Abstract
Abstract We present observations of the optical afterglow of GRB 170817A, made by the Hubble Space Telescope, between 2018 February and August, up to one year after the neutron star merger GW170817. The afterglow shows a rapid decline beyond 170 days, and confirms the jet origin for the observed outflow, in contrast to more slowly declining expectations for “failed-jet” scenarios. We show here that the broadband (radio, optical, X-ray) afterglow is consistent with a structured outflow where an ultra-relativistic jet, with a Lorentz factor of Γ ≳ 100, forms a narrow core (∼5°) and is surrounded by a wider angular component that extends to ∼15°, which is itself relativistic (Γ ≳ 5). For a two-component model of this structure, the late-time optical decline, where F ∝ t −α , is α = 2.20 ± 0.18, and for a Gaussian structure the decline is α = 2.45 ± 0.23. We find the Gaussian model to be consistent with both the early ∼10 days and late ≳290 days data. The agreement of the optical light curve with the evolution of the broadband spectral energy distribution, and its continued decline, indicates that the optical flux is arising primarily from the afterglow and not any underlying host system. This provides the deepest limits on any host stellar cluster with a luminosity ≲4000 L ⊙ (M F606W ≳ −4.3).
Highlights
The first binary neutron star merger detected via gravitational waves (GW170817) was accompanied by a weak shortduration gamma-ray burst (GRB 170817A Abbott et al 2017a), a radioactively powered kilonova, and a long-lived afterglow (e.g., Abbott et al 2017b)
We have presented optical observations made by Hubble Space Telescope (HST) of the afterglow to gamma-ray bursts (GRBs) 170817A between 2018 February and August
Hubble Space Telescope observations of the afterglow of GRB 170817A, taken from 171 days to one year from merger GW170817, show it to be rapidly declining in flux
Summary
The first binary neutron star merger detected via gravitational waves (GW170817) was accompanied by a weak shortduration gamma-ray burst (GRB 170817A Abbott et al 2017a), a radioactively powered kilonova, and a long-lived afterglow (e.g., Abbott et al 2017b). The steady rise of the afterglow from a ∼10 days post-merger, which was traced at radio, X-ray, and optical wavelengths (e.g., Hallinan et al 2017; Margutti et al 2017, 2018; Troja et al 2017, 2018; Alexander et al 2018; D’Avanzo et al 2018; Dobie et al 2018; Lyman et al 2018; Mooley et al 2018c, 2018a, 2018b; Nynka et al 2018; Resmi et al 2018; van Eerten et al 2018; Piro et al 2019), distinguished GRB 170817A (alongside its intrinsic low-luminosity) from cosmological short gamma-ray bursts (GRBs) This called into question the link between GW170817 and the progenitors of other short GRBs. Following a neutron star merger, a jet, launched due to the rapid accretion of ejected matter onto a compact remnant, will propagate through the merger ejecta medium. We supplement these data with radio and X-ray frequency observations to investigate the behavior of the declining afterglow within the structured jet scenario.
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