Results 1 - 10 of 13
Results 1 - 10 of 13. Search took: 0.025 seconds
|Sort by: date | relevance|
[en] The huge optical brightness of GRB 080319B (the 'Naked Eye Burst') makes this event really challenging for models of the prompt GRB emission. In the framework of the internal shock model, we investigate a scenario where the dominant radiative process is synchrotron emission and the high optical flux is due to the dynamical properties of the relativistic outflow : if the initial Lorentz factor distribution in the jet is highly variable, many internal shocks will form within the outflow at various radii. The most violent shocks will produce the main gamma-ray component while the less violent ones will contribute at lower energy, including the optical range.
[en] The association of GRB 980425 with the nearby supernova SN 1998bw at z 0.0085 implies the existence of a population of GRBs with an isotropic-equivalent luminosity which is about 104 times smaller than in classical cosmic GRBs. We investigate two scenarios to explain the peculiar properties of GRB 980425; a normal (intrinsically bright) GRB seen off-axis or an intrinsically weak GRB seen on-axis
[en] We compare the dynamics of the internal and reverse shocks in the standard scenario of GRBs. We show that the two series of shocks are very similar and should a priori contribute in the same energy range. If internal shocks (IS) are responsible far the gamma-ray emission and the reverse shock (RS) far the early optical signal, the post shock physical conditions must somehow differ between the two cases. We briefly discuss different possibilities far this to occur
[en] The Compton Cube is a 4π γ-ray telescope operating in the energy range of 100 keV-2 MeV onboard a microsatellite. Submitted in response to a CNES announcement of opportunity, this experiment makes use of the CdTe technology developed for ISGRI, the low-energy γ-ray camera of the IBIS telescope onboard INTEGRAL. It also takes advantage of the newly developed detectors with screening effect that offer an energy resolution better than 2% above 0.5 MeV. Spatial and spectral performances cooperate to provide a very compact experiment with very good performances in terms of sensitivity, spectral resolution and angular resolution. Thanks to its field of view, it is ideal for the study of transient events such as classical novae, γ-ray bursts, X-ray novae and AGNs. It offers also very long observing time in any direction, enabling an unbiased and high sensitivity mapping of the celestial sphere, particularly well suited to the study of the interstellar emission (lines and continuum) and the extragalactic background
[en] We present a mechanism based on internal self-annihilation of dark matter accreted from the galactic halo in the inner regions of neutron stars that may trigger full or partial conversion into a quark star. We explain how this effect may induce a gamma-ray burst (GRB) that could be classified as short, according to the usual definition based on time duration of the prompt gamma-ray emission. This mechanism differs in many aspects from the most discussed scenario associating short GRBs with compact object binary mergers. We list possible observational signatures that should help distinguish between these two possible classes of progenitors.
[en] Using a time-dependent numerical model in which the prompt gamma-ray burst (GRB) emission is calculated in the framework of the internal shock model on a broad energy range (from soft X-ray to GeV energies), we compute GRB lightcurves and time-evolving spectra. We show how the spectral evolution in this model is determined by the evolution of the physical conditions in the shocked regions and by the dominant radiative process for the effective microphysics parameters. The predictions of a model are confronted by the observations in the standard sub-MeV energy range, as well as with high energy bands observed by Fermi.
[en] This school, dedicated to young researchers, will clarify our present knowledge of the X-ray sky and give the opportunity to learn about the observatories and tools which are available. The contributions have been organized into 3 issues: -) fundamental physics, -) X-ray and Gamma-ray instruments and analysis techniques, and -) astrophysical objects. This document gathers only the slides of the presentations
[en] The most energetic gamma-ray bursts (GRBs) are remarkable sources releasing huge amounts of energy on short timescales. Their prompt emission, which usually lasts a few seconds, is so bright that it is visible across the whole observable universe. Studying these extreme events may provide clues on the nature of GRB progenitors and on the physical processes at work in relativistic jets. In this paper, we study the bright end of the isotropic energy distribution of long GRBs. We use two samples of long GRBs with redshift detected by Fermi/GBM or Konus-Wind, two instruments that measure the spectral shape and the energetics of the prompt emission accurately. We focus on GRBs within a range of redshifts z = 1–5, a volume that contains a large number of energetic GRBs, and we propose a simple method to reconstruct the bright end of the GRB energy distribution from the observed one. We find that the GRB energy distribution cannot be described by a simple power law but requires a strong cutoff above erg. We attribute this feature to an intrinsic limit on the energy per unit of solid angle radiated by GRBs.
[en] Over the past few years, evidence has been accumulated in support of the existence of a thermal-like component during the prompt phase of gamma-ray bursts (GRBs). However, this component, which is often associated with the GRB jet's photosphere, is usually subdominant compared to a much stronger non-thermal one. The prompt emission of GRB 131014A—detected by the Fermi Gamma-ray Space Telescope (hereafter Fermi)—provides a unique opportunity to trace the history of this thermal-like component. Indeed, the thermal emission in GRB 131014A is much more intense than in other GRBs and a pure thermal episode is observed during the initial 0.16 s. The thermal-like component cools monotonically during the first second while the non-thermal emission kicks off. The intensity of the non-thermal component progressively increases until being energetically dominant at late time, similar to what is typically observed. This is a perfect scenario to disentangle the thermal component from the non-thermal component. The initial decaying and cooling phase of the thermal-like component is followed by a strong re-brightening and a re-heating episode; however, despite a much brighter second emission phase, the temperature of the thermal component does not reach its initial value. This re-brightening episode is followed by a global constant cooling until the end of the burst. We note that there is a shallower low-energy spectral slope than the typical index value +1, corresponding to a pure Planck function, which better matches with the thermal-like spectral shape; a spectral index around +0.6 seems to be in better agreement with the data. The non-thermal component is adequately fitted with a Band function whose low- and high-energy power-law indices are ∼−0.7 and <∼−3, respectively; this is also statistically globally equivalent to a cutoff power law with a ∼−0.7 index. This is in agreement with our previous results. Finally, a strong correlation is observed between the time-resolved energy flux, and the corresponding spectral peak energy, of the non-thermal component with a slope similar to the one reported in our previous articles. Assuming a universal relation between the time-resolved luminosity of the non-thermal component, and its rest frame which we derived from a limited sample of GRBs detected by Fermi, we estimate a redshift of ∼1.55 for GRB 131014A, which is a typical value for long GRBs. These observational results are consistent with the models in which the non-thermal emission is produced well above the GRB jet photosphere but they may also be compatible with other scenarios (e.g., dissipative photosphere) that are not discussed in this article.
[en] We compute the expected GRB luminosity function in the internal shock model. We find that if the population of GRB central engines produces all kind of relativistic outflows, from very smooth to highly variable, the luminosity function has to branchs: at low luminosity, the distribution is dominated by low efficiency GRBs and is close to a power law of slope -0.5, whereas at high luminosity, the luminosity function follows the distribution of injected kinetic power. Using Monte Carlo simulations and several observational constrains (BATSE logN-logP diagram, peak energy distribution of bright BATSE bursts, fraction of XRFs in the HETE2 sample), we show that it is currently impossible to distinguish between a single power law or a broken power law luminosity function. However, when the second case is considered, the low-luminosity slope is found to be -0.6±0.2, which is compatible with the prediction of the internal shock model