Abstract
ABSTRACT The afterglows to gamma-ray bursts (GRBs) are due to synchrotron emission from shocks generated as an ultrarelativistic outflow decelerates. A forward and a reverse shock will form, however, where emission from the forward shock is well studied as a potential counterpart to gravitational wave-detected neutron star mergers the reverse shock has been neglected. Here, we show how the reverse shock contributes to the afterglow from an off-axis and structured outflow. The off-axis reverse shock will appear as a brightening feature in the rising afterglow at radio frequencies. For bursts at ∼100 Mpc, the system should be inclined ≲20° for the reverse shock to be observable at ∼0.1–10 d post-merger. For structured outflows, enhancement of the reverse shock emission by a strong magnetic field within the outflow is required for the emission to dominate the afterglow at early times. Early radio photometry of the afterglow could reveal the presence of a strong magnetic field associated with the central engine.
Highlights
The structure of the outflows that drive the shock system responsible for gamma-ray burst (GRB) afterglows is well discussed in the literature (e.g. Rossi, Lazzati & Rees 2002; Panaitescu 2005; Granot 2005; Salafia, Ghisellini, Pescalli, Ghirlanda & Nappo 2015)
Gravitational wave (GW) detected mergers involving at least one neutron star will typically be seen off the central rotational axis and will act as a probe for the structure of the jet or outflow that is likely responsible for the cosmological population of short-duration GRBs (Lamb & Kobayashi 2017; Lazzati, Deich, Morsony & Workman 2017; Jin, et al 2018; Kathirgamaraju, Barniol Duran & Giannios 2018)
For short-duration GRBs, the low characteristic frequency and the early peak time for the reverse-shock emission means that fast response and deep radio photometry of GW triggered neutron star mergers is critical in identifying the reverse shock contribution
Summary
The structure of the outflows that drive the shock system responsible for gamma-ray burst (GRB) afterglows is well discussed in the literature (e.g. Rossi, Lazzati & Rees 2002; Panaitescu 2005; Granot 2005; Salafia, Ghisellini, Pescalli, Ghirlanda & Nappo 2015). Gravitational wave (GW) detected mergers involving at least one neutron star will typically be seen off the central rotational axis and will act as a probe for the structure of the jet or outflow that is likely responsible for the cosmological population of short-duration GRBs (Lamb & Kobayashi 2017; Lazzati, Deich, Morsony & Workman 2017; Jin, et al 2018; Kathirgamaraju, Barniol Duran & Giannios 2018). The phenomenology of the reverse shock emission can be used as a probe for the magnetisation of the central engine (e.g. Fan, Dai, Huang & Lu 2002; Zhang, Kobayashi & Meszaros 2003; Zhang & Kobayashi 2005; Giannios, Mimica & Aloy 2008; Gomboc, et al 2008; Steele, Mundell, Smith, Kobayashi & Guidorzi 2009; Mimica, Giannios & Aloy 2010; Granot 2012; Harrison & Kobayashi 2013; Japelj, et al 2014; Guidorzi, et al 2014; Fraija 2015; Gao, Wang, Meszaros & Zhang 2015; Kopac, et al 2015; Zhang, Jin & Wei 2015; Huang, et al 2016; Liu, Wang & Dai 2016; Alexander, et al 2017; Laskar, et al 2016, 2018; Lamb, et al 2019b) and potentially assist in identifying the likely outflow structure
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