The first two decades of GRBs studies (Hurley 1989) were led by observations from different mis-sions such as the Konus (Venera) experiment (Aptekar et al. 1995),Ginga (Swinbanks 1987) or Solar Maximum Mission (Bohlin et al. 1980). The observed variability of the light curves and high-energy emission component of the GRBs pointed towards a compact source as a possible progenitor. The observed isotropic distribution of the GRBs (Mazets et al. 1981) suggested an ex-tragalactic origin (e.g., Hakkila et al. 1994; Briggs et al. 1996) instead of a galactic one (e.g., Atteia et al. 1987). The non-thermal nature of the spectrum was associated with a dominant synchrotron emission2 and secondary radiation effects such as inverse-Compton radiation (e.g., Golenetskii et al. 1983; Fenimore et al. 1988). The spectrum is described by the Band Function (Cline et al.
1973; Band et al. 1993). This function is a combination of a power-law and an exponential law joined at a specific transition energy. The peak of the function is observed in the sub-MeV energy range. Finally, a temporal bimodal distribution of the GRBs was established. The distribution is based onT90, i.e., the time it takes for 90% of the total fluence to be detected. GRBs are classified into long (LGRB,T90>2 s) and short (SGRB,T90<2 s) (Kouveliotou et al. 1993).
1GeV emission has been detected for a few burst by theFermisatellite.
2David Yu, PhD Thesis 2016, TUM
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1.1 Overview
In 1991 theCompton Gamma-Ray Observatory(CGRO, Fishman 1992) was launched. CGRO had 4 instruments on board: the Energetic Gamma-Ray Experiment Telescope EGRET had an improved sensitivity (>10 times) compared to other instruments operating in the same energy range (200 MeV - 10 GeV). This allowed the detection of the hardest GRBs and, for the first time a detection of GeV emission. TheBurst and Transient Experiment BATSE (1 keV - 1 MeV) was used to detect, localise and measure the energy of the GRBs. BATSE observed more than 2700 GRBs that were used to produce the first homogenous and unbiased GRB sample. Due to the high quality of the data and high statistics of the sample, it was used to confirmed some of the main properties of GRBs previously suggested. The sample confirmed the high variability (Fig. 1.1) and the lack of periodicity of the GRB light curves.
Figure 1.1:Light curves of 2 GRBs observed by BATSE (Fishman & Meegan 1995). The horizontal axes is in seconds and the vertical axes is in 103counts/s.
The non-thermal nature of the GRB spectra was confirmed by a spectral study based on the BATSE sample and observations from EGRET, theComptel Telescopeand theOriented Scintilla-tion Spectrometer(OSSE). The spectrum was confirmed to be described by the band function (e.g., Fig. 1.2a) with its peak energy at around a few MeV, and it was observed to be harder towards high energies (Band et al. 1993). The bimodal distribution of the GRBs (Kouveliotou et al. 1993) was confirmed as seen in Fig. 1.2b.
(a)Spectral energy distribution of GRB 910503 (b) Bimodal distribution of GRB based onT90.
Figure 1.2: Left: GRB 910503 detected by CGRO. The spectrum is described by the Band function with the peak energy in the MeV range (Schaefer et al. 1994; Fishman & Meegan 1995).Right: Histogram with the bimodal distribution of GRBs based on the durationT90(Kouveliotou et al. 1993).
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1.1 Overview The BATSE sample proved the isotropic angular distribution (Fig. 1.3, Meegan et al. 1992) and the in-homogeneity on the intensity distribution3 (Fenimore et al. 1993; Mao & Mo 1998) of the GRBs. This was the first unambiguous proof against a galactic origin of the GRBs. It was supported by results from independent studies such as the first evidence of time delation (Nemiroff 1994; Wijers & Paczynski 1994; Davis et al. 1994). After the confirmation of the extragalactic origin (Usov & Chibisov 1975; van den Bergh 1983) further studies were based mainly on cosmo-logical theories (e.g., Meszaros et al. 1993; Fenimore et al. 1993; Rees & Meszaros 1994).
Figure 1.3:Spatial isotropic distribution of a 2704 GRBs detected by BATSE (Michael S. Briggs 2014).
CGRO opened a new era of GRB science after the confirmation of their cosmological origin.
However, the mechanisms responsible for the gamma-ray emission had not been understood yet and there had been no detection of the fading multi-wavelength radiation (afterglow; e.g., Paczyn-ski & Rhoads 1993; Mészáros & Rees 1997) that was predicted to follow the gamma-ray (prompt) emission. The first X-ray counterpart of a GRB (GRB 960720, Piro et al. 1996) was detected in July 20 1996, by the recently launched italian-dutch satelliteBeppoSAX (Boella et al. 1997).
The improved accuracy in the position of the source (∼1 arcmin) was an important step to allow ground-based follow-up observations of the GRB afterglows. On February 27, 1998 BeppoSAX detected the X-ray counterpart of the GRB 970228 (Fig. 1.4a; Costa et al. 1997; van Paradijs et al.
1997), however, the observations were not deep enough to uniquely associate this host galaxy to the GRB. On May 8th 1997, the counterpart of GRB 970508 was observed in a multi-wavelength range (e.g., Frail et al. 1997; Djorgovski et al. 1997; Galama et al. 1998a; Bremer et al. 1998).
A break in the light curve (LC), known as a jet break, was observed associated to the collimated nature of the outflow (Rhoads 1999; Sari et al. 1999). Furthermore, the determination of the red-shift of the host galaxy was possible (z=0.835, Metzger et al. 1997) strengthening the theory of cosmological origin of the GRBs.
3Deviation of the relation between intensity (I) and number of sources (N) from the expected one in an Euclidean space (N∝I−3/2)
1.1 Overview
(a)LC of GRB 970228. (b)Spectrum GRB 030329 and SN 1998bw.
Figure 1.4: Left: Light curve of GRB 979228 detected by BeppoSAX and observed later in the optical wavelength range (Wijers et al. 1997).Right: Optical afterglow spectrum of GRB 030329. The comparison with the spectrum of SN 1998bw shows the GRB-SN connection (Stanek et al. 2003).
So far, long GRBs have been associated with the deaths of a massive stars collapsing into black holes (BH), while short GRBs are associated with mergers of neutron stars (NS) and either other NS or BH. In both cases, long and short GRBs, the accretion disk around the final BH is thought to give rise to the ultra-relativistic collimated outflow (jet). The spectra of the long-GRBs and the afterglows are non-thermal spectra associated with synchrotron emission from Fermi accelerated electrons (Rees & Meszaros 1994; Nemiroff 1994). The predicted connection of the long-GRBs to core collapse supernovae (e.g., Woosley 1993) had the first evidence from the observations of the afterglow of GRB 980425 (Woosley 1993; Galama et al. 1998b) and the supernova SN1998bw. A stronger confirmation of this GRB-SN connection was obtained from observations of GRB 030329 with the High Energy Transient Explorer II - HETE II satellite (Eichler et al. 2010) and the super-nova SN2003dh (Hjorth et al. 2003; Stanek et al. 2003). The follow-up of the afterglow of GRB 030329 with ground-based telescopes led to the measurement of its Lorentz factorΓat late times confirming the ultra-relativistic nature of the outflows (e.g., Paczynski 1986).
In 2004 theSwiftsatellite (Gehrels et al. 2004) was launched with three instruments on board:
the Burst Alert Telescope (BAT, Barthelmy et al. 2005), the X-ray Telescope (XRT) and the UV-Optical Telescope (UVOT, Roming et al. 2005). BAT detects the GRB and measures its position with an accuracy of 2 - 3 arcmin. Seconds after this detectionSwiftslews to the position of the GRB provided by BAT and starts the observations with the XRT and UVOT. These instruments obtain an accurate measurement of the afterglow energy and an enhanced position of the GRB with an accuracy of a few arcsec. The fast communication between the satellite and the ground-based stations allows the observations of the early light curve evolution in a multi-wavelength range.
These early afterglow observations set the first basis for a different origin between the GRB prompt emission and the afterglow. The detailed structure of the new sample of X-ray light curves (Zhang et al. 2006) of GRB afterglows presents a late decay in agreement with theoretical predictions.
They also have a break associated to a jet break and the collimated nature of the outflow and plateau phases (Nousek et al. 2006; Racusin et al. 2009). The detection of the afterglow of short
1.2 Current state