926 research outputs found

    Afterglows from precursors in Gamma Ray Bursts. Application to the optical afterglow of GRB 091024

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    About 15% of Gamma Ray Bursts have precursors, i.e. emission episodes preceding the main event, whose spectral and temporal properties are similar to the main emission. We propose that precursors have their own fireball, producing afterglow emission due to the dissipation of the kinetic energy via external shock. In the time lapse between the precursor and the main event, we assume that the central engine is not completely turned off, but it continues to eject relativistic material at a smaller rate, whose emission is below the background level. The precursor fireball generates a first afterglow by the interaction with the external circumburst medium. Matter injected by the central engine during the "quasi-quiescent" phase replenishes the external medium with material in relativistic motion. The fireball corresponding to the main prompt emission episode crashes with this moving material, producing a second afterglow, and finally catches up and merges with the first precursor fireball. We apply this new model to GRB 091024, an event with a precursor in the prompt light curve and two well defined bumps in the optical afterglow, obtaining an excellent agreement with the existing data.Comment: 11 pages, 6 figures, 3 tables. Accepted for publication in MNRAS, Main Journa

    GRB 140206A: the most distant polarized Gamma-Ray Burst

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    The nature of the prompt gamma-ray emission of Gamma-Ray Bursts (GRBs) is still far from being completely elucidated. The measure of linear polarization is a powerful tool that can be used to put further constraints on the content and magnetization of the GRB relativistic outflows, as well as on the radiation processes at work. To date only a handful of polarization measurements are available for the prompt emission of GRBs. Here we present the analysis of the prompt emission of GRB 140206A, obtained with INTEGRAL/IBIS, Swift/BAT, and Fermi/GBM. Using INTEGRAL/IBIS as a Compton polarimeter we were able to constrain the linear polarization level of the second peak of this GRB as being larger than 28% at 90% c.l. We also present the GRB afterglow optical spectroscopy obtained at the Telescopio Nazionale Galileo (TNG), which allowed us the measure the distance of this GRB, z=2.739. This distance value together with the polarization measure obtained with IBIS, allowed us to derive the deepest and most reliable limit to date (xi <1x10-16) on the possibility of Lorentz Invariance Violation, measured through the vacuum birefringence effect on a cosmological source.Comment: 9 pages, 5 figures, 3 tables, accepted for publication in MNRAS. arXiv admin note: text overlap with arXiv:1303.418

    The faster the narrower: characteristic bulk velocities and jet opening angles of Gamma Ray Bursts

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    The jet opening angle theta_jet and the bulk Lorentz factor Gamma_0 are crucial parameters for the computation of the energetics of Gamma Ray Bursts (GRBs). From the ~30 GRBs with measured theta_jet or Gamma_0 it is known that: (i) the real energetic E_gamma, obtained by correcting the isotropic equivalent energy E_iso for the collimation factor ~theta_jet^2, is clustered around 10^50-10^51 erg and it is correlated with the peak energy E_p of the prompt emission and (ii) the comoving frame E'_p and E'_gamma are clustered around typical values. Current estimates of Gamma_0 and theta_jet are based on incomplete data samples and their observed distributions could be subject to biases. Through a population synthesis code we investigate whether different assumed intrinsic distributions of Gamma_0 and theta_jet can reproduce a set of observational constraints. Assuming that all bursts have the same E'_p and E'_gamma in the comoving frame, we find that Gamma_0 and theta_jet cannot be distributed as single power-laws. The best agreement between our simulation and the available data is obtained assuming (a) log-normal distributions for theta_jet and Gamma_0 and (b) an intrinsic relation between the peak values of their distributions, i.e theta_jet^2.5*Gamma_0=const. On average, larger values of Gamma_0 (i.e. the "faster" bursts) correspond to smaller values of theta_jet (i.e. the "narrower"). We predict that ~6% of the bursts that point to us should not show any jet break in their afterglow light curve since they have sin(theta_jet)<1/Gamma_0. Finally, we estimate that the local rate of GRBs is ~0.3% of all local SNIb/c and ~4.3% of local hypernovae, i.e. SNIb/c with broad-lines.Comment: 15 pages, 8 figures, 1 table. Accepted for publication in MNRA

    Bulk Lorentz factors of Gamma-Ray Bursts

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    Knowledge of the bulk Lorentz factor őď0\Gamma_{0} of GRBs allows us to compute their comoving frame properties shedding light on their physics. Upon collisions with the circumburst matter, the fireball of a GRB starts to decelerate, producing a peak or a break (depending on the circumburst density profile) in the light curve of the afterglow. Considering all bursts with known redshift and with an early coverage of their emission, we find 67 GRBs with a peak in their optical or GeV light curves at a time tpt_{\rm p}. For another 106 GRBs we set an upper limit tpULt_{\rm p}^{\rm UL}. We show that tpt_{\rm p} is due to the dynamics of the fireball deceleration and not to the passage of a characteristic frequency of the synchrotron spectrum across the optical band. Considering the tpt_{\rm p} of 66 long GRBs and the 85 most constraining upper limits, using censored data analysis methods, we reconstruct the most likely distribution of tpt_{\rm p}. All tpt_{\rm p} are larger than the time tp,gt_{\rm p,g} when the prompt emission peaks, and are much larger than the time tpht_{\rm ph} when the fireball becomes transparent. The reconstructed distribution of őď0\Gamma_0 has median value ‚ąľ\sim300 (150) for a uniform (wind) circumburst density profile. In the comoving frame, long GRBs have typical isotropic energy, luminosity, and peak energy ‚ü®Eiso‚ü©=3(8)√ó1050\langle E_{\rm iso}\rangle=3(8)\times 10^{50} erg, ‚ü®Liso‚ü©=3(15)√ó1047\langle L_{\rm iso}\rangle=3(15) \times 10^{47} erg s‚ąí1^{-1} , and ‚ü®Epeak‚ü©=1(2)\langle E_{\rm peak}\rangle =1(2) keV in the homogeneous (wind) case. We confirm that the significant correlations between őď\Gamma and the rest frame isotropic energy (EisoE_{\rm iso}), luminosity (LisoL_{\rm iso}) and peak energy (EpeakE_{\rm peak}) are not due to selection effects. Assuming a typical opening angle of 5 degrees, we derive the distribution of the jet baryon loading which is centered around a few 10‚ąí6M‚äô10^{-6} {\rm M_{\odot}}.Comment: 19 pages, 11 figures, 6 tables. Accepted for publication on Astronomy & Astrophysic

    Prompt optical emission as a signature of synchrotron radiation in gamma-ray bursts

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    Information on the spectral shape of prompt emission in gamma-ray bursts (GRB) is mostly available only at energies greater than or similar to 10 keV, where the main instruments for GRB detection are sensitive. The origin of this emission is still very uncertain because of the apparent inconsistency with synchrotron radiation, which is the most obvious candidate, and the resulting need for considering less straight-forward scenarios. The inclusion of data down to soft X-rays (similar to 0.5 keV), which are available only in a small fraction of GRBs, has firmly established the common presence of a spectral break in the low-energy part of prompt spectra, and even more importantly, the consistency of the overall spectral shape with synchrotron radiation in the moderately fast-cooling regime, the low-energy break being identified with the cooling frequency. In this work we further extend the range of investigation down to the optical band. In particular, we test the synchrotron interpretation by directly fitting a theoretically derived synchrotron spectrum and making use of optical to gamma-ray data. Secondly, we test an alternative model that considers the presence of a black-body component at similar to keV energies, in addition to a non-thermal component that is responsible for the emission at the spectral peak (100 keV-1 MeV). We find that synchrotron radiation provides a good description of the broadband data, while models composed of a thermal and a non-thermal component require the introduction of a low-energy break in the non-thermal component in order to be consistent with optical observations. Motivated by the good quality of the synchrotron fits, we explore the physical parameter space of the emitting region. In a basic prompt emission scenario we find quite contrived solutions for the magnetic field strength (5G &lt; B' &lt; 40 G) and for the location of the region where the radiation is produced (R-gamma &gt; 10(16) cm). We discuss which assumptions of the basic model would need to be relaxed in order to achieve a more natural parameter space
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