Follow up of GW170817 and its electromagnetic counterpart by Australian-led observing programs

TL;DR

This paper reports on multi-wavelength follow-up observations of GW170817 and its electromagnetic counterpart by Australian telescopes, providing insights into the merger process and host galaxy environment in the context of multi-messenger astronomy.

Contribution

It presents detailed observational data from Australian-led programs on GW170817, including optical, infrared, and radio measurements, enhancing understanding of neutron star mergers.

Findings

The transient cooled from 6400K to 2100K over 7 days.

Spectral profiles and light curves match binary neutron star merger models.

Host galaxy likely experienced a merger, with star formation ceased over a Gyr ago.

Abstract

The discovery of the first electromagnetic counterpart to a gravitational wave signal has generated follow-up observations by over 50 facilities world-wide, ushering in the new era of multi-messenger astronomy. In this paper, we present follow-up observations of the gravitational wave event GW170817 and its electromagnetic counterpart SSS17a/DLT17ck (IAU label AT2017gfo) by 14 Australian telescopes and partner observatories as part of Australian-based and Australian-led research programs. We report early- to late-time multi-wavelength observations, including optical imaging and spectroscopy, mid-infrared imaging, radio imaging, and searches for fast radio bursts. Our optical spectra reveal that the transient source afterglow cooled from approximately 6400K to 2100K over a 7-day period and produced no significant optical emission lines. The spectral profiles, cooling rate, and photometric light curves are consistent with the expected outburst and subsequent processes of a binary neutron star merger. Star formation in the host galaxy probably ceased at least a Gyr ago, although there is evidence for a galaxy merger. Binary pulsars with short (100 Myr) decay times are therefore unlikely progenitors, but pulsars like PSR B1534+12 with its 2.7 Gyr coalescence time could produce such a merger. The displacement (about 2.2 kpc) of the binary star system from the centre of the main galaxy is not unusual for stars in the host galaxy or stars originating in the merging galaxy, and therefore any constraints on the kick velocity imparted to the progenitor are poor.