MAGIC Telescopes Trace Origin of Rare Cosmic Neutrino
For the first time, astrophysicists have localised the source of a cosmic neutrino originated outside of the Milky Way. With high probability, the neutrino comes from a blazar, an active black hole at the centre of a distant galaxy in the Orion constellation. The scientists reached this interesting finding by combining a neutrino signal from IceCube with measurements from the Fermi-LAT and MAGIC telescopes as well as other instruments. This multi-messenger observation could also provide a clue to an unsolved mystery: the origin of cosmic rays. Researchers of the University of Turku have had a significant role in the study.
Neutrinos are elementary particles hardly interacting with the surrounding world. Although difficult to detect, neutrinos are important cosmic messengers since they carry unique information on the regions where they are produced.
The largest detector specialised in hunting the shy particle species is IceCube, located at the South Pole. It detects about 200 neutrinos per day, however, most of them have low energy and are produced by cosmic rays interacting with the Earth’s atmosphere.
Neutrino Triggers Multi-messenger Observations
On 22 September 2017 IceCube detected a neutrino that was special: Its very high energy (roughly 290 teraelectronvolt) indicated that the particle originated from a distant celestial object. Scientists were also able to tell its incoming direction with high precision.
– Theories predict that the emission of neutrinos will be accompanied by the release of light particles, also called photons, explains Razmik Mirzoyan, MAGIC Collaboration spokesperson and scientist at the Max Planck Institute for Physics.
Photons are electromagnetic radiation and can be traced by telescopes. Therefore, the neutrino alert was promptly disseminated to numerous instruments hoping that their observations might disclose the source of the neutrino.
In fact, Fermi-LAT, a space observatory performing all-sky surveys, reported that the direction of the neutrino was in line with a known gamma-ray source in an active state: the blazar TXS 0506+056. What is more, MAGIC, a 17-meter twin telescope probing high energy gamma-rays from ground, revealed that the radiation from the blazar reaches energies of at least 400 gigaelectronvolt.
These findings combined with the neutrino direction make the blazar a likely candidate for the neutrino source. TXS 0506+056 is an active galactic nucleus, the energetic core of a galaxy at a distance of 3.7 billion light years from Earth. It hosts a supermassive black hole ejecting so-called jets – outflows of particles and energetic radiation moving close to the speed of light.
Hot Trail to Cosmic Radiation
As the birth of neutrinos is always linked to proton interactions, the observations may help to solve an old mystery: The so far unknown birthplace of cosmic radiation, discovered by the physicist Victor Hess in 1912. Cosmic rays largely consist of high-energy protons.
– The cosmic neutrino tells us that the blazar is capable of accelerating protons to very high energies – and therefore may actually be one source of cosmic radiation, says Elisa Bernardini, scientist at DESY in Zeuthen.
There is a reason why cosmic ray sources are so hard to find.
– Positively charged protons are deflected by magnetic fields in space. So they do not travel along straight lines, we cannot see which direction they come from, Bernardini continues.
In contrast, neutrinos and photons possess no charge, which is why they travel through the universe without detours. This means that objects they originate from can be reliably identified.
Born to Proton Parents in the Jet
However, there are still a lot of questions on the underlying processes in the blazar.
– We are looking for the specific site and the mechanism able to accelerate protons, making them the parents for both high-energy neutrinos and photons, says Mirzoyan.
A follow-up study by MAGIC delivers some possible answers. After the alert the telescopes observed the flaring blazar for about 41 hours. The data indicate that the protons are interacting in the blazar’s jets.
– What is more, the results confirm that besides the neutrino a part of the gamma-rays are produced by high-energy protons – and not by other particle interactions in the jet. This is the very first time we can confirm that both neutrinos and gamma rays stem from proton parents, adds Mirzoyan.
The scientists found a very distinctive fingerprint in the spectrum of high-energy gamma-rays from TXS 0506+056.
– We see a loss of photons within a certain energy range meaning these particles must have been absorbed, says Bernardini.
This fingerprint also implies that the IceCube neutrino may be the result of interactions of protons with photons in the jets of the blazar.
– This result corroborates a genuine connection between the different particle messengers: the neutrino and the photons, says Mirzoyan.
Gamma radiation provides information on how the ‘power plants’ in supermassive holes work: that is, how the extremely high energy output comes about and which particle physics processes take place.
MAGIC Telescope Celebrates Its 15 Years in Operation with Finnish Researchers
Researchers of the University of Turku have participated in the operations of the MAGIC telescope since the very beginning. The participants of the collaboration from the Finnish Centre for Astronomy with ESO (FINCA) include Docents Elina Lindfors and Kari Nilsson, and from the Department of Physics and Astronomy of the University of Turku, Docent Aimo Sillanpää and Doctoral Candidates Vandad Fallah Ramazani and Auni Somero. Somero is currently working at the MAGIC telescope in La Palma in Spain.
The research group from Turku has had a significant role in the current blazar study. Lindfors has led an observation programme as a result of which over ten new blazars with very high energy gamma radiation have been found. Using the KVA telescope located in La Palma, the research group has also observed optical radiation from blazars which MAGIC detects in areas with high gamma radiation.
– The recent finding is the outcome of persistent team effort. We have invested in utilising the capacity of the MAGIC telescope to the max, and observation strategies have been optimised. Already for years, there has been discussion on the jets ejected by the blazars probably being sources of very high energy neutrinos, but detecting neutrinos is very difficult. It is even more difficult to get everything to fall into place, which means localising the potential source of the neutrino and following it with the highest gamma energies possible. Now we made it, Lindfors concludes.
The research will be published in two major international scientific publications:
- ”Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A,” The IceCube, Fermi-LAT, MAGIC, AGILE, ASAS-SN, HAWC, H.E.S.S, INTEGRAL, Kanata, Kiso, Kapteyn, Liverpool telescope, Subaru, Swift/NuSTAR, VERITAS, and VLA/17B-403 teams. Science 361, eaat1378 (2018). DOI:10.1126/science.aat1378, http://science.sciencemag.org/cgi/doi/10.1126/science.aat1378
- “The blazar TXS 0506+056 associated with a high-energy neutrino: insights into extragalactic jets and cosmic ray acceleration,” The MAGIC Collaboration: M. L. Ahnen et al. The Astrophysical Journal Letters. DOI: 10.3847/2041-8213/aad083
Text: University of Turku & TJ/AJ
Photo: Vandad Fallah Ramazani