ASKAP casts first big radio ‘net’ for gravitational-wave suspects

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 signal’s 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.)


Figure 1: ASKAP image of the localisation region of S190814bv centered on 00:50:37.5, −25:16:57.371, observed two days post-merger. The 30 deg2 field of view covers 89% of the localisation region. The dashed and solid lines mark 50% and 90% contours respectively. Galaxy NGC 253 is prominent near the field’s centre.

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).


Figure 2: Radio constraints on viewing angle and circum-merger density for S190814bv, assuming an isotropic equivalent energy of 1051 erg and an initial jet opening angle of 10°. Shaded regions are parts of the parameter space ruled out by the ASKAP observations, for the median distance (267 Mpc) ± 1σ (52 Mpc). (From Dobie et al. 2019)

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.



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