We present a systematic analysis of the spectral and temporal properties of 17 gamma-ray bursts (GRBs) co-detected by Gamma-Ray Monitor (GBM) and Large Area Telescope (LAT) on board the Fermi satellite by May 2010. We performed a timeresolved spectral analysis of all the bursts with the finest temporal resolution allowed by statistics, in order to reduce temporal smearing of different spectral components. We found that the time-resolved spectra of 14 out of 17 GRBs are best modeled with the classical "Band" function over the entire Fermi spectral range, which may suggest a common origin for emissions detected by LAT and GBM. GRB 090902B and GRB 090510 require the superposition of a MeV component and an extra power law component, with the former having a sharp cutoff above E p . For GRB 090902B, this MeV component becomes progressively narrower as the time bin gets smaller, and can be fit with a Planck function as the time bin becomes small enough. In general, we speculate that phenomenologically there may be three elemental spectral components that shape the time-resolved GRB spectra: a Band-function component (e.g. in GRB 080916C) that extends in a wide energy range and does not narrow with decreasing time bins, which may be of non-thermal origin; a quasi-thermal component (e.g. in GRB 090902B) with the spectra progressively narrowing with reducing time bins; and another non-thermal power law component extending to high energies. The spectra of different bursts may be decomposed into one or more of these elemental components. We compare this sample with the BATSE sample and investigate some correlations among spectral parameters. We discuss the physical implications of the data analysis results for GRB prompt emission, including jet composition (matter-dominated vs. Poyntingflux-dominated outflow), emission sites (internal shock, external shock or photosphere), -2as well as radiation mechanisms (synchrotron, synchrotron self-Compton, or thermal Compton upscattering).

X-ray afterglow light curves have been collected for over 400 Swift gamma-ray bursts (GRBs) with nearly half of them having X-ray flares superimposed on the regular afterglow decay. Evidence suggests that gamma-ray prompt emission and X-ray flares share a common origin and that at least some flares can only be explained by long-lasting central engine activity. We have developed a shell model code to address the question of how X-ray flares are produced within the framework of the internal shock model. The shell model creates randomized GRB explosions from a central engine with multiple shells and follows those shells as they collide, merge, and spread, producing prompt emission and X-ray flares. We pay special attention to the time history of central engine activity, internal shocks, and observed flares, but do not calculate the shock dynamics and radiation processes in detail. Using the empirical E p -E iso (Amati) relation with an assumed Band function spectrum for each collision and an empirical flare temporal profile, we calculate the gamma-ray (Swift/BAT band) and X-ray (Swift/XRT band) lightcurves for arbitrary central engine activity and compare the model results with the observational data. We show that the observed X-ray flare phenomenology can be explained within the internal shock model. The number, width, and occurring time of flares are then used to diagnose the central engine activity, putting constraints on the energy, ejection time, width, and number of ejected shells. We find that the observed X-ray flare time history generally reflects the time history of the central engine, which reactivates multiple times after the prompt emission phase with progressively reduced energy. The same shell model predicts an external shock X-ray afterglow component, which has a shallow decay phase due to the initial pile-up of shells onto the blast wave. However, the predicted X-ray afterglow is too bright as compared with the observed flux level, unless e is as low as 10 −3 .

Recent observations of Gamma‐Ray Bursts (GRBs) by the Fermi Large Area Telescope (LAT) revealed a power‐law decay feature of the high‐energy emission (above 100 MeV), which led to the suggestion that it originates from an external shock. We analyse four GRBs (080916C, 090510, 090902B and 090926A) jointly detected by Fermi LAT and Gamma‐ray Burst Monitor (GBM), which have high‐quality light curves in both instrument energy bands. Using the MeV prompt emission (GBM) data, we can record the energy output from the central engine as a function of time. Assuming a constant radiative efficiency, we are able to track energy accumulation in the external shock using our internal/external shell model code. By solving for the early evolution of both an adiabatic and a radiative blastwave, we calculate the high‐energy emission light curve in the LAT band and compare it with the observed one for each burst. The late time LAT light curves after T90 can be well fitted by the model. However, due to continuous energy injection into the blastwave during the prompt emission phase, the early external shock emission cannot account for the observed GeV flux level. The high‐energy emission during the prompt phase (before T90) is most likely a superposition of a gradually enhancing external shock component and a dominant emission component that is of an internal origin.

GRB 090926A was detected by both the Gamma-ray Burst Monitor and Large Area Telescope (LAT) instruments on board the Fermi Gamma-ray Space Telescope. Swift follow-up observations began ∼13 hr after the initial trigger. The optical afterglow was detected for nearly 23 days post trigger, placing it in the long-lived category. The afterglow is of particular interest due to its brightness at late times, as well as the presence of optical flares at T0+10 5 s and later, which may indicate late-time central engine activity. The LAT has detected a total of 16 gamma-ray bursts; nine of these bursts, including GRB 090926A, also have been observed by Swift. Of the nine Swift-observed LAT bursts, six were detected by UVOT, with five of the bursts having bright, long-lived optical afterglows. In comparison, Swift has been operating for five years and has detected nearly 500 bursts, but has only seen ∼30% of bursts with optical afterglows that live longer than 10 5 s. We have calculated the predicted gamma-ray fluence, as would have been seen by the Burst Alert Telescope (BAT) on board Swift, of the LAT bursts to determine whether this high percentage of long-lived optical afterglows is unique, when compared to BAT-triggered bursts. We find that, with the exception of the short burst GRB 090510A, the predicted BAT fluences indicate that the LAT bursts are more energetic than 88% of all Swift bursts and also have brighter than average X-ray and optical afterglows.

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