Abstract
Abstract We present the results of our year-long afterglow monitoring of GW170817, the first binary neutron star (NS) merger detected by advanced LIGO and advanced Virgo. New observations with the Australian Telescope Compact Array (ATCA) and the Chandra X-ray Telescope were used to constrain its late-time behavior. The broadband emission, from radio to X-rays, is well-described by a simple power-law spectrum with index β ∼0.585 at all epochs. After an initial shallow rise ∝ t0.9, the afterglow displayed a smooth turn-over, reaching a peak X-ray luminosity of LX≈5 ×1039 erg s−1 at 160 d, and has now entered a phase of rapid decline, approximately ∝ t−2. The latest temporal trend challenges most models of choked jet/cocoon systems, and is instead consistent with the emergence of a relativistic structured jet seen at an angle of ≈22○ from its axis. Within such model, the properties of the explosion (such as its blastwave energy EK ≈ 2 × 1050 erg, jet width θc ≈4○, and ambient density n ≈3 × 10−3 cm−3) fit well within the range of properties of cosmological short GRBs.
Highlights
On 2017 August 17 the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) interferometers detected the first gravitational wave (GW) signal from a binary neutron star merger, GW 170817, followed 1.7 s later by a short-duration gamma-ray burst (GRB), GRB 170817A (Abbott et al 2017b)
A standard uniform jet viewed off-axis could account for the early afterglow emission, Troja et al (2017) and Kasliwal et al (2017) noted that it could not account for the observed gamma-ray signal and proposed two alternative models: a structured jet, i.e. a jet with an angular profile of Lorentz factors and energy, and a mildly relativistic isotropic cocoon
The post-peak temporal slope is a shallow decay of α ≈ 1.0– 1.2 up to at least 300 d for GRB 170817A, as can be inferred from semi-analytical modelling of the evolution of a trans-relativistic shell (Troja et al 2018b)
Summary
On 2017 August 17 the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) interferometers detected the first gravitational wave (GW) signal from a binary neutron star merger, GW 170817, followed 1.7 s later by a short-duration gamma-ray burst (GRB), GRB 170817A (Abbott et al 2017b). A standard uniform jet viewed off-axis could account for the early afterglow emission, Troja et al (2017) and Kasliwal et al (2017) noted that it could not account for the observed gamma-ray signal and proposed two alternative models: a structured jet, i.e. a jet with an angular profile of Lorentz factors and energy (see Abbott et al 2017b; Kathirgamaraju, Barniol Duran & Giannios 2018), and a mildly relativistic isotropic cocoon (see Kasliwal et al 2017; Lazzati et al 2017) In the latter model, the jet may never emerge from the merger ejecta (choked jet). The rich broad-band data set allows us to tightly constrain the afterglow parameters, and to compare the explosion properties of GW 170817 to canonical short GRBs (Section 3.2)
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