A Radio Counterpart to a Neutron Star Merger
On 17 August 2017, the Advanced Laser Interferometer Gravitational Wave Observatory (Advanced LIGO) detected a gravitational wave signal, GW170817, which was rapidly identified to be associated with the inspiral and coalescence of two neutron stars. A burst of gamma-rays, GRB170817A, was detected ~2 seconds after the gravitational wave detection by the Gamma-ray Burst Monitor (GBM) of the Fermi Gamma-ray Space Telescope. With the addition of data from the Advanced Virgo interferometer, the gravitational waves source was localized to an area of 28 deg2 (90% confidence region) and a distance of 40 ± 8 Mpc. There were 49 cataloged galaxies within this volume, allowing astronomers to rapidly search for electromagnetic counterparts. An optical counterpart, designated SSS17a, was detected within ~11 hours of the event by astronomers using the Swope telescope, localizing the merger to the S0-type galaxy NGC 4993 at a distance of 40 Mpc and soon independently confirmed. Following the optical detections, targeted observing campaigns were initiated across the electromagnetic spectrum. Subsequent optical and infrared spectroscopic observations firmly established this optical counterpart to be associated with the neutron star merger GW170817.
In this work, Hallinan et al. report a coordinated effort to use the Jansky VLA, the VLA Low Band Ionosphere and Transient Experiment (VLITE), the Australia Telescope Compact Array (ATCA) and the Giant Metrewave Radio Telescope (GMRT) to constrain the early time radio properties of the neutron star merger. Companion papers report the ultraviolet and X-ray properties and interpret the panchromatic behavior of the transient. The multi-wavelength counterpart to GW170817 is designated EM170817.
Hallinan et al. discovered the radio afterglow of the neutron star merger using the Jansky VLA 16 days after the gravitational wave burst. This radio detection was made possible by the ultra-sensitivity of the Jansky VLA. The radio afterglow is the key element in interpreting the event as a likely relativistic jet driven by the neutron star merger. Continuing observations with the VLBA and the Jansky VLA will solidify the interpretation of this event, heralding a new field of astronomy.
Image: [Left] Radio image created using VLA observations (6 GHz) on 9 September 2017, with the radio counterpart to EM170817 highlighted. Its flux density is 23 ± 3.4 μJy. [Right] A combined image from four VLA observations at 6 GHz spanning 22.6 August–1 September 2017. The flux density at the position of EM170817 is 7.8 ± 2.6 μJy, consistent with a marginal or non-detection.
Publication: G. Hallinan (California Institute of Technology) et al., A Radio Counterpart to a Neutron Star Merger, 16 October 2017, Science, 358, 1579.
