Afterglow light curves from misaligned structured jets

Abstract

GRB 170817A / GW 170817 is the first GRB clearly viewed far from the GRB jet's symmetry axis. Its afterglow was densely monitored over a wide range of frequencies and times. It has been modeled extensively, primarily numerically, and although this endeavour was very fruitful, many of the underlying model parameters remain undetermined. We provide analytic modelling of GRB afterglows observed off-axis, considering jets with a narrow core (of halfopening angle θ c ) and power-law wings in energy per unit solid angle, as well as briefly discuss Gaussian jets. Our study reveals qualitatively different types of lightcurves that can be viewed in future off-axis GRBs, with either single or double peaks, depending on the jet structure and the viewing angle. Considering the lightcurve shape rather than the absolute normalizations of times and / or fluxes, removes the dependence of the lightcurve on many of the highly degenerate burst parameters. This study can be easily used to determine the underlying jet structure, significantly reduce the effective parameter space for numerical fitting attempts and provide physical insights. As an illustration, we show that for GRB 170817A, there is a strong correlation between the allowed values of Γ c,0 and b, leading to a narrow strip of allowed solutions in the Γ c,0 -b plane above some minimal values Γ c,0 40, b 1.2. Furthermore, the Lorentz factor of the material dominating the early lightcurve can be constrained by three independent techniques to be Γ 0 (θ min,0 ) ≈ 5 − 7.

Constraining the Kilonova Rate with Zwicky Transient Facility Searches Independent of Gravitational Wave and Short Gamma-Ray Burst Triggers

“…The lack of a detected GRB and the observation of the rise phase of the transient make it unlikely for ZTF19aanhtzz to be the afterglow of a relativistic explosion viewed on-axis. Although a detail analysis using off-axis GRB models (e.g., van Eerten et al 2010; Beniamini et al 2020) is outside the scope of this paper, we suggest that a slightly off-axis relativistic explosion might explain the behaviour of this transient. In summary, with the data in hand, a conclusive answer regarding the nature of ZTF19aanhtzz/AT2019aacu is yet to be found, but a cosmological afterglow origin scenario is favored.…”

Ready, Set, Launch: Time Interval between a Binary Neutron Star Merger and Short Gamma-Ray Burst Jet Formation

“…The detection of a short gamma-ray burst (sGRB; Nakar 2007; Berger 2014), GRB 170817, from the BNS merger has renewed the community's interest in these enigmatic explosions (see, e.g., Nakar 2019 for a recent review on sGRBs from BNS mergers). GRB 170817 has forced us to revisit several important properties of GRB jets, such as their angular structure (e.g., Granot et al 2017;Lamb & Kobayashi 2017;Kathirgamaraju et al 2018;Beniamini et al 2020b), as well as possible implications for some as-of-yet mysterious properties of "standard" GRB afterglow observations such as X-ray plateaus (Oganesyan et al 2020;Beniamini et al 2020a). More importantly, it has highlighted our need to understand how jets propagate through external media.…”

Short Duration Gamma-Ray Bursts and Their Outflows in Light of GW170817

“…Unfortunately, telling whether a misaligned relativistic jet is present is not easy, since all the radiation is relativistically beamed away from the line of sight. The slow but steady brightening was shown to be consistent with the presence of a jet, its energy contribution along the line of sight growing with the deceleration of the external shock (Xiao et al, 2017;De Colle et al, 2018;Finstad et al, 2018;Granot et al, 2018b;Lamb and Kobayashi, 2018;Lazzati et al, 2018;Fraija et al, 2019a;Fraija et al, 2019b;Beniamini et al, 2020b;Oganesyan et al, 2020). However, a radially stratified spherical outflow could reproduce the observations as well, albeit at the price of adding a never observed before component to the models (Li et al, 2018;Mooley et al, 2018c;Nakar and Piran, 2018;Salafia et al, 2018).…”

“…A few robust inferences can however be made (Lazzati et al, 2020). Overall, the measured delay was fairly small, since GW170817 ejected a significant amount of energy toward the observer but its Lorentz factor could be at most moderate (Γ < 7) (Beniamini et al, 2020b). These combine to a large photospheric radius and a photospheric delay…”