Radio Flares From Gamma-Ray Bursts

Abstract

We present predictions of centimeter and millimeter radio emission from reverse shocks in the early afterglows of gamma-ray bursts with the goal of determining their detectability with current and future radio facilities. Using a range of GRB properties, such as peak optical brightness and time, isotropic equivalent gamma-ray energy and redshift, we simulate radio light curves in a framework generalized for any circumburst medium structure and including a parametrization of the shell thickness regime that is more realistic than the simple assumption of thick-or thin-shell approximations. Building on earlier work by Mundell et al. (2007) and Melandri et al. (2010) in which the typical frequency of the reverse shock was suggested to lie at radio, rather than optical wavelengths at early times, we show that the brightest and most distinct reverse-shock radio signatures are detectable up to 0.1−1 day after the burst, emphasizing the need for rapid radio follow-up. Detection is easier for bursts with later optical peaks, high isotropic energies, lower circumburst medium densities, and at observing frequencies that are less prone to synchrotron self-absorption effects -typically above a few GHz. Given recent detections of polarized prompt gamma-ray and optical reverse-shock emission, we suggest that detection of polarized radio/mm emission will unambiguously confirm the presence of low-frequency reverse shocks at early time.

A Reverse Shock in GRB 160509a

“…We find a low circumburst density in the context of longlasting reverse shock emission for GRB 160509A, leading credence to this hypothesis. However, we also note that additional considerations such as high f ν,m,RS or late deceleration times may also contribute to stronger RS signatures; therefore, the detectability of a RS remains a complex question (Kopac et al 2015).…”

“…However, this signature has only been seen in a few cases in the Swift era, despite optical follow-up observations as early as a few minutes after γ-ray triggers (see Japelj et al 2014, for a review). The dearth of bright optical flashes suggests RS emission may instead be easier to observe at longer wavelengths (Mundell et al 2007;Laskar et al 2013;Kopac et al 2015). We have therefore initiated a program at the Karl G. Jansky Very Large Array (VLA) for radio RS studies, and here present the detection of a reverse shock in the Fermi GRB 160509A.…”

First ALMA Light Curve Constrains Refreshed Reverse Shocks and Jet Magnetization in GRB 161219B

“…We have previously speculated that the low-density medium may be responsible for a slow-cooling RS, allowing the RS emission to be detectable for longer (Laskar et al 2013(Laskar et al , 2016Kopac et al 2015;Alexander et al 2017). Indeed, we find a slow-cooling RS in a low-density medium in the case of GRB 161219B also, lending credence to this hypothesis.…”

A Reverse Shock and Unusual Radio Properties in GRB 160625B

“…Bright optical flashes are now ruled out by observations in many events, while other events show complicated optical light curves that, like the prompt γ-ray emission, may originate instead from internal shocks (Kopač et al 2013;Japelj et al 2014). Some authors have proposed that RS emission may be easier to observe at longer wavelengths, where the emission peaks on timescales of days (Mundell et al 2007;Melandri et al 2010;Kopač et al 2015). This approach was successfully adopted in trailblazing multi-frequency radio studies of GRB 130427A that characterized the RS emission at multiple epochs in detail (Laskar et al 2013;Perley et al 2014).…”