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Register a new userThe Minimum Variability Time Scale and its Relation to Pulse Profiles of Fermi GRBs
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We present a direct link between the minimum variability time scales extracted through a wavelet decomposition and the rise times of the shortest pulses extracted via fits of 34 Fermi GBM GRB light curves comprised of 379 pulses. Pulses used in this study were fitted with log-normal functions whereas the wavelet technique used employs a multiresolution analysis that does not rely on identifying distinct pulses. By applying a corrective filter to published data fitted with pulses we demonstrate agreement between these two independent techniques and offer a method for distinguishing signal from noise.
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We have investigated the time variations in the light curves from a sample of long and short Fermi/GBM Gamma ray bursts (GRBs) using an impartial wavelet analysis. The results indicate that in the source frame, the variability time scales for long bursts differ from that for short bursts, that variabilities on the order of a few milliseconds are not uncommon, and that an intriguing relationship exists between the minimum variability time and the burst duration.
It has been suggested that the prompt emission in gamma-ray bursts consists of several components giving rise to the observed spectral shape. Here we examine a sample of the 8 brightest, single pulsed {it Fermi} bursts whose spectra are modelled by using synchrotron emission as one of the components. Five of these bursts require an additional photospheric component (blackbody). In particular, we investigate the inferred properties of the jet and the physical requirements set by the observed components for these five bursts, in the context of a baryonic dominated outflow, motivated by the strong photospheric component. We find similar jet properties for all five bursts: the bulk Lorentz factor decreases monotonously over the pulses and lies between 1000 and 100. This evolution is robust and can neither be explained by a varying radiative efficiency nor a varying magnetisation of the jet assuming the photosphere radius is above the coasting radius. Such a behaviour challenges several dissipation mechanisms, e.g., the internal shocks. Furthermore, in all 8 cases the data clearly reject a fast-cooled synchrotron spectrum (in which a significant fraction of the emitting electrons have cooled to energies below the minimum injection energy), inferring a typical electron Lorentz factor of $10^4 - 10^7$. Such values are much higher than what is typically expected in internal shocks. Therefore, while the synchrotron scenario is not rejected by the data, the interpretation does present several limitations that need to be addressed. Finally, we point out and discuss alternative interpretations.
We used a sample of GRBs detected by Fermi and Swift to reanalyze the correlation discovered by Amati et al. (2002) between Epi, the peak energy of the prompt GRB emission, and Eiso, the energy released by the GRB assuming isotropic emission. This correlation has been disputed by various authors, and our aim is to assess whether it is an intrinsic GRB property or the consequence of selection effects. We constructed a sample of Fermi GRBs with homogeneous selection criteria, and we studied their distribution in the Epi-Eiso plane. Our sample is made of 43 GRBs with a redshift and 243 GRBs without a redshift. We show that GRBs with a redshift follow a broad Epi-Eiso relation, while GRBs without a redshift show several outliers. We use these samples to discuss the impact of selection effects associated with GRB detection and with redshift measurement. We find that the Epi-Eiso relation is partly due to intrinsic GRB properties and partly due to selection effects. The lower right boundary of the Epi-Eiso relation stems from a true lack of luminous GRBs with low Epi. In contrast, the upper left boundary is attributed to selection effects acting against the detection GRBs with low Eiso and large Epi that appear to have a lower signal-to-noise ratio. In addition, we demonstrate that GRBs with and without a redshift follow different distributions in the Epi-Eiso plane. GRBs with a redshift are concentrated near the lower right boundary of the Epi-Eiso relation. This suggests that it is easier to measure the redshift of GRBs close to the lower Epi-Eiso boundary. In this context, we attribute the controversy about the reality of the Amati relation to the complex nature of this relation resulting from the combination of a true physical boundary and biases favoring the detection and the measurement of the redshift of GRBs located close to this boundary.
Studying the GRBs gamma-ray spectra may reveal some physical information of Gamma-ray Bursts. The Fermi satellite observed more than two thousand GRBs. The FERMIGBRST catalog contains GRB parameters (peak energy, spectral indices, intensity) estimated for both the total emission (fluence), and the emission during the interval of the peak flux. We found a relationship with linear discriminant analysis between the spectral categories and the model independent physical data. We compared the Swift and Fermi spectral types. We found a connection between the Fermi fluence spectra and the Swift spectra but the result of the peak flux spectra can be disputable. We found that those GRBs which were observed by both Swift and Fermi can similarly discriminate as the complete Fermi sample. We concluded that the common observation probably did not mean any trace of selection effects in the spectral behavior of GRBs.
Time-resolved spectroscopy is performed on eight bright, long gamma-ray bursts (GRBs) dominated by single emission pulses that were observed with the {it Fermi Gamma-ray Space Telescope}. Fitting the prompt radiation of GRBs by empirical spectral forms such as the Band function leads to ambiguous conclusions about the physical model for the prompt radiation. Moreover, the Band function is often inadequate to fit the data. The GRB spectrum is therefore modeled with two emission components consisting of optically thin nonthermal synchrotron radiation from relativistic electrons and, when significant, thermal emission from a jet photosphere, which is represented by a blackbody spectrum. To produce an acceptable fit, the addition of a blackbody component is required in 5 out of the 8 cases. We also find that the low-energy spectral index alpha is consistent with a synchrotron component with alpha = -0.81pm 0.1. This value lies between the limiting values of alpha = -2/3 and alpha = -3/2 for electrons in the slow and fast-cooling regimes, respectively, suggesting ongoing acceleration at the emission site. The blackbody component can be more significant when using a physical synchrotron model instead of the Band function, illustrating that the Band function does not serve as a good proxy for a nonthermal synchrotron emission component. The temperature and characteristic emission-region size of the blackbody component are found to, respectively, decrease and increase as power laws with time during the prompt phase. In addition, we find that the blackbody and nonthermal components have separate temporal behaviors.
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