ASKAP has made its first large-scale follow-up of a gravitational-wave event – the merger of a neutron star and a black hole – looking for a radio source generated by the merger. Such a source would localise where the event took place.
The observations showed how valuable ASKAP’s wide field of view is for gravitational-wave follow-up. They were also the most sensitive widefield search to date for radio transients.
The LIGO and Virgo gravitational-wave detectors picked up an event, S190814bv, on 14 August 2019.
The signals characteristics showed one object was of more than five solar masses, the other less than three, marking the event as a likely neutron star-black hole (NSBH) merger.
An NSBH merger is less than 1% likely to leave matter outside the merged entity, so S190814bv may have had no electromagnetic counterpart.
High-energy detectors saw no sign that the merger had generated a short gamma-ray burst.
Optical observations found candidate counterparts, but these were later ruled out.
However, an optical counterpart could have formed but gone unseen for many reasons: the system’s inclination angle, for instance, or the mass ratio of the two bodies, the remnant’s short lifetime, the lack of polar ejecta, or obscuring dust.
In such cases, radio emission might be the only way to localise the merger.
University of Sydney/CSIRO PhD student Dougal Dobie and his collaborators observed the field at two, nine and 33 days after the merger, at a central frequency of 943 MHz.
The 90% localisation region was 23 deg2. With its 30 deg2 field of view, ASKAP covered the main part of this area, in a single pointing (Figure 1) – something no other existing radio telescope can do. (A second, smaller area of localisation lay outside ASKAP’s footprint.)
The researchers searched for highly variable sources in the field, turning up 285 candidates. Inspection by eye, then further filtering, whittled the number to 21.
The most promising candidate was AT2019osy.
Follow-up observations of this object at radio, optical and X-ray wavelengths (made with the VLA, DECam and Chandra respectively) suggest AT2019osy is a variable low-luminosity AGN, unrelated to S190814bv.
But these investigations were notable because ASKAP had the lead role as the discovery instrument.
Radio lightcurves from NSBH mergers are predicted to vary greatly, their fluxes and timescales depending on several factors: whether the radio emission is dominated by ejecta or a jet; the system’s inclination angle; the circum-merger density; the mass ratio of the merging objects; the black hole’s spin; and the neutron star equation of state. The non-detection of a radio counterpart to S190814bv let Dobie et al. place constraints on the viewing angle and circum-merger density (Figure 2).
In 2017 ATCA and other telescopes determined the radio lightcurve of a binary neutron-star merger, GW170817, a much closer event. If that lightcurve were scaled to 943 MHz and the system placed at the distance of S190814bv, 267±52 Mpc, its peak flux density would be ~5µJy, well below Dobie et al.’s detection threshold.
Nevertheless, Dobie et al. will revisit the field of S190814bv over months and years: the radio emission from GW170817 peaked ~150 days after the merger, so if there is a radio counterpart to S190814bv, it may brighten on that timescale. In any case, more observations will further constrain the circum-merger density and inclination angle, which in turn may help better localise the gravitational-wave event.
Publications
Dobie, D. et al., arxiv.org/abs/1910.13647
Hobbs, G., Heywood, I., Bell, M. E., et al. 2016, MNRAS, 456, 3948, doi: 10.1093/mnras/stv2893
Mooley, K. P., Frail, D. A., Ofek, E. O., et al. 2013, ApJ, 768, 165, doi: 10.1088/0004-637X/768/2/165
Swinbank, J. D., Staley, T. D., Molenaar, G. J., et al. 2015, Astronomy and Computing, 11, 25, doi: 10.1016/j.ascom.2015.03.002