Fermi Gamma-ray Space Telescope successfully detected high-energy emission from 20 GRBs so far. Thanks to its unprecedented very wide energy coverage of Large Area Telescope (LAT: from 25MeV to > 300 GeV) and Gammaray

Burst Monitor (GBM: from 8 keV to 40 MeV), Fermi provided new observational pictures of GRBs. Here we review some of the GRB properties seen by the LAT instrument such as the delayed onset and longer durations of high-energy emission

compared with low-energy emission of the GBM. An extra spectral component in high and low energy is detected in some GRBs and moreover for the first time a cut-off in the spectral extra component is seen. These temporal and spectral distinct behaviors inspire many implications on the emission mechanism, including leptonic, hadronic and afterglow origin. Fermi also placed constraints both on the

bulk Lorentz factor of the relativistic jet, larger than 1000 for bright LAT GRBs, and on outside-GRB topics such as quantum gravity

Moretti, E.

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DOI 10.1393/ncc/i2011-10880-3Colloquia: Scineghe2010IL NUOVO CIMENTO Vol. 34 C, N. 3 Maggio-Giugno 2011Observations of GRBs with the Fermi Large Area TelescopeE. Moretti on behalf of the Fermi GBM/LAT CollaborationsRoyal Institute of Technology, AlbaNova University Center - 106 91 Stockholm, SwedenConsorzio Interuniversitario per la Fisica Spaziale, Viale Settimio Severo - 10133 Torino, Italy(ricevuto il 25 Febbraio 2011; pubblicato online il 18 Maggio 2011)Summary. — Fermi Gamma-ray Space Telescope successfully detected high-energyemission from 20 GRBs so far. Thanks to its unprecedented very wide energycoverage of Large Area Telescope (LAT: from 25MeV to > 300GeV) and Gamma-ray Burst Monitor (GBM: from 8 keV to 40MeV), Fermi provided new observationalpictures of GRBs. Here we review some of the GRB properties seen by the LATinstrument such as the delayed onset and longer durations of high-energy emissioncompared with low-energy emission of the GBM. An extra spectral component inhigh and low energy is detected in some GRBs and moreover for the first timea cut-off in the spectral extra component is seen. These temporal and spectraldistinct behaviors inspire many implications on the emission mechanism, includingleptonic, hadronic and afterglow origin. Fermi also placed constraints both on thebulk Lorentz factor of the relativistic jet, larger than 1000 for bright LAT GRBs,and on outside-GRB topics such as quantum gravity.PACS 98.70.Rz – γ-ray sources; γ-ray bursts.1. – IntroductionThe Gamma-Ray Bursts (GRBs) were first discovered by the Vela satellites in 1969.Since then many discoveries were made, but still there is no clear evidence on the GRBprogenitors and the emission mechanisms that can produce high-energy photons up totens of GeV.The high-energy detector EGRET on board of the CGRO satellite detected 5 GRBswith energy > 50MeV seen also by the other detector on board: BATSE. Two importantdiscoveries were made: a very late emission of high-energy photons (up to 90 min) inthe GRB 940217 [1] and a spectral extra component at energies > 100MeV lasted formore than 200 s in the GRB 941017 [2]. Both these properties are inconsistent with apure electron synchrotron model. More recently, also the Astro-rivelatore Gamma adImmagini LEggero (AGILE) detected four GRBs with energy > 30MeV: 080514B [3]with a high-energy delayed component, 090401B (GCN 9069), 090510 [4] which showsan extended high-energy emission and the brightest 100224B (GCN 1096).c© Societa` Italiana di Fisica 261262 E. MORETTI on behalf of the FERMI GBM/LAT COLLABORATIONSFig. 1. – Left: multi-detector light curve of GRB 080916C. The insets are a zoom of the first 15seconds after the GBM trigger. Right: time-resolved spectral fits in the 5 time bins. Both thedelayed onset (time bin a) and the long-lived emission (time bin d) are present in other burstsdetected by Fermi-LAT.Eight years after the end of the CGRO mission, Fermi was launched on June 11th2008. The Fermi/LAT instrument has an effective area 10 times larger than the EGRETone and together with a larger field of view and a smaller dead time it collects morestatistics to perform a detailed temporal-spectral analysis [5]. The possibility to re-point itself autonomously when a bright GRB happens and the synergy with the GBMinstrument dedicated to the GRBs monitoring, make the LAT a perfect instrument toinvestigate the problems opened by EGRET on the GRB high-energy emission.In the first two years of science operations more than 500 bursts were detected withthe Gamma-ray Burst Monitor also on board of the Fermi satellite. Among those, 18were detected also by the LAT instrument (as of August 2010), both long and shortwith at least 10 photons > 100MeV, but only few GRBs have also photons at energies> 1GeV.2. – Properties from the Fermi-LAT dataThanks to the Fermi-LAT large field of view and its timing capabilities a new featureof GRB was discovered: a delayed onset of the > 100MeV component. In almost all thebursts the delay onset is clearly detected as shown in fig. 1. In the first 3 seconds of GRB080916C the light curve from the GBM detectors shows a first peak, instead there are nophotons in the LAT light curve, when the > 100MeV “transient” [5] events are selected.In this first time interval also the spectrum shows a different behavior with respect theothers time bins as shown in fig. 1 (right panel) there is a soft-to-hard evolution frombin a to bin b, but a hard-to-soft evolution later on [6].OBSERVATIONS OF GRBs WITH THE FERMI LARGE AREA TELESCOPE 263Fig. 2. – The time-integrated and time-resolved spectra for GRB 090510 [8]. Panel (a): best-fitspectral model (Band+PL) for the time-integrated spectrum. Panel (b): spectral evolution.The spectral extra component is clearly present in the time-integrated spectrum and all but one(the first time bin) the time-resolved spectra.The first GBM peak is missing in the LAT > 100MeV events also in the short GRB081024B (GCN 8407). The emission in the GBM lasts for less than 1 second and theLAT data still shows a signal 3 seconds after the trigger. As for the burst 080916C thespectrum of this first peak has a softer high-energy spectral index with respect to laterspectra and the spectra of the later emission are hardening [7].A feature the EGRET GRBs already showed is the presence of an extra componentin the spectrum, so far among the LAT sample only thee bursts clearly show it, for twoit is statistically disfavored, while for the other bursts the extra-component hypothesisis not statistically significant.The GRBs 090510 (GCN 9334) and 090902 (GCN 9867) are, respectively, a short anda long burst but they have very similar characteristics. With more than 150 photonsabove 100MeV and more than 20 above 1GeV, they are two of the three brightest GRBsseen so far by the Fermi/LAT. For both the highest-energy photon is ∼ 30GeV butemitted at different times: in the short one, this photon comes in the first second whenthe GBM emission is still present, while for the long burst, this photon is received 82seconds after the trigger when the GBM emission is no more present. As well as otherbursts also these two have an extended emission lasting longer than the GBM emission,but the feature that is clearly detected in these bursts is the spectral extra componentsshown in fig. 2. This component is present not only in the highest energies (> 100MeV),but also in the lowest GBM energy band (8–20 keV) [8, 9].A cut-off of the extra component was not measured so far, but the GRB 090926Ashows a clear cut-off at 1.4GeV with 5σ significance. This long GRB is one of thethree brightest bursts detected by the LAT with more than 150 photons with energy> 100MeV and more than 50 photons with energy grater than 1GeV. If the cut-off isdue to the changing of the optical depth and pair production, this lets the first directmeasurement of Γ of ∼ 630 [10].The extended emission is another GRB property identified by EGRET and confirmedfrom most of the Fermi/LAT GRBs. It is still present after several tens of secondsfrom the trigger when the low-energy component is returned to the background level.This property is not ubiquitous for all the bursts: for example 081215 (GCN 8684)264 E. MORETTI on behalf of the FERMI GBM/LAT COLLABORATIONSand 090217 (GCN 8903) do not show this feature. For the first one, 081215, not manystudies were possible, because it was detected at large incident angle and no spatial orspectroscopic information are therefore available. The “featureless” burst 090217 insteadhas a temporal coincidence between the low energy detected by GBM and the high energyin the LAT data, as well as it shows no spectral extra component and no photons above1GeV [11]. The GRBs 090323 (GCN 9021) and 090328 (GCN 9077), on the contrary,show a very long-lasting high-energy tail still present 3 hours after the trigger. Thestudies on these bursts will be presented into an upcoming paper.3. – ConclusionAfter a few months from its launch, the LAT telescope has detected around 10 gamma-ray bursts per year above 100MeV. The high-energy emission is, indeed, not commonfor GRBs: the long ones have a fluence from few percent to few tens percent in the band100MeV–10GeV with respect to the band from 20 keV to 2MeV. There are commontrends in the bursts observed so far, some already seen by EGRET and some discoveredby Fermi/LAT. We believe that the understanding of these properties is needed for theoverall understanding of gamma-ray burst phenomena. As proposed in [6], the delayedonset can occur if the pulses originate in two distinct physical regions, with differentphysical conditions. In the framework of internal shocks, this naturally happens as thetwo emission episodes are related to different sets of shells. This could be explained asan additional hard component that arises later and lasts longer than the GBM emission.This component emerges distinctly in the late part of the LAT signal, where the GBMflux has decreased below detectability. The existence of an extra component was firmlydetected by Fermi in both long and short bursts, suggesting analogies between these twoclasses of objects. Different interpretations span from synchrotron emission from inter-nal shocks [12], to Self-Synchrotron Compton in internal shocks [12], or to high-energyemission from an external shock [12-14]. Hadronic models are also proposed [15], as wellas thermal emission from the jet photosphere combined with non-thermal emission [16].Temporally extended emission also appears to be common in LAT gamma-ray bursts,suggesting that the high-energy emission mechanism lasts longer than previously be-lieved. Future detections will enlarge our high-energy GRB sample, this will allow usto study deeply the properties illustrated and maybe to discover new ones. These stud-ies on the detected GRBs might or not confirm the proposed acceleration and emissionmechanisms, providing new elements to distinguish among those.∗ ∗ ∗The Fermi LAT Collaboration acknowledges support from a number of agenciesand institutes for both development and the operation of the LAT as well as scien-tific data analysis. These include NASA and DOE in the United States, CEA/Irfu andIN2P3/CNRS in France, ASI and INFN in Italy, MEXT, KEK, and JAXA in Japan,and the K. A. Wallenberg Foundation, the Swedish Research Council and the NationalSpace Board in Sweden. Additional support from INAF in Italy and CNES in Francefor science analysis during the operations phase is also gratefully acknowledged.REFERENCES[1] Hurley K. et al., Nature, 372 (1994) 652.[2] Gonzalez M. et al., Nature, 424 (2003) 749.OBSERVATIONS OF GRBs WITH THE FERMI LARGE AREA TELESCOPE 265[3] Giuliani A. et al., Astron. Astrophys., 491 (2008) L25.[4] Giuliani A. et al., arXiv:0908.1908 (2009).[5] Atwood W. B. et al., Astrophys. J., 697 (2009) 1071.[6] Abdo A. et al., Science, 323 (2009) 1688.[7] Abdo A. et al., Astrophys. J., 712 (2010) 558.[8] Abdo A. A. et al., Nature, 462 (2009) 331.[9] Abdo A. A. et al., Astrophys. J. Lett., 706 (2009) L138.[10] Ackermann M. et al., Astrophys. J., 729 (2011) L114.[11] Ackermann M. et al., Astrophys. J. Lett., 717 (2010) L127.[12] Corsi A., Guetta D. and Piro L., Astrophys. J., 720 (2010) 1008.[13] Ghirlanda G., Ghisellini G. and Nava L., Astron. Astrophys., 510L (2010) 7G.[14] Gao W.-H., Mao J., Xu D. and Fan Y.-Z., Astrophys. J. Lett., 706 (2009) L33.[15] Asano K., Guiriec S. and Me´sza´ros P., Astrophys. J. Lett., 705 (2009) L191.[16] Ryde F. et al., Astrophys. J., 709L (2010) 172R.

Observations of GRBs with the Fermi Large Area Telescope

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Observations of GRBs with the Fermi Large Area Telescope

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