Off-axis emission of short γ-ray bursts and the detectability of electromagnetic counterparts of gravitational-wave-detected binary mergers

Abstract

We present calculations of the wide angle emission of short-duration gamma-ray bursts from compact binary merger progenitors. Such events are expected to be localized by their gravitational wave emission, fairly irrespective of the orientation of the angular momentum vector of the system, along which the gamma-ray burst outflow is expected to propagate. We show that both the prompt and afterglow emission are dim and challenging to detect for observers lying outside of the cone within which the relativistic outflow is propagating. If the jet initially propagates through a baryon contaminated region surrounding the merger site, however, a hot cocoon forms around it. The cocoon subsequently expands quasi-isotropically producing its own prompt emission and external shock powered afterglow. We show that the cocoon prompt emission is detectable by Swift BAT and Fermi GBM, We also show that the cocoon afterglow peaks a few hours to a few days after the burst and is detectable for up to a few weeks at all wavelengths. The timing and brightness of the transient are however uncertain due to their dependence on unknown quantities such as the density of the ambient medium surrounding the merger site, the cocoon energy, and the cocoon Lorentz factor. For a significant fraction of the gravitationally-detected neutron-star-binary mergers, the cocoon afterglow could possibly be the only identifiable electromagnetic counterpart, at least at radio and X-ray frequencies.

SwiftandNuSTARobservations of GW170817: Detection of a blue kilonova

“…Instead it must arise from some previously unseen process, such as a heated cocoon (e.g. (26,27)); and we are likely viewing the event off-axis (unless there was no jet at all), as indeed our modelling of the UV emission (above) suggests. In this case, if there is an ultrarelativistic jet which is oriented away from us, we can expect X-ray emission at late times.…”

“…But even using the observed correlation (14) between E γ,iso and X-ray afterglow luminosity, the predicted X-ray flux at ∆t = 0.6 d is still above our Swift and NuSTAR upper limits. This requires an alternative explanation for the observed γ-ray emission, such as a (typical) short GRB viewed (slightly) off-axis, or the emission from a cocoon formed by the interaction of a jet with the merger ejecta (26)(27)(28). We return to this issue below in the context of late-time (∆t >∼ 10 d) X-ray emission (see also (10) and (9)).…”

“…One way of explaining the observed prompt gamma-rays may be via the 'cocoon model' (26,27). In this model, if the GRB jet propagates through a baryon-contaminated site around the merger, a heated cocoon is formed from which prompt gamma-rays may be seen.…”

The Peculiar Physics of GRB 170817A and Their Implications for Short GRBs

“…Another possibility is that thermal emission is produced by a nearly spherical outflow, which expands at sub-relativistic speeds, the so-called cocoon (Lazzati et al 2017;Kasliwal et al 2017). The peak energy and the luminosity constrain the size of the emitting region to be ∼ 2 × 10 7 cm.…”

“…Indeed, Troja et al (2017), interpret an off-axis afterglow via an observed delay in the X-ray emission. Additionally, a slowly expanding cocoon can also be formed by the material pushed away during the propagation of the jet (Lazzati et al 2017).…”