ASKAP set for the next gravitational-wave event

Researchers have now worked out the best strategy for using ASKAP to follow up gravitational-wave detections.

Gravitational-wave detectors Advanced LIGO and Advanced Virgo are expected to start their next observing run, the O3 run, in April 2019.

In 2017 these detectors spotted a history-making event, GW 170817, the merger of two neutron stars. This was the first time astronomers were able to obtain both gravitational-wave and electromagnetic observations of an event, linking gravitational-wave studies with the rest of astronomy.

Since then the detectors have been off-air for upgrading.

The Australia Telescope Compact Array was one of the telescopes that detected GW 170817’s radio emission and followed its evolution.

ASKAP was not at full sensitivity at the time of GW 170817: if it had been, it would have seen the emission from 40 days after the merger and the peak flux density would have been a 15-σ detection.

Radio observations of a neutron-star merger can tell us the total energy of the event and the density of material around the merging objects.

A neutron star merger – artist’s impression. Credit: NASA/CXC/M.Weiss

Surprises from GW 170817

The radio emission from GW 170817 was surprising.

Astronomers had expected to see either emission from a relativistic jet that peaked within days of the merger, or emission from sub-relativistic ejecta that peaked after thousands of days.

Instead, the radio emission was detectable a few days after the merger, peaked around 150 days, then declined. Mooley et al. (2018) concluded that the decline was the signature of a successful relativistic jet (one that had broken through a ‘cocoon’ of mildly relativistic material ejected by the merger). Recent VLBI observations (Ghirlanda et al. 2019) corroborate this.

Radio observations of GW 170817. Black points are observations from Hallinan et al. (2017), Mooley et al. (2018a), Dobie et al. (2018) and Mooley et al. (2018c) (scaled to 1.4 GHz); the blue line is the power-law fit from Dobie et al. (2018). Also shown are the ASKAP 5-σ detectability limits in 2017 (shaded region) and as per design specifications (dotted line). Vertical lines mark ASKAP observations of GW170817. (From Dobie et al. 2019)

Was GW 170817 typical of neutron-star mergers? At present we don’t know. But we can expect the O3 observing run to detect more such events.

In addition, LIGO and Virgo will shortly be joined by LIGO–India and KAGRA (the Kamioka Gravitational Wave Detector) in Japan. Soon we could be looking at tens, perhaps hundreds, of events a year, all localised to tens of square degrees.

ASKAP’s advantages

ASKAP’s field of view, angular resolution and sensitivity mean it is able to follow up poorly localised gravitational-wave events that have no other electromagnetic counterparts. This follow-up will be efficient, too: areas that other gigahertz-frequency telescopes would take hundreds of pointings to cover, ASKAP can handle with just a few.

So what is ASKAP best observing strategy for following up gravitational-wave events? Dougal Dobie (University of Sydney) has led a paper that models different options, with the aim of finding those that minimise telescope time.

Dobie et al. focused on detecting the late-time radio emission – that which occurs tens to thousands of days after the initial event.

Detecting late-time emission

Targeting individual galaxies is sub-optimal, the researchers find. This was the initial strategy for ATCA observations of GW 170817, but it was abandoned when an optical detection became available. GW 170817 was initially localised to within 31 deg2: localisations in the O3 observing run may be hundreds of square degrees, presenting too many galaxies to target one by one.

An obvious approach is to entirely cover a given probability contour (e.g. the 90%) contour. But this too is a sub-optimal strategy because it will cover a lot of area outside the contour if the localisation ellipse is narrow.

Dobie et al. find that the best strategy for detecting late-time emission is ‘shifted ranked tiles’ (Ghosh et al. 2016). This involves moving groups of adjacent tiles (telescope pointings) in an optimum fashion. Calculating each optimum shift takes minutes rather than seconds, but that is not a handicap for late-time observations.

The shifted ranked tiling strategy. The gravitational-wave skymap is covered with a pre-defined grid of non-overlapping tiles. The ranked tiles required to cover the desired probability level are selected and grouped into strips of constant Declination. Each group is iteratively shifted in Right Ascension until the desired probability is enclosed within the minimum number of tiles. The tiles are then ranked and any extraneous tiles removed. (From Dobie et al. 2019)

Detecting prompt emission

Neutron star mergers may also produce prompt, coherent radio emission.

At present, the time taken to receive LIGO alerts, plus ASKAP’s slewing rate, would make it difficult for ASKAP to detect this prompt emission.

However, LIGO detected the gravitational-wave signal of the inspiral that led to GW 170817 for about 100 seconds before the event. If events could be detected before the actual merger, ASKAP and other telescopes could be on-source when the merger occurs.

Should this be possible, Dobie et al. find that ASKAP’s best observing strategy would be ‘greedy ranked tiles’ (Ghosh et al. 2017). This is quicker to compute than ‘shifted ranked tiles’, and so better suited to fast-evolving prompt emission.

Finding hidden kilonovae

The initial outburst of energy from a neutron-star merger is called a kilonova.

Optical and X-ray searches for kilonovae can be hindered by factors such as the inclination angle of a merging system, dust obscuration, proximity to the Galactic plane or solar angle.

ASKASP may be able to detect long-term synchrotron radio emission even in such ‘hidden’ kilonovae, thus giving a more complete picture of the population.

Other
Access: 
Public