A Strong Jet Signature in the Late-time Light Curve of GW170817

Abstract

We present new 0.6-10 GHz observations of the binary neutron star merger GW170817 covering the period up to 300 days post-merger, taken with the upgraded Karl G. Jansky Very Large Array, the Australia Telescope Compact Array, the Giant Metrewave Radio Telescope and the MeerKAT telescope. We use these data to precisely characterize the decay phase of the late-time radio light curve. We find that the temporal decay is consistent with a power-law slope of t −2.2 , and that the transition between the power-law rise and decay is relatively sharp. Such a slope cannot be produced by a quasi-isotropic (cocoon-dominated) outflow, but is instead the classic signature of a relativistic jet. This provides strong observational evidence that GW170817 produced a successful jet, and directly demonstrates the link between binary neutron star mergers and short-hard GRBs. Using simple analytical arguments, we derive constraints on the geometry and the jet opening angle of GW170817. These results are consistent with those from our companion Very Long Baseline Interferometry (VLBI) paper, reporting superluminal motion in GW170817.

LOFAR 144-MHz follow-up observations of GW170817

“…The constraints are α 610 144 −2.5, α 1390 144 −1.8, and α 7250 144 −1.2. These limits are still significantly steeper than the fitted 0.6-10 GHz radio spectral index of −0.53 ± 0.04 as determined by Mooley et al (2018c) (see Section 1). Therefore, we can only rule out that the radio spectrum of AT 2017gfo does not steepen below 610 MHz to an extreme degree.…”

“…In the case of AT 2017gfo, our first set of observations is either bookended by or close in time to a selection of the 610-and 1390-MHz GMRT observations, as well as Australia Telescope Compact Array (ATCA; Frater, Brooks & Whiteoak 1992;Wilson et al 2011) observations carried out at 5500 and 9000 MHz (Dobie et al 2018). For the 1390-MHz observations that bracketed our data, we averaged the corre- In the left panel, the solid line with surrounding shading is the light curve at 144 MHz extrapolated from the observations at higher frequencies with α = −0.53 ± 0.04 (Mooley et al 2018c). The triangles are our 3σ LOFAR upper limits, and the median 3σ sensitivity of LOFAR for routine 8-h observations for declinations at or near zenith (see Section 4.2) is depicted as a dashed horizontal line.…”

“…sponding flux densities using inverse-variance weighting; we also did this for the ATCA data, but using the 7250-MHz flux densities with a correction factor applied from Mooley et al (2018c). We can then combine these (averaged) flux densities with our LOFAR 3σ upper limit from Run 1 to calculate approximate constraints on a number of twopoint spectral indices, roughly 125-150 days post merger.…”

“…Margutti et al 2018;Troja et al 2018Troja et al , 2019Alexander et al 2018;Hajela et al 2019). Mooley et al (2018c) also found power-law dependencies for the rise and decay phases of approximately t 0.8 and t −2.4 , respectively, where t is the time since the merger. Within the associated uncertainties, these results are consistent with the broadband evolution of AT 2017gfo (e.g.…”

“…At these frequencies, a general picture emerged in which the radio light curve was first observed to steadily rise, before it turned over and began a more rapid decay. Using a compilation of 0.6-10 GHz radio data from 17-298 days post merger, Mooley et al (2018c) derived both a fitted time for the radio peak of 174 +9 −6 days and a fitted 3-GHz peak flux density of 98 +8 −9 µJy (also see similar analyses in Dobie et al 2018 andAlexander et al 2018). The fitted radio spectral index α 1 from this study is −0.53 ± 0.04, consistent with broadband spectral indices determined using radio, optical and X-ray data at various epochs, where the typical value is approximately −0.58 (e.g.…”