The Optical Afterglow of GW170817 at One Year Post-merger

Abstract

We present observations of the optical afterglow of GRB 170817A, made by the Hubble Space Telescope, between February and August 2018, 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 ultrarelativistic jet, with Lorentz factor 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 4000L (M F606W −4.3).

LOFAR 144-MHz follow-up observations of GW170817

“…Within the associated uncertainties, these results are consistent with the broadband evolution of AT 2017gfo (e.g. Alexander et al 2018;Lamb et al 2019;Troja et al 2019;Hajela et al 2019).…”

Science case for the Einstein telescope

“…Nine and sixteen days after the GW observation of the merger, X-ray and radio emissions were also detected [8,9]. Over longer timescale the radio, optical, and X-ray observations showed a slow achromatic flux increase until about 150 days before starting to decline [91][92][93][94]. High-resolution radio observations [10,11] were able to constrain the source size and to show a source displacement consistent with the launch of a jet which successfully breaks through the ejecta developing an angular structure, i.e.…”

The Optical Afterglow of GW170817: An Off-axis Structured Jet and Deep Constraints on a Globular Cluster Origin

“…We present a revised light curve of the optical afterglow of GW170817, relative to previous studies which have used subsets of HST observations to derive measurements and upper limits of the afterglow in the F606W filter (Alexander et al 2018;Margutti et al 2018;Lyman et al 2018;Lamb et al 2019a;Piro et al 2019). We calculate the difference ∆m between the published values and the values presented in this work (Figure 2).…”

“…When GW170817 emerged from solar conjunction at ≈ 100 days, the non-thermal afterglow emission, which results from relativistic material interacting with the surrounding medium, outshined the kilonova. The study of this second phase in the optical band was enabled by the Hubble Space Telescope (HST) (Alexander et al 2018;Lyman et al 2018;Margutti et al 2018;Lamb et al 2019a;Piro et al 2019), which was the only facility with the sensitivity to securely detect the source at these epochs, due to a combination of intrinsic faintness of the afterglow and contaminating light from NGC4993.…”

“…However, the morphology of NGC4993 is complex, and characterized by dust lanes and concentric shells (Blanchard et al 2017;Levan et al 2017;Palmese et al 2017), making accurate and uniform photometry extremely challenging. Thus, previous studies which utilized optical data suffer from a combination of imperfect galaxy subtraction and nonuniform photometric methods (Alexander et al 2018;Lyman et al 2018;Margutti et al 2018;Lamb et al 2019a;Piro et al 2019).…”