In April 2019 the Zwicky Transient Facility (ZTF) detected a transient source in the middle of a distant galaxy, 700 million light years away. What first looked like a supernova, something the ZTF has now detected thousands of, instead turned out to be something more unusual, a tidal disruption event (TDE). TDE:s are stars that come too close to the supermassive black hole lurking at the centre of their galaxy. The star is then stretched out – spaghettified – and part of it is drawn into the black hole while the rest is shredded in a luminous outburst. During its first three years of operation, ZTF has discovered more than a dozen of these phenomena, and these results were recently published.
But there is more to this story. Six months later, the IceCube instrument deep in the Antarctic ice suddenly detected a high-energy neutrino. Neutrinos are ghost-like particles that require huge detectors to catch them, and for this particular neutrino the high energy indicated that it came from a source outside our Milky Way galaxy. High-energy extragalactic neutrinos have only recently been discovered and their origin is one of the major unsolved puzzles in astrophysics.
Turning the optical telescopes to the part of the sky where the neutrino came from, astronomers identified one interesting transient, the above mentioned TDE! Could they be connected?
Further observations of this TDE system in other regimes of the electromagnetic spectrum revealed an active and unusual system. A recent paper in the journal Nature Astronomy (arXiv version), led by graduate student Robert Stein from DESY, Berlin, argues that this is a good candidate for an extragalactic neutrino source.
As the star approaches the black hole, the enormous tidal forces stretch it more and more until it is finally shredded. Half of the stellar debris is flung back into space, while the remaining part forms a rotating accretion disk from which two strong outflows of matter shoot up and down. The system acts as a powerful natural particle accelerator. Scientists have caught a single neutrino hurled towards Earth by the system. Animation: DESY, Science Communication Lab (broadcast quality, please contact presse@desy.de)
"This is only the second such claim," explains Chad Finley, researcher at IceCube and the Oskar Klein Centre, who helped establish the first such connection back in 2018. The previous connection was with a blazar, which also contains a central supermassive black hole.
Researchers at the OKC are leading much of the transient investigations with the ZTF. Ariel Goobar and Jesper Sollerman are co-authors on the paper in Nature Astronomy.
"The last word is not said yet, but it is very exciting that we now have the tools needed to both find extragalactic neutrinos and quickly hunt for their origins," says Ariel Goobar. "This is probably not the end of this story, but rather the beginning of solving a long-standing mystery."
In the upcoming years IceCube will continue searching the neutrino sky while undergoing an expansion to become the much larger IceCube-Gen2 detector, with construction of the first Upgrade phase starting in 2023. ZTF has just launched its sequel: ZTF-2, to operate at least three years, until the Vera Rubin Observatory, an 8-meter survey telescope in Chile sees first light at about the same time as the upgraded neutrino detector. The future looks very promising!
