Magnetic Fields in Relativistic Collisionless Shocks

Abstract

We present a systematic study on magnetic fields in Gamma-Ray Burst (GRB) external forward shocks (FSs). There are 60 (35) GRBs in our X-ray (optical) sample, mostly from Swift. We use two methods to study ǫ B (fraction of energy in magnetic field in the FS). 1. For the X-ray sample, we use the constraint that the observed flux at the end of the steep decline is ≥ X-ray FS flux. 2. For the optical sample, we use the condition that the observed flux arises from the FS (optical sample light curves decline as ∼ t −1 , as expected for the FS). Making a reasonable assumption on E (jet isotropic equivalent kinetic energy), we converted these conditions into an upper limit (measurement) on ǫ B n 2/(p+1) for our X-ray (optical) sample, where n is the circumburst density and p is the electron index. Taking n = 1 cm −3 , the distribution of ǫ B measurements (upper limits) for our optical (X-ray) sample has a range of ∼ 10 −8 − 10 −3 (∼ 10 −6 − 10 −3 ) and median of ∼ few × 10 −5 (∼ few × 10 −5 ). To characterize how much amplification is needed, beyond shock compression of a seed magnetic field ∼ 10µG, we expressed our results in terms of an amplification factor, AF , which is very weakly dependent on n (AF ∝ n 0.21 ). The range of AF measurements (upper limits) for our optical (X-ray) sample is ∼ 1 − 1000 (∼ 10 − 300) with a median of ∼ 50 (∼ 50). These results suggest that some amplification, in addition to shock compression, is needed to explain the afterglow observations.

Temporal Evolution of the Gamma-ray Burst Afterglow Spectrum for an Observer: GeV–TeV Synchrotron Self-Compton Light Curve

“…Sironi & Spitkovsky 2009;Lemoine & Pelletier 2010;Sironi et al 2013) from the theoretical point of view, the low magnetization implied from recent broadband observations of the afterglows (e.g. Kumar & Barniol Duran 2009;Lemoine et al 2013;Santana et al 2014;Beniamini et al 2015;Zhang et al 2015) seems to be consistent with the theoretical argument.…”

“…The observed samples (gray) with error bars are taken from Willingale et al (2007). Santana et al 2014;Zhang et al 2015) suggest much lower magnetization. Beniamini et al (2015Beniamini et al ( , 2016 pointed out that the high-efficiency problem in the prompt emission will be resolved by a very small ǫ B .…”

Radio SNRs in the Magellanic Clouds as probes of shock microphysics

“…Given that the magnetic field in the MCs is ∼ 1 µG (e.g., Gaensler et al 2005;Mao et al 2008), it appears that the amplification needed in MCs SNRs is by a factor of > ∼ 10, although we cannot constrain B uniquely. Interestingly, a similar amplification factor has been found for gamma-ray burst relativistic shocks (e.g., Barniol Duran 2014; Santana, Barniol Duran & Kumar 2014).…”

Radio Observations of a Sample of Broad-Line Type Ic Supernovae Discovered by PTF/Iptf: A Search for Relativistic Explosions

“…The exclusion regions obtained in this way are shown in Figure 6. In this Figure 0.33 (Santana et al 2014). As evident from Equation (3), for a given upper limit on the flux at a certain epoch, the smaller the value of the product   e B 3 4 , the larger the minimum E 51 excluded for each value of n 0 (thus, off-axis emission from lower-energy fireballs / low-luminosity GRBs is less constrained).…”