A simulation of a neutron star merging with a black hole. Image credit: Stephan Rosswog (OKC, SU Ast
A simulation of a neutron star merging with a black hole. Image credit: Stephan Rosswog (OKC, SU Astronomy).


Gravitational-wave detectors have indeed found many new spectacular mergers over the past year or so, including the recently announced massive black-hole merger. However, despite enormous efforts to find more optical counterparts, using for example the Zwicky Transient Facility (ZTF) at Palomar Observatory, none have been discovered in the past years.

Ana Sagués Carracedo started her PhD studies at the Oskar Klein Centre as part of the GREAT research environment, with the aim to find the next electromagnetic counterpart of a binary merger involving a neutron star. “It has been many nights waking up to a gravitational-wave alarm where we have triggered follow-up telescopes and searched the sky,” describes Ana. “We have scrutinized thousands of candidates, but unfortunately none have been similar to that strong event that was found back in 2017.”

Now, researchers have started to exploit the past years ‘non-detections’ to actually learn something about the merging neutron stars, such as how common such phenomena could be and what we can say about their brightness.

“It is incredible to see how much information we can extract from a non-detection. The fact that we got zero photons from these events already tells us that the electromagnetic counterparts can’t be too bright (or we would have seen them!)” explains Mattia Bulla who is working at Nordita and developed some of the models that were used in these investigations. “These non-detections rule out bright models and give us precious information on the key parameters governing these spectacular mergers.”

A recent paper published this week in Nature Astronomy, led by US astronomers Shreya Anand and Michael Coughlin, focuses on the search for counterparts for a new type of systems -- those where a black hole merges with a neutron star.
”We found nothing bright this time either,” says Jesper Solllerman, who is co-author on this paper together with OKC researchers Mattia, Ana and Erik Kool. ”But in the paper we manage to show that we are indeed able to detect these things with the strategy and equipment we currently have, and that such a detection would allow us to learn quite a lot about the properties of the merging black hole and the neutron star.”
Scientists at the OKC continue to work hard in this field, which includes a combination of fundamental hydrodynamic modeling and observations. They are also engaged in collaborations to upgrade and build more follow-up telescopes, including the ZTF-II project and the Vera Rubin telescope. The gravitational wave detectors are currently undergoing updates and are expected to start up again at the end of 2021 for their fourth observing run (O4). According to Erik, "the increased sensitivity of the detectors in O4 will improve the localization of gravitational wave sources, which will help us in finding the electromagnetic counterparts of these events -- if there are any to be found."