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CORDIS - Resultados de investigaciones de la UE

Explosive phenomena in the Universe: Gamma-Ray Bursts and SuperNovaRemnants as high-energy particle acceleration sites

Final Report Summary - SHALOM (Explosive phenomena in the Universe: Gamma-Ray Bursts and SuperNovaRemnants as high-energy particle acceleration sites)

The acceleration of high energy particles at collisionless shocks in astrophysical plasmas is a common phenomenon in the Universe that takes place in a variety of systems, from the heliosphere up to the most distant cosmic sources. The goal of this project is to investigate the results of shock acceleration in two different classes of Astrophysical sources: Gamma-Ray Bursts (GRB) and SuperNova Remnants (SNR). Both GRBs and SNRs are the product of explosive stellar phenomena that inject matter into the surrounding medium. These expanding ejecta develop a forward shock that accelerates the particles present in the environment.
The project consists of two parts: i) acceleration of Cosmic Ray protons at SNRs shock and their diffusion into the Galaxy and ii) acceleration of electrons at GRBs' external shocks and the resulting radiative emission.

Standard propagation codes assume that the CR sources are distributed axisymmetrically within the Galaxy. However, SNR are more likely concentrated in the spiral arms. In the past years, researchers at the Host Institutions have shown that a source concentration in galactic arms (hereafter spiral model) has the potential to solve some inconsistency between the standard diffusion models and the observations. To this aim, A Monte Carlo code that simulates the propagation of CRs in the dynamic galactic disk, taking into account both the inhomogeneous distribution and the motion of the galactic arms relative to the Galaxy (and to the sun) has been developed.
The main aim of this project is to derive the implications of having a spiral arm distribution for the CR sources, derive implications for particle acceleration models, and compare model predictions with available observations.

The Researcher has focused her investigation on the implications of this model for gamma-ray observations and has compared the model predictions with observations. Gamma-ray radiation is the product of the decay of the neutral pion, a particle produced in interactions between the CR protons and ambient protons. The Researcher has considered the output results of the spiral arm propagation code (i.e. the spectrum of CR protons in each point of the Galaxy) and estimated the interaction with the ambient medium and the subsequent production of neutral pions. Then the local photon spectrum produced by the decay of neutral pions has been computed. The gamma-ray emission has then been integrated along different lines of sight. A map of the gamma-ray emission from neutral pion decay has been inferred. The decay of the neutral pion is expected to dominate the diffuse gamma-ray radiation at small latitudes (<10 degrees) and at low energies (<10 GeV). In this range of energies and latitudes the simulation results can then be compared with observations. A map of the spectral slope of the diffuse gamma-ray radiation is available thanks to observations by the Fermi/LAT. The spectral slope is seen to change considerably and exhibits clear trends as a function of the galactic longitudes and latitudes. A contribution from how outflows superimposed to neutral pion decay has been invoked to explain the presence of soft and hard regions in the observed map. Comparison between the model predictions derived in this project and Fermi/LAT observations have shown instead that in the context of a dynamical spiral model the main features present in the observed spectral map can be fully reproduced without the need to invoke an additional spectral component. The main trend visible in the observed spectral map are recovered in the simulated map and can be explained as due to the dynamics of the spiral arms (see the attached plot). The simulation results and comparison with observations will be presented in a paper currently in preparation (Nava, Benyamin, Shaviv & Piran, 2016).

GRB outflows drive a relativistic shock into the external medium, accelerating ambient particles. Particle acceleration at relativistic shocks is poorly understood. The basic picture usually considered in GRB studies assumes that a fraction \epsilon_e of the energy dissipated at the shock is given to all the swept-up electrons, which are accelerated into a power-law distribution. Collisionless shocks also amplify the magnetic field so that it contains a fraction epsilon_B of the energy dissipated at the shock. Shocked electrons in a strong magnetic field radiate their energy via synchrotron mechanism. This radiation powers a slowly fading broadband emission, identified as the afterglow radiation. Broad-band modelling of afterglow data allows to derive some clues about the properties of the acceleration mechanism. Our present knowledge on shock acceleration parameters, however, dates back to the first pioneering studies based on a small number of bursts. Since these first studies, the values epsilon_e=0.1 and \epsilon_B=0.01 have been taken as typical values and very few efforts to derive their distributions have been made, also because of the degeneracy between the several parameters entering the afterglow modeling.
In 2008 the Fermi satellite was launched, allowing the study of GRB at high energies and adding information to this picture. By now, more than 100 GRB have been detected by the Fermi/LAT (tens of MeV--300 GeV). In around half of these events the high-energy emission lasts much longer than the prompt. This temporally extended emission decays in time as a power-law. Its physical origin is still a matter of debate. The most promising model invokes emission from electrons accelerated at the external shock via synchrotron and inverse Compton radiation.

The project aims to 1) understand if the high-energy (GeV) radiation is originated at the shock with the external matter, if the dominant radiative process is synchrotron or Inverse Compton radiation, and if and how the proposed external shock model can explain observations of high-energy photons (>10GeV) detected within a few hundred seconds, 2) perform broadband (optical, X-ray and GeV) modelling of external shock (afterglow) radiation to infer the properties of the parameters describing the shock acceleration process, and 3) compare results inferred from afterglow modelling with predictions from theory of particle acceleration in collisionless relativistic shocks.

The Researcher has completed the proposed investigation. A code for modeling of the synchrotron and inverse Compton radiation from electrons accelerated at the external shock radiating in presence of an amplified magnetic field has been developed and used to perform modeling of broad band (from optical to GeV) data. The main results can be summarised as follows.

*Origin of the GeV radiation: data modelling has been successfully performed, demonstrating that synchrotron radiation from external shocks is a viable mechanism to produce the observed high-energy emission. The contribution from Inverse Compton radiation has been necessary in few cases (Nava et al 2014; Beniamini, Nava et al 2015).

*Fraction of energy in the accelerated electrons: a typical value of 10% is consistent with GeV observations and observations imply that its value must be narrowly distributed among different GRBs (Nava et al. 2014). A comparison with theoretical predictions shows that these findings are in agreement with recent findings from numerical simulations, which show that around 10% of the energy is stored in the accelerated electrons, independently on the properties of the external medium and on the fluid Lorentz factor.

*Fraction of energy in the amplified magnetic field: very small upper limits (from 10^-4 to 10^-6) have been inferred, in agreement with other recent investigations (Beniamini, Nava et al. 2014). The comparison with theoretical expectations has allowed to derive interesting conclusions. The magnetic field is expected to be large (0.1) just behind the shock and should decay rapidly at a given distance from the shock, down to shock compressed values of 10^-9. However the decay length is presently not constrained by theoretical arguments or numerical simulations. The results achieved in this project show that the magnetic field has considerably decayed on length scales where electrons emitting X-ray and optical radiation cool.