A cocoon shock breakout as the origin of the γ-ray emission in GW170817
By O. Gottlieb, E. Nakar, T. Piran, K. Hotokezaka. Monthly notices of the Royal Astronomical Society, 2017
175 citations recorded by the source corpus.
The short Gamma-Ray Burst, GRB170817A, that followed the binary neutron star merger gravitational waves signal, GW170817, is not a usual sGRB. It is weaker by three orders of magnitude than the weakest sGRB seen before and its spectra, showing a hard early signal followed by a softer thermal spectrum, is unique. We show, first, that the $\gamma$-rays must have emerged from at least mildly relativistic outflow, implying that a relativistic jet was launched following the merger. We then show that the observations are consistent with the predictions of a mildly relativistic shock breakout: a minute $\gamma$-ray energy as compared with the total energy and a rather smooth light curve with a hard to soft evolution. We present here a novel analytic study and detailed numerical 2D and 3D relativistic hydrodynamic and radiation simulations that support the picture in which the observed $\gamma$-rays arose from a shock breakout of a cocoon from the merger's ejecta (Kasliwal 2017). The cocoon can be formed by either a choked jet which does not generate a sGRB (in any direction) or by a successful jet which generates an undetected regular sGRB along the system's axis pointing away from us. Remarkably, for the choked jet model, the macronova signal produced by the ejecta (which is partially boosted to high velocities by the cocoon's shock) and the radio that is produced by the interaction of the shocked cocoon material with the surrounding matter, agree with the observed UV/optical/IR emission and with current radio observations. Finally, we discuss the possibility that the jet propagation within the ejecta may photodissociate some of of the heavy elements and may affect the composition of a fraction of ejecta and, in turn, the opacity and the early macronova light.
Subfields: Astronomy and Astrophysics
View Paper: https://www.semanticscholar.org/paper/945f31d44ef7d8c8c3dd116848d51c7edad42159
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Citations in the corpus
- The Zwicky Transient Facility: Science Objectives (2019, background): “Such emission may arise naturally from the “cocoon” formed when a jet fails to penetrate the neutronrich material dynamically ejected prior to the merger (Kasliwal et al. 2017; Gottlieb et al. 2018).”
- A Decline in the X-Ray through Radio Emission from GW170817 Continues to Support an Off-axis Structured Jet (2018, background): “…its energy into a mildly relativistic cocoon that dominates the emission (referred to as a “pure cocoon” model in this Letter; Kasliwal et al. 2017; Gottlieb et al. 2018; Mooley et al. 2018b) or the emission arises from the fastest component of the dynamical ejecta expelled during the merger…”
- Constraints on the neutron star equation of state from AT2017gfo using radiative transfer simulations (2018, background): “…parts of the ejecta (Kulkarni 2005; Metzger et al. 2015), or additional thermal energy added to the ejecta by a relativistic jet [‘cocoon’ emission (Gottlieb et al. 2018; Kasliwal et al. 2017; Piro & Kollmeier 2018), however, see Duffell et al. (2018), who find that relatively little thermal…”
- The Binary Neutron Star Event LIGO/Virgo GW170817 160 Days after Merger: Synchrotron Emission across the Electromagnetic Spectrum (2018, methodology): “This model was favored by Gottlieb et al. (2018); Hallinan et al. (2017); Kasliwal et al. (2017); Mooley et al. (2017); Nakar & Piran (2018).”