Abstract
Abstract The short gamma-ray burst (GRB) 170817A was the first GRB associated with a gravitational-wave event. Due to the exceptionally low luminosity of the prompt γ-ray and the afterglow emission, the origin of both radiation components is highly debated. The most discussed models for the burst and the afterglow include a regular GRB jet seen off-axis and the emission from the cocoon encompassing a “choked” jet. Here, we report low radio frequency observations at 610 and 1390 MHz obtained with the Giant Metrewave Radio Telescope. Our observations span a range of ∼7 to ∼152 days after the burst. The afterglow started to emerge at these low frequencies about 60 days after the burst. The 1390 MHz light curve barely evolved between 60 and 150 days, but its evolution is also marginally consistent with an F ν ∝ t 0.8 rise seen in higher frequencies. We model the radio data and archival X-ray, optical, and high-frequency radio data with models of top-hat and Gaussian structured GRB jets. We performed a Markov Chain Monte Carlo analysis of the structured-jet parameter space. Though highly degenerate, useful bounds on the posterior probability distributions can be obtained. Our bounds of the viewing angle are consistent with that inferred from the gravitational-wave signal. We estimate the energy budget in prompt emission to be an order of magnitude lower than that in the afterglow blast wave.
Highlights
The Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo gravitational-wave (GW) detectors detected on 2017 August 17 for the first time the emission from two inspiraling neutron stars (GW170817; Abbott et al 2017c)
We present our continuing low-frequency observations of the radio transient using the Giant Metrewave Radio Telescope (GMRT), located in Pune, India (Section 2), covering the time interval from 7 to 152 days after GW170817
We present the low radio frequency observations of the afterglow of gamma-ray burst (GRB) 170817A/GW170817 with the GMRT
Summary
The Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo gravitational-wave (GW) detectors detected on 2017 August 17 for the first time the emission from two inspiraling neutron stars (GW170817; Abbott et al 2017c). 9 days after GW170817, a new source at the position of AT 2017gfo emerged at X-ray frequencies (Troja et al 2017) and a week later at radio frequencies (Hallinan et al 2017). The emission at longer and shorter wavelengths is of nonthermal origin The brightness of this component increased since its discovery (Fν ∝ t0.8; Haggard et al 2017; Hallinan et al 2017; Margutti et al 2017a; Mooley et al 2017; Troja et al 2017), while the shape of the spectral energy distribution remained constant with time (Fν ∝ ν−0.6; Mooley et al 2017). We assume the distance to GW170817 to be 42.5 Mpc (Hjorth et al 2017)
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