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Register a new userBlack-body components in Gamma-Ray Bursts spectra?
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Added by
Gabriele Ghisellini
Publication date
2007
fields
Physics
and research's language is
English
Authors
G. Ghirlanda
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We study 7 Gamma Ray Bursts (GRBs), detected both by the BATSE instrument, on-board the Compton Gamma Ray Observatory, and by the Wide Field Camera (WFC), on-board BeppoSAX. These bursts have measured spectroscopic redshifts and are a sizeable fraction of the bursts defining the correlation between the peak energy E_peak (i.e. the peak of the vFv spectrum) and the total prompt isotropic energy E_iso (the so called Amati relation). Recent theoretical interpretations of this correlation assume that black-body emission dominates the time resolved spectra of GRBs, even if, in the time integrated spectrum, its presence may be hidden by the change of its temperature and by the dilution of a possible non-thermal power law component. We perform a time resolved spectral analysis, and show that the sum of a power-law and a black-body gives acceptable fits to the time dependent spectra within the BATSE energy range, but overpredicts the flux in the WFC X-ray range. Moreover, a fit with a cutoff power-law plus a black-body is consistent with the WFC data, but the black-body component contributes a negligible fraction of the total flux. On the contrary, we find that fitting the spectra with a Band model or a simple cutoff power-law model yields an X-ray flux and spectral slope which well matches the WFC spectra.
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We study the structure and evolution of the hyperaccreting disks and outflows in the gamma ray bursts central engines. The torus around a stellar mass black hole is composed of free nucleons, Helium, electron-positron pairs, and is cooled by neutrino emission. Accretion of matter powers the relativistic jets, responsible for the gamma ray prompt emission. The significant number density of neutrons in the disk and outflowing material will cause subsequent formation of heavier nuclei. We study the process of nucleosynthesis and its possible observational consequences. We also apply our scenario to the recent observation of the gravitational wave signal, detected on September 14th, 2015 by the two Advanced LIGO detectors, and related to an inspiral and merger of a binary black hole system. A gamma ray burst that could possibly be related with the GW150914 event was observed by the Fermi satellite. It had a duration of about 1 second and appeared about 0.4 seconds after the gravitational-wave signal. We propose that a collapsing massive star and a black hole in a close binary could lead to the event. The gamma ray burst was powered by a weak neutrino flux produced in the star remnants matter. Low spin and kick velocity of the merged black hole are reproduced in our simulations. Coincident gravitational-wave emission originates from the merger of the collapsed core and the companion black hole.
Principal component analysis is a statistical method, which lowers the number of important variables in a data set. The use of this method for the bursts spectra and afterglows is discussed in this paper. The analysis indicates that three principal components are enough among the eight ones to describe the variablity of the data. The correlation between spectral index alpha and the redshift suggests that the thermal emission component becomes more dominant at larger redshifts.
The radiation from afterglows of gamma-ray bursts (GRB) is generated in collisionless plasma shocks. The two main ingredients behind the radiation are high-energy, non-thermal electrons and a strong magnetic field. I argue that in order to make the right conclusions about gamma-ray burst and afterglow parameters from observations, it is crucial to have a firm understanding of the microphysics of collisionless shock. I present the results of self-consistent, three-dimensional particle-in-cell computational simulations of the collision of weakly magnetized plasma shells: The experiments show how a plasma instability generates a magnetic field in the shock. The field has strength up to percents of the equipartition value. The experiments also reveal a new, non-thermal electron acceleration mechanism that differs substantially from Fermi acceleration. Finally, I present the results from a new numerical tool that enables us to extract synthetic radiation spectra directly from the experiments. The preliminary results differ from synchrotron radiation but are consistent with GRB afterglow observations. I conclude that strong magnetic field generation, non-thermal particle acceleration and the emission of radiation that is consistent with GRB afterglow observations, are all unavoidable consequences of the collision between two relativistic plasma shells.
Prompt extra power-law (PL) spectral component is discovered in some bright gamma-ray bursts (GRBs), which usually dominates the spectral energy distribution below tens of keV or above about 10 MeV. However, its origin is still unclear. In this paper, we present a systematic analysis 13 Fermi short GRBs as of August 2020, with the contemporaneous keV-MeV and GeV detections at the prompt emission phase. We find that the extra PL component is a ubiquitous spectral feature for short GRBs, showing up in all 13 analyzed GRBs. The PL indices are mostly harder than -2.0, which may be well reproduced by considering the electromagnetic cascade induced by ultra-relativistic protons or electrons accelerated in the prompt emission phase. The average flux of these extra PL components positively correlates with that of the main spectral components, which implies they may share the same physical origin.
Timing analysis is a powerful tool with which to shed light on the still obscure emission physics and geometry of the prompt emission of GRBs. Fourier power density spectra (PDS) characterise time series as stochastic processes and can be used to search for coherent pulsations and to investigate the dominant variability timescales. Because of the limited duration and of the statistical properties, modelling the PDS of individual GRBs is challenging, and only average PDS of large samples have been discussed in the literature. We characterise the individual PDS of GRBs in terms of a stochastic process, and carry out for the first time a systematic search for periodic signals and for a link between the PDS and other observables. We present a Bayesian procedure that uses a Markov chain Monte Carlo technique and apply it to study 215 bright long GRBs detected with the Swift Burst Alert Telescope from January 2005 to May 2015. The PDS are modelled with a power-law either with or without a break. Two classes of GRBs emerge: with or without a unique dominant timescale. A comparison with active galactic nuclei (AGNs) reveals similar distributions of PDS slopes. Unexpectedly, GRBs with subsecond-dominant timescales and duration longer than a few tens of seconds in the source frame appear to be either very rare or altogether absent. Three GRBs are found with possible evidence for a periodic signal at ~3 sigma (Gaussian) significance, corresponding to a multitrial chance probability of ~1%. Thus, we found no compelling evidence for periodic signals. The analogy between the PDS of GRBs and of AGNs could tentatively indicate similar stochastic processes that rule BH accretion across different BH mass scales and objects. In addition, we find evidence that short dominant timescales and duration are not completely independent of each other, in contrast with commonly accepted paradigms (abridged).
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