Reading a Message Carried on a Distant Tide

Some 700 million years ago, as it was being ripped to pieces by a black hole, a distant star sent out “messages” in the form of subatomic particles – neutrinos. In 2019, one of those neutrinos slammed into a detector at the South Pole with unprecedented force. In only the second instance in which researchers were able to trace a high-energy neutrino back to its cosmic source, international research teams working with telescopes on the ground and in space, an array of detectors and theoretical models found that this tiny, ageless particle had a story to tell.

Prof. Avishay Gal-Yam of the Weizmann Institute of Science’s Particle Physics and Astrophysics Department was part of that team, working with researchers at DESY in Germany. This particular research focused on so-called tidal disruption events (TDEs), which occur when stars venture too close to the supermassive black holes at the centers of their galaxies. These dramatic events were first observed around a decade ago, and Gal-Yam, along with his then graduate student Iair Arcavi, was among the first to study this class of cosmic events – yet they are still not well understood. The gravitational force acting on a doomed star – in this case from a black hole with a mass of 30 million suns – creates a tidal effect. That is, its pull is stronger on the near side than on the far one. A tide on Earth, resulting from the gravitational pull of the Moon, is strong enough to raise and lower water levels. The tides in a star coming under the gravitational pull of a black hole stretch it out like taffy until the star is completely pulled apart. Gal-Yam and the German researchers had been surveying the sky for evidence of cosmic explosions when they just came across such a TDE.

The distant TDE in question was first picked up by the Zwicky Transient Facility (ZTF) on Mount Palomar, California, in April 2019. Weizmann Institute astrophysicists also had a hand in setting up the facility, essentially a set of computerized telescopes trained to spot new sources of bright light in the night sky that indicate a supernova, TDE or other such occurrence. The software used to detect this – and all other sudden light sources found by ZTF – is based on an algorithm developed by Weizmann scientists Dr. Barak Zackay and Prof. Eran Ofek, together with Gal-Yam.

In TDEs, the temporary brightening is an effect of the accretion disk – gaseous matter ripped from the star that swirls around the black hole – heating up before being sucked into the black hole’s darkness. When such an event is identified in the Zwicky Facility, other telescopes and detectors are directed toward that patch of sky to figure out what kind of event has been spotted and to gather as many details as possible.

Around half a year later, in October 2019, a neutrino with super-high energy was clocked passing through the IceCube Neutrino Observatory’s detector, an array of detectors buried under the ice in the South Pole. The fact that this neutrino was detected at all was something of a minor miracle. Neutrinos are nearly massless and don’t normally interact with matter, so trillions of them pass through the Earth each day without a trace. A cosmic neutrino from a distant place in the Universe must hit one of the detectors in just the right way to be recorded, and the IceCube detector therefore reports on the discovery of just a handful of such neutrinos each year.

The particle’s extreme energy was produced when the matter it came from swirled around the black hole as if it were in an immense particle accelerator

This particular neutrino hit the detector with an incredible energy of over 100 teraelectronvolts. That told researchers that it must have come from an unusual source. When they traced it back toward its origin, they found it had come from the same patch of sky where the TDE had earlier been detected. The research team realized that if these events were indeed related, the particle’s extreme energy was produced when the matter it came from swirled around the black hole as if it were in an immense particle accelerator – much more powerful than any created on Earth – and the neutrino was flung out with an intense force that sped it straight (over 700 million years) in our direction.

Prof. Anna Franckowiak of DESY calls her field “multi-messenger astronomy.” The team there had developed special software specifically to link data from the Zwicky Transient Facility and the IceCube neutrino detector. In addition to information from a range of optical telescopes, including one on NASA’s Swift satellite, data from a radio telescope showed that even when the visible signs of the TDE died down fairly quickly, other forms of radiation continued to rise, and the theoretical analysis was able to show how the neutrino could have been formed and thrown out in the later stages of the tidal event.

“This was a really exciting demonstration of the power of combining many instruments and diverse research teams,” says Gal-Yam. The scientists believe that more such discoveries will arise in the future, helping scientists to understand how this giant particle accelerator is created and how the Universe’s high energy neutrinos form. This will be facilitated, in part, by a planned expansion of the IceCube array. But Weizmann and DESY scientists intend to make another contribution in the form of ULTRASAT, a mini-satellite developed at the Institute under the leadership of Prof. Eli Waxman, now being planned with significant input from DESY. After its launch sometime in 2024, ULTRASAT’s highly sensitive telescopes will look for transient phenomena in a range that is currently barely explored: the ultraviolet. Sweeping a large field, the satellite may be able to pick up currently undetectable signals from some of the Universe’s most spectacular events, from TDEs to the gravitational waves emitted by crashing neutron stars, supernovae and more, when they are in the very earliest, pre-cataclysmic stages.