Gamma-Ray Bursts From Magnetic Reconnection: Variability and Robustness of Light Curves

Abstract

The dissipation mechanism that powers gamma-ray bursts (GRBs) remains uncertain almost half a century after their discovery. The two main competing mechanisms are the extensively studied internal shocks and the less studied magnetic reconnection. Here we consider GRB emission from magnetic reconnection accounting for the relativistic bulk motions that it produces in the jetʼs bulk rest frame. Far from the source the magnetic field is almost exactly normal to the radial direction, suggesting locally quasi-spherical thin reconnection layers between regions of oppositely directed magnetic field. We show that if the relativistic motions in the jetʼs frame are confined to such a quasi-spherical uniform layer, then the resulting GRB light curves are independent of their direction distribution within this layer. This renders previous results for a delta-function velocity-direction distribution applicable to a much more general class of reconnection models, which are suggested by numerical simulations. Such models that vary in their velocity-direction distribution differ mainly in the size of the bright region that contributes most of the observed flux at a given emission radius or observed time. The more sharply peaked this distribution, the smaller this bright region, and the stronger the light curve variability that may be induced by deviations from a uniform emission over the thin reconnection layer, which may be expected in a realistic GRB outflow. This is reflected both in the observed image at a given observed time and in the observer-frame emissivity map at a given emission radius, which are calculated here for three simple velocity-direction distributions.

Fermi and Swift Observations of GRB 190114C: Tracing the Evolution of High-Energy Emission from Prompt to Afterglow

“…The red shaded region represents a non-observable period for GRB 190114C due to Earth occultation. magnetic field dissipation and jet acceleration can occur on a time scale much shorter than the diffusion time (Lyutikov et al 2003;Kumar & Narayan 2009;Lazar et al 2009;Granot 2016).…”

A detailed radio study of the energetic, nearby, and puzzling GRB 171010A

“…Frail et al 2000a;van der Horst et al 2008). The prompt emission is thought to be produced in processes internal to the out-flowing material, with magnetic reconnection and internal shocks both invoked as the physical process responsible (Rees & Meszaros 1994;Spruit et al 2001;Granot 2016). The afterglow is the result of the ejected material interacting with the circumburst environment, which results in broadband synchrotron emission from shock accelerated E-mail: joe.bright@physics.ox.ac.uk electrons (Wijers et al 1997;Wijers & Galama 1999;Sari et al 1998).…”

Prompt gamma-ray burst emission from gradual magnetic dissipation

“…Many studies (Giannios 2008;Zhang & Yan 2011;McKinney & Uzdensky 2012;Sironi et al 2015;Beniamini & Granot 2016;Granot 2016;Kagan et al 2016;Bégué et al 2017) have therefore recently considered reconnection models as strong candidates for reproducing the prompt sub-MeV emission. In MHD jets, the energy dissipation through reconnection occurs gradually over a wide range of radii (Drenkhahn 2002).…”