by Anna Franckowiak (Ruhr-University Bochum)

Neutrinos are the most elusive elementary particles we know of. They have no charge, almost no mass and almost never interact. In summary: they are really hard to catch. At the same time they are unique messengers from the Universe, directly tracing the acceleration of protons and heavier nuclei in distant sources. So it is worth the effort to build giant detectors to catch at least some of the ghostly particles.

The IceCube observatory at the South Pole is currently the largest operating neutrino detector. It comprises a volume of one cubic kilometer of clear ice with a grid of more than 5000 photomultipliers with the goal to detect Cherenkov light from secondary charged particles produced in the rare neutrino interactions. A breakthrough was achieved in 2013 with the first detection of a diffuse cosmic neutrino flux at TeV to PeV energies. Since then we are trying to figure out where those neutrinos are coming from.

The sources have to be powerful proton accelerators and have a target for the protons to interact with (ambient matter or photon fields) in order to produce the TeV to PeV neutrinos. Candidate sources are blazars, gamma-ray bursts, choked-jet and interacting supernova and tidal disruption events. Those all have electromagnetic counterparts.

To find the electromagnetic counterparts of transient or variable neutrino sources the IceCube collaboration has implemented a real-time system. Interesting neutrino events are identified within tens of seconds at the South Pole and their direction is broadcasted to the community in form of a GCN message.

We perform an optical follow up of the typically few square degree sized neutrino error contour with the wide-field-of-view optical survey instrument, ZTF (Zwicky Transient Facility). ZTF provides us with photometric information of possible source candidates. After making a pre-selection based on the ZTF light curves of the candidates, we are typically left with a handful of sources. To really know if the sources are potential neutrino emitters, we need to classify them spectroscopically. Our candidates are typically at magnitudes of 19-21, therefore we need a considerable larger  telescope to get a spectroscopic classification. The Nordic Optical Telescope (NOT) is an ideal instrument for this purpose. It’s located at a similar latitude as ZTF, so sources discovered by ZTF will mostly be visible for NOT too, and it is sensitive enough to obtain spectra even for sources at ZTF’s detection limit.

On July 6 2022 we triggered NOT to follow up ZTF19adgzidh/AT2021bei, an optical candidate discovered by ZTF in the follow-up of the 200 TeV neutrino IceCube-220624A (Santander et. al, GCN 32260; Stein et al., GCN 32357). The optical light curve matched the typical evolution of a Supernova. Thanks to the NOT spectrum we identified the source as a flaring AGN at z=0.48. The optical flare was not exceptional, therefore this is not a promising neutrino source candidate. Let’s hope we have more luck next time!

Anna Franckowiak is a full professor at Ruhr-University Bochum. Her research focuses on identifying the sources of high-energy neutrinos by detecting their electromagnetic counterparts. She is a member of the IceCube collaboration, who operates the largest neutrino telescope in the world.