Gamma-Ray Burst Jets and their Radio Observations

Abstract

Radio observations play a key role in studying the jets that power GRBs, the most luminous cosmic explosions. They are crucial for determining the GRB jet energy, the external density, and the microphysical parameters of relativistic collisionless shocks, from afterglow broadband modeling. Radio image size measurements are rare, but provide extremely useful information. The "radio flare" peaking after ß1 day helps constrain the magnetisation and magnetic-field structure of GRB outflows. This review discusses the current observational and modeling status, focusing on the afterglow and outlining prompt radio emission searches, along with recent theoretical progress in GRB jet dynamics, focusing on magnetic acceleration, jet propagation inside a massive star progenitor (for long GRBs), the reverse shock, and the late afterglow. Great progress has been made in our understanding of magnetic acceleration, collimation and later sideways expansion of GRB jets, with interesting implications for the prompt, reverse shock, and afterglow emission. We outline how theory and observations were combined to study GRB jet physics and their immediate environment. Finally, potential paths are suggested for combining theory and observations to achieve greater progress, and some prospects for the future are discussed in light of the expected improvements in observational capabilities and theoretical advances.

The Arcminute Microkelvin Imager catalogue of gamma-ray burst afterglows at 15.7 GHz

“…However, 15.7 GHz is usually above the scattering frequency in a given direction and therefore in the weak scattering regime. This implies that scintillation effects are likely to be minimal (see Table 1 of Granot & van der Horst 2014). Of course, this assumption is predicated upon the simple framework of Walker (1998), which assumes a single scattering screen at a fixed distance.…”

“…These equations assume that the self-absorption frequency is below 15.7 GHz (for more details on GRB radio light curve modelling see Granot & van der Horst 2014, and references therein). Equations 1 and 2 demonstrate that the forward-shock peak is highly dependent on many parameters that can only be calculated through broadband modelling.…”

Magnetars in Ultra-Long Gamma-Ray Bursts and GRB 111209A

“…Setting ν = ν 0 in Equation 17 gives the Fresnel angle associated with ν 0 . This allows us to calculate the amplitude of the three types of scintillation using the equations in Table 1 from Granot & van der Horst (2014). Diffractive scintillation is a narrow-band phenomenon, and there is a suppression factor of (∆ν obs /ν) −1/2 (ν/ν 0 ) 17/10 (Granot & van der Horst 2014) for cases where the observing bandwidth ∆ν obs is greater than the de-correlation bandwidth ∆ν dc = ν(ν/ν 0 ) 17/5 because the brightness variations are averaged over a large frequency range and cannot be individually resolved.…”

The SKA View of Gamma-Ray Bursts

“…These studies indicate that there is a large spread in the physical parameters, which has implications for models of GRB progenitors, particle acceleration, and magnetic field amplification by shocks (Granot & van der Horst 2014). A striking example is GRB 130427A, another nearby, high-luminosity event with extremely good temporal and spectral coverage, over 10 orders of magnitude in time and 16 orders of magnitude in observing frequency (Ackermann et al 2014;Maselli et al 2014).…”

“…We have performed simulations to give prospects for GRB studies in the SKA era, for GRBs whose jets point towards the Earth (Section 4), and also those which are pointed away and from which we can detect late-time emission when the GRB outflow enters our line of sight (Section 5). For a more comprehensive review on radio observations of GRBs and their role in GRB jet studies we refer to Granot & van der Horst (2014).…”