💥 A worldwide hunt for cosmic neutrino sources

For the first time, an international collaboration of more than 800 scientists has joined forces to track down the sources of high-energy cosmic neutrinos. By combining neutrino observations with gamma-ray observations, this multi-messenger approach opens a new window onto the most violent phenomena in the Universe.

This study marks a turning point: four networks of atmospheric Cherenkov telescopes (FACT, H.E.S.S., MAGIC, VERITAS) and the Fermi satellite's Large Area Telescope (LAT) have pooled their data for the first time to search their data for gamma-ray events that would come from the same source as the neutrinos detected by IceCube in 2017. An unprecedented cooperation between facilities usually in competition!


The analysis includes follow-up observations of high-energy neutrino events observed by the four observatories between September 2017 (after the IceCube-170922A event) and January 2021. The study found no association between the gamma-ray sources and the observed neutrino events but was able to provide combined upper limits on the very-high-energy gamma-ray flux that these sources could emit. These limits are more constraining than those obtained by a single telescope, because they are based on increased sensitivity thanks to the combination of data.

These limits allow to rule out certain theoretical models of cosmic ray acceleration. If a model predicts a gamma-ray flux higher than the established limits, it must be revised.

Why look for gamma rays associated with neutrinos?

Astronomy has entered the era of "multi-messenger" astrophysics, where cosmic phenomena are studied not only using electromagnetic radiation, but also gravitational waves and neutrinos.

When a neutrino is produced in an astrophysical source (such as a blazar or a supernova remnant), it should be accompanied by high-energy gamma rays. These gamma rays, unlike neutrinos, are easier to detect, and if gamma rays are detected, it confirms that the source is indeed a cosmic accelerator. If nothing is detected, an upper limit can still be established on the gamma-ray flux that this source could emit.


The researchers have:
- searched for known gamma-ray sources in connection with neutrino events detected by IceCube between September 2017 and January 2021.
- coordinated observations from several gamma-ray telescopes (H.E.S.S. for the southern sky, MAGIC, VERITAS and FACT for the northern sky, as well as Fermi's Large Area Telescope (LAT)).
- then combined their data to establish joint upper limits on the gamma-ray flux associated with each neutrino event.

Result: If the telescopes had detected gamma rays in coincidence with the neutrinos, it would have confirmed the identity of the source.

Since no significant detection was made in most cases, the researchers were able to establish upper limits on the gamma-ray flux that these sources could emit. These limits are more constraining than those obtained by a single telescope, because they are based on complete sky coverage and increased sensitivity thanks to the combination of data.

Concrete example: The case of the blazar 1ES 1312-423

In the study, one particular case drew attention: in March 2019, H.E.S.S. detected very-high-energy gamma rays from the blazar 1ES 1312-423, after IceCube detected a cluster of neutrinos in the same region of the sky. However, after a thorough analysis, the researchers concluded that:
- The observed gamma-ray flux was consistent with the "normal" emission from this blazar.
- The neutrinos detected by IceCube were probably not related to this blazar, but rather to a random fluctuation of the neutrino background.
- The upper limits established for the other sources led to the conclusion that no source showed a clear correlation between neutrinos and gamma rays during the studied period.

What these limits mean for science:
- Rule out theoretical models: The study provides a "reference dataset" that constrains theoretical models of neutrino emission. For example, if a model predicts that a source should emit a gamma-ray flux higher than the upper limit set by the joint analysis, then that model is invalid and must be revised.
- It demonstrates the ability of the current generation of Cherenkov telescopes to operate as a global network, reacting quickly to alerts from the IceCube neutrino observatory.
- This work paves the way for the future of time-domain astronomy. The lessons learned from this coordinated research are essential for the future Cherenkov Telescope Array Observatory (CTAO), which will offer improved sensitivity and fast reaction times.
- Furthermore, the global network is preparing to ensure follow-up of alerts from next-generation neutrino observatories such as KM3NeT, which will significantly increase our ability to catch these violent cosmic events as they happen.