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Astronomers are developing ways to rapidly search the sky after a gravitational wave detection so they can see the light from huge merger events.
The detection of gravitational waves by the LIGO and Virgo experiments has opened up an exciting new avenue of astronomy observations beyond the electromagnetic spectrum. They capture the ripples in space time generated by huge events, such as a neutron star merging with a black hole (an NSBH merger).
However, traditional astronomy still has a vital role to play, and researchers are working to catch the optical signals from events that LIGO and Virgo flag up. Shreya Anand at the California Institute of Technology in Pasadena, and Michael Coughlin at the University of Minnesota, working in a large international team that includes researchers at New York University Abu Dhabi, have described their recent efforts to observe light from NSBH mergers, and how they are rapidly learning from some initial failures to improve their chances of seeing these transient events.
“Every time the LIGO/Virgo interferometers receive a promising signal, they issue an alert to the astronomy community, with information about the signal’s likely position, distance, and the probability of the event producing an electromagnetic counterpart,” says Anand.
The trouble is, LIGO/Virgo cannot pinpoint exactly where the gravitational wave signal came from. There is a lot of uncertainty. When an alert comes out, Anand, Coughlin and co-workers must immediately search hundreds to thousands of square degrees using the Zwicky Transient Facility (ZTF) at Palomar Observatory in California, a unique camera designed to detect objects that rapidly change in brightness.
On 5 and 15 January 2020, LIGO/Virgo detected NSBH candidate signals, both of which were deemed likely to leave some light-emitting neutron star material outside the final black hole. Over the following three nights, ZTF scanned continuously across the probable sky regions, but found no signals that could plausibly be associated with either of the gravitational wave events.
“While the non-detections are a bit disappointing, they can also inform us about the nature of the transients,” says Coughlin. “For example, it might imply that the black hole has simply swallowed the neutron star, not disrupting much material in a way that we could detect. This tells us something about how much larger the black hole is than the neutron star.”
Anand, Coughlin and co-workers have used their experience of those observations to inform many computer simulations, with results suggesting it may be useful to focus on obtaining redder rather than bluer observations.
“We should also note that luck plays an important role here; some nights of observations were limited by clouds over Palomar, so simply having more clear nights and therefore sensitive observations will help us in the future,” says Anand.
Anand, S., Coughlin, M.W. et al. Optical follow-up of the neutron star–black hole mergers S200105ae and S200115j. Nat. Astron. http://dx.doi.org/10.1038/s41550-020-1183-3 (2020).