It is now more than 40 years since the discovery of gamma-ray bursts (GRBs) and in the last two decades there has been major progress in the observations of bursts, the afterglows and their host galaxies. This recent progress has been fueled by the a
bility of gamma-ray telescopes to quickly localise GRBs and the rapid follow-up observations with multi-wavelength instruments in space and on the ground. A total of 674 GRBs have been localised to date using the coded aperture masks of the four gamma-ray missions, BeppoSAX, HETE II, INTEGRAL and Swift. As a result there are now high quality observations of more than 100 GRBs, including afterglows and host galaxies, revealing the richness and progress in this field. The observations of GRBs cover more than 20 orders of magnitude in energy, from 10^-5 eV to 10^15 eV and also in two non-electromagnetic channels, neutrinos and gravitational waves. However the continuation of progress relies on space based instruments to detect and rapidly localise GRBs and distribute the coordinates.
(Abridged) GRB 061122 is one of the brightest GRBs detected within INTEGRALs field of view to date. The two gamma-ray detectors on INTEGRAL were used to investigate the spectral characteristics of GRB 061122. A search for linear polarisation in the p
rompt emission was carried out using the SPI multiple event data in the energy range 100 keV-1 MeV. The prompt spectrum was best fit by a combination of a blackbody and a power--law model (the quasithermal model), with evidence for high energy emission continuing above 8 MeV. A pseudo-redshift value of pz = 0.95 +/- 0.18 was determined using the spectral fit parameters. The isotropic energy at this pseudo-redshift is 8.5 x 10^{52} erg. The jet opening angle was estimated to be smaller than 2.8 deg or larger than 11.9 deg from the X-ray lightcurve. An upper limit of 60% polarisation was determined for the prompt emission of GRB 061122, using the multiple event data. The high energy emission observed in the spectrum may be due to the reverse shock interacting with the GRB ejecta when it is decelerated by the circumburst medium. This behaviour has been observed in a small fraction of GRBs to date, but is expected to be more commonly observed by the Fermi Gamma-ray Space Telescope. The conditions for polarisation are met if the jet opening angle is less than 2.8 deg, but further constraints on the level of polarisation are not possible.
INTEGRAL has observed 47 long-duration GRBs (T_90 > 2s) and 1 short-duration GRB (T_90 < 2s) in five years of observation since October 2002. This work presents the properties of the prompt emission of GRB 070707, which is the first short hard GRB ob
served by INTEGRAL. The spectral and temporal properties of GRB 070707 were determined using the two sensitive coded-mask gamma-ray instruments on board INTEGRAL, IBIS and SPI. The T_90 duration was 0.8s, and the spectrum of the prompt emission was obtained by joint deconvolution of IBIS and SPI data to yield a best fit power-law with photon index alpha = -1.19 +0.14 -0.13, which is consistent with the characteristics of short-hard gamma-ray bursts. The peak flux over 1 second was 1.79 photons/cm^2/s and the fluence over the same interval was 2.07 x 10^-7 erg/cm^2 in the energy range 20-200keV. The spectral lag measured between 25-50keV and 100-300keV is 20 +/- 5ms, consistent with the small or negligible lags measured for short bursts. The spectral and temporal properties of GRB 070707 are comparable to those of the short hard bursts detected by other gamma-ray satellites, including BATSE and Swift. We estimate a lower limit on the Lorentz factor Gamma >~ 25 for GRB 070707, assuming the typical redshift for short GRBs of z=0.35. This limit is consistent with previous estimates for short GRBs and is smaller than the lower limits of Gamma >~ 100 calculated for long GRBs. If GRB 070707 is a member of the recently postulated class of short GRBs at z ~ 1, the lower limit on Gamma increases to Gamma >~ 35.
INTEGRAL has two sensitive gamma-ray instruments that have detected 46 gamma-ray bursts (GRBs) up to July 2007. We present the spectral, spatial, and temporal properties of the bursts in the INTEGRAL GRB catalogue using data from the imager, IBIS, an
d spectrometer, SPI. Spectral properties of the GRBs are determined using power-law, Band model and quasithermal model fits to the prompt emission. Spectral lags, i.e. the time delay in the arrival of low-energy gamma-rays with respect to high-energy gamma-rays, are measured for 31 of the GRBs. The photon index distribution of power-law fits to the prompt emission spectra is consistent with that obtained by Swift. The peak flux distribution shows that INTEGRAL detects proportionally more weak GRBs than Swift because of its higher sensitivity in a smaller field of view. The all-sky rate of GRBs above ~0.15 ph cm^-2 s^-1 is ~1400 yr^-1 in the fully coded field of view of IBIS. Two groups are identified in the spectral lag distribution, one with short lags <0.75 s (between 25-50 keV and 50-300 keV) and one with long lags >0.75 s. Most of the long-lag GRBs are inferred to have low redshifts because of their long spectral lags, their tendency to have low peak energies and their faint optical and X-ray afterglows. They are mainly observed in the direction of the supergalactic plane with a quadrupole moment of Q=-0.225+/-0.090 and hence reflect the local large-scale structure of the Universe. The rate of long-lag GRBs with inferred low luminosity is ~25% of Type Ib/c supernovae. Some of these bursts could be produced by the collapse of a massive star without a supernova or by a different progenitor, such as the merger of two white dwarfs or a white dwarf with a neutron star or black hole, possibly in the cluster environment without a host galaxy.
