Can an off-axis gamma-ray burst jet in GW170817 explain all the electromagnetic counterparts?

Abstract

Gravitational waves from a merger of two neutron stars (NSs) were discovered for the first time in GW170817, together with diverse electromagnetic (EM) counterparts. To make constraints on a relativistic jet from the NS merger, we calculate the EM signals in (1) the short gamma-ray burst sGRB 170817A from an off-axis jet, (2) the optical-infrared macronova (or kilonova), especially the blue macronova, from a jetpowered cocoon, and (3) the X-ray and radio afterglows from the interaction between the jet and interstellar medium. We find that a typical sGRB jet is consistent with these observations, and there is a parameter space to explain all the observations in a unified fashion with an isotropic energy ∼ 10 51 -10 52 erg, opening angle ∼ 20 • , and viewing angle ∼ 30 • . The off-axis emission is less de-beamed than the point-source case because the viewing angle is comparable to the opening angle. We also analytically show that the jet energy accelerates a fair fraction of the merger ejecta to a sub-relativistic velocity ∼ 0.3-0.4c as a cocoon in a wide parameter range. The ambient density might be low ∼ 10 −3 -10 −6 cm −3 , which can be tested by future observations of radio flares and X-ray remnants.

Jet Propagation in Neutron Star Mergers and GW170817

“…(iii) The jet head breakout velocity was constrained as 0.52 < v b /c < 1. This result is in agreement with Ioka & Nakamura (2018).…”

“…Because sGRB 170817A is several orders-ofmagnitude fainter than ordinary sGRBs (Abbott et al 2017c), the jet associated with GW170817 is not fully understood yet. This peculiar faintness has been interpreted as most likely due to the large off-axis viewing angle (see: Abbott et al 2017c;Kasliwal et al 2017;Granot et al 2017;Ioka & Nakamura 2018). With radio afterglow obser- Figure 1.…”

“…Hence, analytic modeling is indispensable, especially as several key parameters remain beyond reach (e.g., parameters of the central engine). Analytic modeling of the jet propagation in a BNS merger has been presented using ideas from the collapsar case (Murguia-Berthier et al 2017;Ioka & Nakamura 2018;Matsumoto & Kimura 2018;Gill et al 2019;Geng et al 2019;Lazzati & Perna 2019). These studies tried to make constraints on the equation of state of neutron stars (Lazzati & Perna 2019), on the properties of the ejecta (or the wind) (Murguia-Berthier et al 2017;Lazzati & Perna 2019), and on the timescale between the merger and BH formation (Murguia-Berthier et al 2017;Matsumoto & Kimura 2018;Gill et al 2019;Geng et al 2019).…”

Black hole-neutron star mergers using a survey of finite-temperature equations of state

“…A key difference between NSNS and BHNS systems is that NSNS mergers eject material away from the equatorial plane of the binary, while BHNS mergers do not. A relativistic jet from a NSNS central remnant may break through this surrounding material or may be choked inside it; various scenarios of cocoon-jet interaction have been considered in models of GW170817/GRB170817A [3][4][5][6][7]. The production of standard short GRBs thus may proceed somewhat differently, and is perhaps easier, for BHNS mergers.…”

Linking extended and plateau emissions of short gamma-ray bursts

“…The VLBI observations revealed a super-luminal motion of a radio point source, which is well explained by an emission from a relativistic jet (Mooley et al 2018; Ghirlanda et al 2019). It should be noted that the γ-ray counterpart, GRB 170817A (Abbott et al 2017c;Goldstein et al 2017;Savchenko et al 2017), is unlikely to be emitted by a core of the jet (see, e.g., Ioka & Nakamura 2018, for such an off-axis scenario), which is usually observed for regular SGRBs, and this GRB is not decisive evidence that the binary NS merger produced a SGRB (Kasliwal et al 2017;Matsumoto et al 2019a,b).…”