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.
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 T90 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 T90 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 Ep=(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.
LIN Lin1,LIANG EnWei2 & ZHANG ShuangNan3 1 Department of Physics and Center for Astrophysics,Tsinghua University,Beijing 100084,China
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 Ep 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.
LV HouJun,LIANG EnWei & TONG XiaoFeng Department of Physics,Guangxi University,Nanning 530004,China
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 transi- tion 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.
XU RenXin1 & LIANG EnWei2 1 School of Physics and State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China
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.
LU RuiJing & LIANG EnWei Physical Science & Technology College, Guangxi University, Nanning 530004, China
Swift GRB 100418A is a long burst at z = 0.624 without detection of any associated supernova （SN）. Its light curves in both the prompt and afterglow phases are similar to GRB 060614, a nearby long GRB without an associated SN. We analyze the observational data of this event and discuss the possible origins of its multiwavelength emission. We show that its joint light curve at 1 keV derived from Swift BAT and XRT observations is composed of two distinguished components. The first component, whose spectrum is extremely soft （Γ = 4.32）, ends with a steep decay segment, indicating the internal origin of this component. The second component is a slowly-rising, broad bump which peaks at ～ 10 5 s post the BAT trigger. Assuming that the late bump is due to onset of the afterglow, we derive the initial Lorentz factor （Γ 0 ） of the GRB fireball and find that it significantly deviates from the relation between the Γ 0 and isotropic gamma-ray energy derived from typical GRBs. We also check whether it follows the same anti-correlation between X-ray luminosity and the break time observed in the shallow decay phase of many typical GRBs, which is usually regarded as a signal of late energy injection from the GRB central engine. However, we find that it does not obey this correlation. We propose that the late bump could be contributed by a two-component jet. We fit the second component with an off-axis jet model for a constant medium density and find the late bump can be represented by the model. The derived jet half-opening angle is 0.30 rad and the viewing angle is 0.315 rad. The medium density is 0.05 cm 3 , possibly suggesting that it may be from a merger of compact stars. The similarity between GRBs 060614 and 100418A may indicate that the two GRBs are from the same population and the late bump observed in the two GRBs may be a signal of a two-component jet powered by the GRB central engine.
