Researchers Point to Supermassive Black Hole as Elusive Neutrino Source

Physicists may have linked the elusive source for the highest energy neutrinos, abundant sub-atomic particles with no electrical charge that race through the universe, to a black hole -- the supermassive black hole at the center of the Milky Way galaxy called Sagittarius A* (pronounced "Sagittarius A-star").

Observations leading to a potential breakthrough in the identification of a source were made with three NASA space telescopes, the 15-year-old Chandra X-ray Observatory,  10-year-old Swift Gamma Ray Burst mission observatory and the 2-year-old NuSTAR (an X-ray observatory), as well as the IceCube Neutrino Observatory, which is positioned under the South Pole. IceCube has recorded 36 high-energy neutrinos since the facility became operational in 2010.

"We now have the first evidence that an astronomical source, the Milky Way's supermassive black hole may be producing these very energetic neutrinos," states University of Wisconsin physicist Yang Bai, in a Nov. 13 NASA announcement. He is among a half dozen scientists from the universities of Wisconsin and Hawaii who published their findings in Physical Review D.

Massive black hole at the center of the Milky Way galaxy, Sagittarius A*, a likely source of super-energetic neutrinos. NASA, University of Wisconsin.

The Earth is showered by lower energy neutrinos whose origin is the sun. However, neutrinos millions to billions of times more energetic emanate from well beyond the solar system. Candidates for their origin have included galactic mergers, gamma ray bursts, stellar explosions, pulsars and material falling into black holes. Their physical properties permit neutrinos to pass easily through most terrestrial materials making the construction of detectors that could point to their source difficult, which explains the relatively low number of overall IceCube detections.

That led the research team to look for correlations between high energy cosmic events detected by Chandra, Swift and NuStar and the high energy neutrinos recorded by IceCube, a sensor network distributed throughout a cubic kilometer of ice beneath the South Pole.

"We checked to see what happened after Chandra witnessed the biggest outburst ever detected from Sagittarius A*", notes University of Wisconsin researcher Andrea Peterson, one of the study's co-authors.  "And less than three hours later, there was a neutrino detection at IceCube.''

Swift and NuSTAR also observed Sagittarius A* flares, followed within a few days by IceCube high energy neutrino detections.

Sagittarius A*, weighing in at 3.6 million solar masses, lies 26,000 light years from the Earth. Most star systems host massive central black holes.

The joint Wisconsin/Hawaii university research team is still trying to develop an explanation for how a massive black hole might generate neutrinos. One idea is that it could happen when particles around the black hole are accelerated by a shock wave (conceptually like a sonic boom) that produces charged particles that then decay to neutrinos.

This latest result may also contribute to the understanding of another major issue puzzling astrophysicists, the source of high-energy cosmic rays. Since the tiny charged particles that make up cosmic rays are deflected by magnetic fields in the Milky Way, scientists have also been handcuffed in their efforts to identify an origin. The charged particles accelerated by a shock wave near Sagitarius A* may be a significant source of very energetic cosmic rays as well as high energy neutrinos.