Kamioka ObservatoryICRR, The University of Tokyo

The Super-Kamiokande Collaboration published a search for astronomical neutrinos from Blazar TXS0506+056 in Astrophysical Journal Letters, which has been featured as a “Research Highlight” in Nature Physics.

Neutrino detectors worldwide are studying astronomical neutrinos

Neutrinos observed at Super-Kamiokande are named based on their point of origin. For instance, neutrinos emitted from nuclear reactors are dubbed “reactor neutrinos”, those generated by particle accelerators are “accelerator neutrinos” and those generated in the collisions of cosmic rays with the atmosphere are called “atmospheric neutrinos”. Similarly, neutrinos which are generated by astronomical events and later arrive at the Earth are called “astronomical neutrinos”. These neutrinos are expected to play a big role in elucidating the nature of those astronomical phenomena. To date, astronomical neutrinos from the Sun and from Supernova as well as ultra-high energy neutrinos, whose origins are as yet unknown, have been observed. At present, neutrino detectors all over the world are searching for new sources of astronomical neutrinos.

Ultra-high energy neutrinos from Blazar TX0506+056 were observed by the IceCube detector

At 20:54 on September 22nd, 2017 the IceCube detector located at the South pole observed an ultra-high energy neutrino signal. This information was immediately announced to astronomical telescopes throughout the world and follow-up observations in direction from which the neutrinos arrived. As a result, the origin of the neutrinos was determined to be an astronomical body known as Blazar TX0506+056.

A blazar is a supermassive black hole that produces an energetic jet of particles directed towards the observer. However, while the exact mechanism of the blazar’s jet is still unknown they are expected to consist of high energy protons. Mesons, such as pions and kaons, are produced in the interactions of protons in the jet interaction with each other or with particles in the surrounding interstellar gas. The decays of those mesons produce neutrinos, which can in principle be observed with neutrino detectors on Earth. Importantly, because neutrinos rarely interact with matter and therefore travel unaltered on their way to terrestrial detectors, the observation of these blazer neutrinos is expected to provide valuable information about the mechanism of jet formation and particle acceleration therein.

Super-Kamiokande’s search for blazar neutrinos yields an upper limit on their flux in the 1 GeV to 10 TeV region

In the published analysis, a search for neutrinos from the direction of Blazar TXS0506+056 was performed using Super-Kamiokande data from 1996 to 2018. Though Super-Kamiokande is smaller than IceCube, it can detect lower energy neutrinos and has operated for longer, giving it a complementary window on the blazar. Unfortunately, no significant signal was observed so an upper limit was placed on the emitted neutrino flux. It is the world’s first limit in the energy region from 1 GeV to 10 TeV.

Super-Kamiokande has an important role in multi-messenger astronomy.

Though this analysis was performed focusing on only one blazar, there are hundreds of other blazers that can be studied. In addition, researchers expect to observe neutrinos from other energetic astronomical objects, such as a gamma ray bursts. Amidst rising interest in studying astronomical processes using their multiple particle “messengers”, such as gamma rays, gravitational waves, and neutrinos, Super-Kamiokande continues to play an important role in understanding these phenomena.

[Link]Published article (pdf)