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Register a new userAcceleration of GRB outflows by Poynting flux dissipation
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Authors
Georg Drenkhahn
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We study magnetically powered relativistic outflows in which a part of the magnetic energy is dissipated internally by reconnection. For GRB parameters, and assuming that the reconnection speed scales with the Alfven speed, significant dissipation can take place both inside and outside the photosphere of the flow. The process leads to a steady increase of the flow Lorentz factor with radius. With an analytic model we show how the efficiency of this process depends on GRB parameters. Estimates are given for the thermal and non-thermal radiation expected to be emitted from the photosphere and the optically thin part of the flow respectively. A critical parameter of the model is the ratio of Poynting flux to kinetic energy flux at some initial radius of the flow. For a large value (> 100) the non-thermal radiation dominates over the thermal component. If the ratio is small (< 40) only prompt thermal emission is expected which can be identified with X-ray flashes.
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We investigate the effects of magnetic energy release by local magnetic dissipation processes in Poynting flux-powered GRBs. For typical GRB parameters (energy and baryon loading) the dissipation takes place mainly outside the photosphere, producing non-thermal radiation. This process converts the total burst energy into prompt radiation at an efficiency of 10-50%. At the same time the dissipation has the effect of accelerating the flow to a large Lorentz factor. For higher baryon loading, the dissipation takes place mostly inside the photosphere, the efficiency of conversion of magnetic energy into radiation is lower, and an X-ray flash results instead of a GRB. We demonstrate these effects with numerical one-dimensional steady relativistic MHD calculations.
We investigate the production of the gamma-ray spectrum of a Poynting-flux dominated GRB ouflow. The very high magnetic field strengths (super-equipartition) in such a flow lead to very efficient synchrotron emission. In contrast with internal shocks, dissipation of magnetic energy by reconnection is gradual and does not produce the spectrum of cooling electrons associated with shock acceleration.We show that a spectrum with a break in the BATSE energy range is produced, instead, if the magnetic dissipation heats a small (sim 10^{-4}) population of electrons.
We investigate roles of magnetic activity in the Galactic bulge region in driving large-scale outflows of size $sim 10$ kpc. Magnetic buoyancy and breakups of channel flows formed by magnetorotational instability excite Poynting flux by the magnetic tension force. A three-dimensional global numerical simulation shows that the average luminosity of such Alfvenic Poynting flux is $10^{40} - 10^{41}$ erg s$^{-1}$. We examine the energy and momentum transfer from the Poynting flux to the gas by solving time-dependent hydrodynamical simulations with explicitly taking into account low-frequency Alfvenic waves of period of 0.5 Myr in a one-dimensional vertical magnetic flux tube. The Alfvenic waves propagate upward into the Galactic halo, and they are damped through the propagation along meandering magnetic field lines. If the turbulence is nearly trans-Alfv{e}nic, the wave damping is significant, which leads to the formation of an upward propagating shock wave. At the shock front, the temperature $gtrsim 5times 10^6$ K, the density $approx 6times 10^{-4}$ cm$^{-3}$, and the outflow velocity $approx 400-500$ km s$^{-1}$ at a height $approx 10$ kpc, which reasonably explain the basic physical properties of the thermal component of the Fermi bubbles.
Particle acceleration in magnetized relativistic jets still puzzles theorists, specially when one tries to explain the highly variable emission observed in blazar jets or gamma-ray bursts putting severe constraints on current models. In this work we investigate the acceleration of particles injected in a three-dimensional relativistic magnetohydrodynamical jet subject to current driven kink instability (CDKI), which drives turbulence and fast magnetic reconnection. Test protons injected in the nearly stationary snapshots of the jet, experience an exponential acceleration up to a maximum energy. For a background magnetic field of $B sim 0.1$ G, this saturation energy is $sim 10^{16}$ eV, while for $B sim 10$ G it is $sim 10^{18}$ eV. The simulations also reveal a clear association of the accelerated particles with the regions of fast reconnection. In the early stages of the development of the non-linear growth of CDKI in the jet, when there are still no sites of fast reconnection, injected particles are also efficiently accelerated, but by magnetic curvature drift in the wiggling jet spine. However, they have to be injected with an initial energy much larger than that required for particles to accelerate in reconnection sites. Finally, we have also obtained from the simulations an acceleration time due to reconnection with a weak dependence on the particles energy $E$, $t_A propto E^{0.1}$. The energy spectrum of the accelerated particles develops a high energy tail with a power law index $p sim$ -1.2 in the beginning of the acceleration, in agreement with earlier works. Our results provide an appropriate multi-dimensional framework for exploring this process in real systems and explain their complex emission patterns, specially in the very high energy bands and the associated neutrino emission recently detected in some blazars.
The ejecta composition is an open question in gamma-ray bursts (GRB) physics. Some GRBs possess a quasi-thermal spectral component in the time-resolved spectral analysis, suggesting a hot fireball origin. Others show a featureless non-thermal spectrum known as the Band function, consistent with a synchrotron radiation origin and suggesting that the jet is Poynting-flux-dominated at the central engine and likely in the emission region as well. There are also bursts showing a sub-dominant thermal component and a dominant synchrotron component suggesting a likely hybrid jet composition. Here we report an extraordinarily bright GRB 160625B, simultaneously observed in gamma-rays and optical wavelengths, whose prompt emission consists of three isolated episodes separated by long quiescent intervals, with the durations of each sub-burst being $sim$ 0.8 s, 35 s, and 212 s, respectively. Its high brightness (with isotropic peak luminosity L$_{rm p, iso}sim 4times 10^{53}$ erg/s) allows us to conduct detailed time-resolved spectral analysis in each episode, from precursor to main burst and to extended emission. The spectral properties of the first two sub-bursts are distinctly different, allowing us to observe the transition from thermal to non-thermal radiation between well-separated emission episodes within a single GRB. Such a transition is a clear indication of the change of jet composition from a fireball to a Poynting-flux-dominated jet.
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