Search

The flare activity that is observed in GRBs soon after the prompt emission with the XRT (0.3–10 KeV) instrument on Board of the Swift satellite is leading to important clues in relation to the physical characteristics of the mechanism generating the emission of energy in Gamma Ray Bursts. We will briefly refer to the results obtained with the recent analysis [1] and [2] and discuss the preliminary results we obtained with a new larger sample of GRBs [limited to early flares] based on fitting of the flares using the Norris 2005 profile. We find, in agreement with previous results, that XRT flares follow the main characteristics observed in [3] for the prompt emission spikes. The estimate of the flare energy for the subsample with redshift is rather robust and an attempt is made, using the redshisft sample, to estimate how the energy emitted in flares depends on time. We used a H0 = 70 km/s/Mpc, ΩΛ = 0.7, Ωm = 0.3 cosmology. [ABSTRACT FROM AUTHOR]

We performed the first systematic search for the minimum variability time scale between 0.3 and 10 keV studying the 28 brightest early (<3000 s) afterglows detected by Swift-XRT up to March 2008. We adopt the power spectrum analysis in the time domain: unlike the Fourier spectrum, this is suitable to study the rms variations at different time-scales. We find that early XRT afterglows show variability in excess of the Poissonian noise level on time-scales as short as ∼1 s (rest frame value), with the shortest tmin associated with the highest energy band. The γ—ray prompt emission of GRB 080319B shows a characteristic average variability time-scale var>∼1 s; this parameter undergoes a remarkable evolution during the prompt emission (BAT observation). [ABSTRACT FROM AUTHOR]

One of the most interesting discoveries of the Swift mission is the wide variety of X-ray behaviors observed in the early GRB afterglows. We selected a large sample of bright X-ray flares observed by XRT up to March 2008. The Norris et al. (2005) profile was adopted to characterize the morphological and timing properties of the flares in different energy bands, such as the peak, rise and decay times, the width and the asymmetry of each flare. These parameters were used to estimate other properties in a homogeneous way, such as the temporal lag between the same pulse seen at different energy bands, or the width of the flare as a function of energy, thus allowing us to derive important clues on the underlying physical mechanism and the connections with the central engine. [ABSTRACT FROM AUTHOR]

We investigated the ability of simple spectral models to describe the early afterglow emission of GRBs. We found that four spectra, belonging to the GRB060502A, GRB060729, GRB060904B, GRB061H0A prompt–afterglow transition phase, can be modeled neither by a single power–law nor by the Band model. Instead we find that the data present high–energy (>3 keV, in the observer frame) statistically significant excesses with respect to these models In all four cases, the deviations can be modeled well by adding either a second power law or a blackbody component to the usual synchrotron power law spectrum. Although the data do not allow an unequivocal physical interpretation, the importance of this analysis consists in showing that a simple power–law model or a Band model is insufficient to describe the X-ray spectra of a small homogeneous sample of GRBs at the end of their prompt phase [1]. [ABSTRACT FROM AUTHOR]