient of 0.98 and a chance probability of p<10⁻⁴. We derive its bolometric flux (F) in the 0.01–10⁴ keV band, and find that its F-E _p relation, with a power-law index of 0.37, is much shallower than that observed in typical GRB pulses. Discussion of this shallowness is presented.

     

Gamma-ray bursts in the Swift-Fermi era: Confronting data with theory

With the prompt slewing capability of the X-ray and UV-optical telescopes onboard the Swift mission and with the gamma-ray large area telescope onboard the Fermi mission, gamma-ray bursts (GRBs) are now accessible in a full time window and in all electromagnetic wavelengths for the events. Many observational breakthroughs have been made in recent years. I present here a brief review of some observational breakthroughs with the two missions, focusing on how these breakthroughs have revolutionized our understanding of the nature of this phenomenon and puzzles as well as challenges of confronting the conventional models with data.     

X-ray flares of γ-ray bursts: Quakes of solid quark stars?

A star-quake model is proposed to understand X-ray flares of both long and short γ-ray bursts (GRBs) in a solid quark star regime. Two kinds of central engines for GRBs are available if pulsar-like stars are actually (solid) quark stars, i.e., the SNE-type GRBs and the SGR-type GRBs. It is found that a quark star could be solidified about 10³ to 10⁶ s later after its birth if the critical temperature of phase transition is a few Metga-electron-volts, and then a new source of free energy (i.e., elastic and gravitational ones, rather than rotational or magnetic energy) could be possible to power GRB X-ray flares. Supported by the National Natural Science Foundation of China (Grant Nos. 10573002, 10778611, and 10873002), the National Basic Research Program of China (Grant No. 2009CB824800), the Research Foundation of Guangxi University (Grant No. M30520), and the LCWR (Grant No. LHXZ200602)