Neutrinos are elementary particles that hardly interact with the surrounding world at all. Although difficult to detect, neutrinos are important cosmic messengers since they carry unique information about the regions where they are produced.
The largest detector specialized in hunting the shy particle species is IceCube, which is located at the South Pole. It detects about 200 neutrinos per day, although 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 might have originated from a distant celestial object. Scientists were also able to identify 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 in the hope that their observations might disclose the source of the neutrino.
In fact, Fermi-LAT, a space observatory that conducts 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 that probes high energy gamma-rays from the ground, revealed that the radiation from the blazar reaches energies of at least 400 gigaelectronvolts.
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 4.5 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.
A hot trail to cosmic radiation
Since the birth of neutrinos is always linked to proton interactions, the observations help to solve an old mystery: The birthplace of cosmic radiation, discovered by the physicist Victor Hess in 1912, which is so far unknown. 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”, explains 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”, Bernardini continues. “So they do not travel along straight lines, we cannot see which direction they come from.” By contrast, neutrinos and photons possess no charge, which is why they travel through the universe without detours. This means that the objects from which they originate 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 black holes work: that is, how the extremely high energy output comes about and which particle physics processes take place.”
Multiwavelength observations of a flaring blazar coincident with an IceCube high-energy neutrino; IceCube, Fermi-LAT, MAGIC, AGILE, ASAS-SN, HAWC, H.E.S.S, INTEGRAL, Kapteyn, Kanata, Kiso, Liverpool, Subaru, Swift, Veritas, VLA; Science, 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; Elisa Bernardini, Wrijupan Bhattacharrya, Susumu Inoue, Konstancja Satalecka, Fabrizio Tavecchio; akzeptiert für die Veröffentlichung in The Astrophysical Journal Letters, https://arxiv.org/abs/1807.04300.