Radiative signature of magnetic fields in internal shocks

Common models of blazars and gamma-ray bursts assume that the plasma underlying the ob- served phenomenology is magnetized to some extent. Within this context, radiative signatures of dissipation of kinetic and conversion of magnetic energy in internal shocks of relativistic magnetized outflows are studied. We model internal shocks as being caused by collisions of homogeneous plasma shells. We compute the flow state after the shell interaction by solving Riemann problems at the contact surface between the colliding shells, and then compute the emission from the resulting shocks. Under the assumption of a constant flow luminosity we find that there is a clear difference between the models where both shells are weakly magne- tized ({\sigma}<\sim0.01) and those where, at least, one shell has a {\sigma}>\sim0.01. We obtain that the radiative efficiency is largest for models in which, regardless of the ordering, one shell is weakly and the other strongly magnetized. Substantial differences between weakly and strongly magne- tized shell collisions are observed in the inverse-Compton part of the spectrum, as well as in the optical, X-ray and 1GeV light curves. We propose a way to distinguish observationally between weakly magnetized from magnetized internal shocks by comparing the maximum frequency of the inverse-Compton and synchrotron part of the spectrum to the ratio of the inverse-Compton and synchrotron fluence. Finally, our results suggest that LBL blazars may correspond to barely magnetized flows, while HBL blazars could correspond to moderately magnetized ones. Indeed, by comparing with actual blazar observations we conclude that the magnetization of typical blazars is {\sigma} <\sim 0.01 for the internal shock model to be valid in these sources.Comment: 15 pages, 11 figures, accepted for publication in MNRA

Efficiency of internal shocks in magnetized relativistic jets

We study the dynamic and radiative efficiency of conversion of kinetic-to-thermal/magnetic energy by internal shocks in relativistic magnetized outflows. A parameter study of a large number of collisions of cylindrical shells is performed. We explore how, while keeping the total flow luminosity constant, the variable fluid magnetization influences the efficiency and find that the interaction of shells in a mildly magnetized jet yields higher dynamic, but lower radiative efficiency than in a non-magnetized flow. A multi-wavelength radiative signature of different shell magnetization is computed assuming that relativistic particles are accelerated at internal shocks.Comment: 4 pages, 2 figures, proceedings of the meeting "HEPRO III: High Energy Phenomena in Relativistic Outflows" (Barcelona, June 2011), fixed the bibliography error

Internal shocks in relativistic outflows: collisions of magnetized shells

(Abridged): We study the collision of magnetized irregularities (shells) in relativistic outflows in order to explain the origin of the generic phenomenology observed in the non-thermal emission of both blazars and gamma-ray bursts. We focus on the influence of the magnetic field on the collision dynamics, and we further investigate how the properties of the observed radiation depend on the strength of the initial magnetic field and on the initial internal energy density of the flow. The collisions of magnetized shells and the radiation resulting from these collisions are calculated using the 1D relativistic magnetohydrodynamics code MRGENESIS. The interaction of the shells with the external medium prior to their collision is also determined using an exact solver for the corresponding 1D relativistic magnetohydrodynamic Riemann problem. Our simulations show that two magnetization parameters - the ratio of magnetic energy density and thermal energy density, \alpha_B, and the ratio of magnetic energy density and mass-energy density, \sigma - play an important role in the pre-collision phase, while the dynamics of the collision and the properties of the light curves depend mostly on the magnetization parameter \sigma. The interaction of the shells with the external medium changes the flow properties at their edges prior to the collision. For sufficiently dense shells moving at large Lorentz factors (\simgt 25) these properties depend only on the magnetization parameter \sigma. Internal shocks in GRBs may reach maximum efficiencies of conversion of kinetic into thermal energy between 6% and 10%, while in case of blazars, the maximum efficiencies are \sim 2%.Comment: 17 pages, 18 figures. 2 new references have been added. Accepted for publication in Astronomy and Astrophysic

GRB afterglow light curves from realistic density profiles

The afterglow emission that follows gamma-ray bursts (GRBs) contains valuable information about the circumburst medium and, therefore, about the GRB progenitor. Theoretical studies of GRB blast waves, however, are often limited to simple density profiles for the external medium (mostly constant density and power-law R^{-k} ones). We argue that a large fraction of long-duration GRBs should take place in massive stellar clusters where the circumburst medium is much more complicated. As a case study, we simulate the propagation of a GRB blast wave in a medium shaped by the collision of the winds of O and Wolf-Rayet stars, the typical distance of which is d /sim 0.1 - 1 pc. Assuming a spherical blast wave, the afterglow light curve shows a flattening followed by a shallow break on a timescale from hours up to a week after the burst, which is a result of the propagation of the blast wave through the shocked wind region. If the blast wave is collimated, the jet break may, in some cases, become very pronounced with the post break decline of the light curve as steep as t-5. Inverse Compton scattering of ultra-violet photons from the nearby star off energetic electrons in the blast wave leads to a bright \simGeV afterglow flare that may be detectable by Fermi.Comment: 7 pages, 7 figures, submitted to MNRA

On the dynamic efficiency of internal shocks in magnetized relativistic outflows

We study the dynamic efficiency of conversion of kinetic-to-thermal/magnetic energy of internal shocks in relativistic magnetized outflows. We model internal shocks as being caused by collisions of shells of plasma with the same energy flux and a non-zero relative velocity. The contact surface, where the interaction between the shells takes place, can break up either into two oppositely moving shocks (in the frame where the contact surface is at rest), or into a reverse shock and a forward rarefaction. We find that for moderately magnetized shocks (magnetization $\sigma\simeq 0.1$), the dynamic efficiency in a single two-shell interaction can be as large as 40%. Thus, the dynamic efficiency of moderately magnetized shocks is larger than in the corresponding unmagnetized two-shell interaction. If the slower shell propagates with a sufficiently large velocity, the efficiency is only weakly dependent on its Lorentz factor. Consequently, the dynamic efficiency of shell interactions in the magnetized flow of blazars and gamma-ray bursts is effectively the same. These results are quantitatively rather independent on the equation of state of the plasma. The radiative efficiency of the process is expected to be a fraction $f_r<1$ of the estimated dynamic one, the exact value of $f_r$ depending on the particularities of the emission processes which radiate away the thermal or magnetic energy of the shocked states.Comment: Accepted for publication in MNRAS. 8 pages, 6 figures. The definitive version is available at http://www.blackwell-synergy.co

A method for computing synchrotron and inverse-Compton emission from hydrodynamic simulations of supernova remnants

The observational signature of supernova remnants (SNRs) is very complex, in terms of both their geometrical shape and their spectral properties, dominated by non-thermal synchrotron and inverse-Compton scattering. We propose a post-processing method to analyse the broad-band emission of SNRs based on three-dimensional hydrodynamical simulations. From the hydrodynamical data, we estimate the distribution of non-thermal electrons accelerated at the shock wave and follow the subsequent evolution as they lose or gain energy by adiabatic expansion or compression and emit energy by radiation. As a first test case, we use a simulation of a bipolar supernova expanding into a cloudy medium. We find that our method qualitatively reproduces the main observational features of typical SNRs and produces fluxes that agree with observations to within a factor of a few. allowing for further use in more extended sets of models.Comment: 15 pages, 3 figures; accepted, HEDLA 2014 special issue of High Energy Density Physic

Multiwavelength afterglow light curves from magnetized GRB flows

We use high-resolution relativistic MHD simulations coupled with a radiative transfer code to compute multiwavelength afterglow light curves of magnetized ejecta of gamma-ray bursts interacting with a uniform circumburst medium. The aim of our study is to determine how the magnetization of the ejecta at large distance from the central engine influences the afterglow emission, and to assess whether observations can be reliably used to infer the strength of the magnetic field. We find that, for typical parameters of the ejecta, the emission from the reverse shock peaks for magnetization $\sigma_0 \sim 0.01 - 0.1$ of the flow, and that it is greatly suppressed for higher $\sigma_0$. The emission from the forward shock shows an achromatic break shortly after the end of the burst marking the onset of the self-similar evolution of the blast wave. Fitting the early afterglow of GRB 990123 and 090102 with our numerical models we infer respective magnetizations of $\sigma_0 \sim 0.01$ and $\sigma_0 \sim 0.1$ for these bursts. We argue that the lack of observed reverse shock emission from the majority of the bursts can be understood if \sigma_0 \simmore 0.1, since we obtain that the luminosity of the reverse shock decreases significantly for $\sigma_0 \sim 1$. For ejecta with \sigma_0 \simmore 0.1 our models predict that there is sufficient energy left in the magnetic field, at least during an interval of ~10 times the burst duration, to produce a substantial emission if the magnetic energy can be dissipated (for instance, due to resistive effects) and radiated away.Comment: 9 pages, 9 figures. Submitted to MNRAS