On the existence of a reverse shock in magnetized gamma-ray burst ejecta

Abstract

The role of magnetic fields in gamma-ray burst (GRB) flows remains controversial. The study of the early afterglow phases and, in particular, of the reverse shock dynamics and associated emission offers a promising probe of the magnetization of the ejecta. In this paper, we derive the conditions for the existence of a reverse shock in arbitrarily magnetized ejecta that decelerate and interact with the circumburst medium. Both constant and wind-like density profiles are considered. We show, in contrast to previous estimates, that ejecta with magnetization σ 0 > ∼ 1 are not crossed by a reverse shock for a large fraction of the parameter space relevant to GRB flows. Allowing for shell spreading, there is always a relativistic or mildly relativistic reverse shock forming in σ 0 < ∼ 0.3 ejecta. From this, we conclude that the paucity of optical flashes, believed to be a distinctive signature of a reverse shock, may be explained by the existence of dynamically important magnetic fields in the ejecta.

Magnetohydrodynamic Effects in Relativistic Ejecta

“…The FS is stronger (due to a higher jump in pressure) and slower (more deceleration relative to the frame of the contact discontinuity), while the RS is weaker but faster. These features are expected from analytical work, 10,14 and agree with 1D relativistic MHD simulations. 17,18 For σ = 10.0 (dashed) and σ = 20.0 (dash-dotted), a prominent left-going rarefaction wave ( ← R) is observed, instead of a left-going shock.…”

“…Exact solutions indicate that the condition for the existence of a reverse shock is σ < σ c . 10,14 The paucity of bright optical flashes in GRBs 22 may, among other interpretions, be attributed to highly magnetized flows. If strongly magnetized jet (Poynting flux jet) is formed and leaves from the dense stellar matter like a collapsar model and goes to rarefied wind media, it is possible to form the bow-shock with the rarefaction wave on the jet head.…”

“…11,12 However, consensus on the conditions required for the existence of the RS or how Poynting flux is transferred to kinetic flux in the interaction region has not yet been achieved. 10,13,14 We present a onedimensional study of the interaction between a magnetized relativistic flow and a static, unmagnetized external medium. A Riemann problem is solved analytically over a broad range of σ.…”

Reverse shock emission driven by post-merger millisecond magnetar winds: Effects of the magnetization parameter

“…There have been a number of analytic and numerical papers on magnetization. The existence of a reverse shock (RS) in magnetized ejecta was suggested (e.g., Fan et al 2004;Zhang & Kobayashi 2005;Giannios et al 2008;Mimica et al 2009;Lyutikov 2011) and the dynamics of arbitrarily magnetized ejecta-medium interaction were studied (e.g., Zhang & Kobayashi 2005;Mimica et al 2009;Mizuno et al 2009;Lyutikov 2011). It was found that the A&A 592, A92 (2016) magnetization influences the RS emission (e.g., Mimica et al 2010;Harrison & Kobayashi 2013).…”

“…The RS, arising from the interaction of the wind with the merger ejecta and external medium, depends on the magnetization of the magnetar wind (Fan et al 2004;Zhang & Kobayashi 2005;Mimica et al 2009Mimica et al , 2010Mao et al 2010). The strong magnetic fields would suppress the RS in the magnetized wind (Fan et al 2004;Zhang & Kobayashi 2005;Giannios et al 2008;Mimica et al 2009Mimica et al , 2010.…”