Binary neutron star (BNS) mergers have long been suspected to produce the central engines of short gamma-ray bursts (sGRBs) (Eichler et al. 1989). The link was firmly established in August 2017, after the combined detection of gravitational waves and an sGRB from the same BNS merger (Abbott et al. 2017a,b,c; Goldstein et al. 2017; Savchenko et al. 2017). The actual origin of the gamma-ray signal is still debated, coming from either an off-axis jet or from the shock breakout of a relativistic cocoon inflated by the jet itself (Kasliwal et al. 2017; Nakar & Piran 2017; Gottlieb, Nakar & Piran 2018a; Gottlieb et al. 2018b; Mooley et al. 2018a; Lundman & Beloborodov 2021). Nevertheless, the multiband observations of a rising afterglow (Alexander et al. 2017; Hallinan et al. 2017; Margutti et al. 2017, 2018; Troja et al. 2017, 2018, 2019; D’Avanzo P. et al. 2018; Lyman et al. 2018; Lamb et al. 2019) together with the detection of superluminal motion (Mooley et al. 2018b; Ghirlanda et al. 2019; Hotokezaka et al. 2019) have settled the presence of a jet that successfully broke out from the surrounding ejecta, observed off-axis with a viewing angle θobs ≈ 19° (Murguia-Berthier et al. 2017; Lamb, Mandel & Resmi 2018; Lazzati et al. 2018; Mooley et al. 2018a; Margutti & Chornock 2020). The information obtainable from afterglow observations is strongly dependent on the angular structure of the emerging jet. This structure might be mainly determined by the launching process (Kathirgamaraju et al. 2019) or it may arise as a consequence of the interaction with the surrounding environment during propagation. Therefore, an understanding of the processes that shape the jet could in principle provide insights into both jet formation, and the post-merger environment. Recent relativistic (magneto-) hydrodynamics simulations (Murguia-Berthier et al. 2014, 2017, 2021b; Geng et al. 2019; Beniamini et al. 2020b; Gottlieb, Levinson & Nakar 2020; Gottlieb & Nakar 2021; Gottlieb et al. 2021a; Gottlieb, Nakar & Bromberg 2021b; Hamidani & Ioka 2021; Lazzati et al. 2021; Nathanail et al. 2021; Pavan et al. 2021; Urrutia et al. 2021) have illustrated the importance of the ambient medium in shaping the jet, thereby highlighting the importance of understanding the remnant structure and ejecta properties. The joint events GW170817 and GRB170817A were followed by an additional electromagnetic transient spanning the spectral bands from UV to optical and IR on time-scales from days to weeks (Abbott et al. 2017b; Arcavi et al. 2017; Cowperthwaite et al. 2017; Drout et al. 2017; Evans et al. 2017; Pian et al. 2017; Smartt et al. 2017; Soares-Santos et al. 2017; Tanvir et al. 2017; Utsumi et al. 2017). The observed properties were consistent with the expectations for a thermal transient powered by the radioactive decay of freshly synthesized r-process elements (a so-called ‘macronova’ or ‘kilonova’ e.g. Li & Paczyński 1998; Kulkarni 2005; Rosswog 2005; Metzger et al. 2010; Roberts et al. 2011; Kasen, Badnell & Barnes 2013; Yu, Zhang & Gao 2013; Kasen, Fernández & Metzger 2015; Kasen et al. 2017; Metzger 2017; Perego, Radice & Bernuzzi 2017; Tanaka et al. 2017; Rosswog et al. 2018). Understanding the properties of this signal requires in-depth investigation of all the processes that can unbind material during and after a BNS merger, together with the available amount of free neutrons provided by each ejection channel. By now, several mass-ejection channels have been identified and they differ in terms of launch time, mass, electron fraction Ye, and velocity. During the merger ∼10−3 − 10−2 M⊙ of material are ejected dynamically (Rosswog et al. 1998, 1999; Rosswog & Davies 2002; Oechslin, Janka & Marek 2007; Bauswein, Goriely & Janka 2013; Radice et al. 2018a). On longer time-scales (∼1 s) a few 10−2 M⊙ of material can be unbound from the torus surrounding the remnant by the action of nuclear heating (Metzger, Piro & Quataert 2008; Fernandez & Metzger 2013; Fernandez et al. 2015; Just et al. 2015), magnetic (Siegel & Ciolfi 2015; Ciolfi et al. 2017; Siegel & Metzger 2017, 2018; Ciolfi & Kalinani 2020; Murguia-Berthier et al. 2021a), and viscous effects (Shibata, Kiuchi & Sekiguchi 2017; Fujibayashi et al. 2018, 2020; Radice et al. 2018b; Shibata & Hotokezaka 2019). Weak interactions play a key role since they can change the initially extremely low electron fraction and they are therefore of paramount importance for nucleosynthesis and the electromagnetic appearance of a BNS merger (Ruffert et al. 1997; Rosswog & Liebendörfer 2003; Dessart et al. 2009; Perego et al. 2014, 2017; Martin et al. 2015, 2018; Miller et al. 2019; Murguia-Berthier et al. 2021a).
Citation: Murguia-Berthier et al. 2014
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