Riddle of cosmic ray hotspot in the Great Bear
Sen—Cosmic rays are one of the most energetic yet mysterious phenomena in the Universe. Despite their name, they are particles rather than rays, and their source is generally uncertain.
Today an observatory run by the University of Utah revealed that it has found a “hotspot” in the constellation of Ursa Major, the Great Bear, which appears to be emitting a particularly high proportion of cosmic rays.
The zone lies beneath the famous pattern of seven stars within the constellation that is commonly known either as the Big Dipper or the Plough.
Low-energy cosmic rays are known to come from the Sun and exploding stars. But those with the highest energy arrive from the depths of the Universe. Astronomers believe they are given off by blazars—jets from active galactic nuclei driven by supermassive black holes—or the most powerful supernovae and gamma ray bursts.
Other sources mooted include noisy radio galaxies, shock waves from colliding galaxies and even some exotic hypothetical sources such as leftover particles from the Big Bang that formed the Universe 13.8 billion years ago.
Unfortunately, it is impossible to tell where the individual rays come from, that bombard Earth all the time, because their paths across the Universe have been deflected by the magnetic fields of our Galaxy, the Solar System and even the Earth itself.
This map of the northern sky shows the hotspot in cosmic ray concentrations, coloured bright red and yellow spot. Image credit: K. Kawata, University of Tokyo Institute for Cosmic Ray Research
However, 90 per cent of the ultrahigh-energy ones are reckoned to come from within a distance of 300 million light-years because those from more distant sources would be greatly weakened by interaction with the cosmic microwave background left by the Big Bang.
The new study by an international research team looked at ultrahigh-energy cosmic rays which are bent the least by magnetic fields in space. They were recorded by the $25 million Telescope Array cosmic ray observatory west of Delta, Utah, which is the Northern Hemisphere’s largest cosmic ray detector, between 11 May, 2008, and 4 May, 2013.
During the five years, only 72 such cosmic rays were detected, confirmed and analyzed for their energy and source direction.
But 19 of those cosmic rays were detected coming from the direction of a hotspot in the southwest corner of the constellation Ursa Major, compared with only 4.5 that would have been expected if the cosmic rays came randomly from all parts of the sky, according to the researchers.
The hotspot is a 40°-diameter circle representing 6 per cent of the northern sky. “We have a quarter of our events in that circle instead of 6 per cent,” said Professor Charlie Jui, of the University of Utah.
Colleague Gordon Thomson, spokesman and co-principal investigator for the Telescope Array, said: “This puts us closer to finding out the sources – but no cigar yet. All we see is a blob in the sky, and inside this blob there is all sorts of stuff – various types of objects—that could be the source. Now we know where to look.”
The hotspot is near the “supergalactic plane”—the rather flattened Virgo supercluster of galaxies. Our Milky Way galaxy is on the outskirts of the supercluster.
One of the 523 table-like scintillation detectors at the Telescope Array cosmic ray observatory which measures the strength and direction of particles that fall to Earth. Image credit: John Matthews, University of Utah
Observations from a second site, the Pierre Auger cosmic ray observatory in Argentina, provide evidence for a weaker Southern Hemisphere hotspot. If that proves real, Thomson says cosmic rays in the northern and southern hotspots must come from different sources.
Jui says a separate study now in progress suggests the distribution of ultrahigh-energy cosmic rays in the northern sky is consistent with the “large-scale structure” of the universe, which means the cosmic rays tend to come from areas of the universe where matter is concentrated in clusters and superclusters of galaxies.
The Telescope Array, jointly financed by Japan and the United States, uses two methods to detect and measure cosmic rays. At three locations spread across the desert, sets of mirrors called fluorescence detectors watch the skies for faint blue flashes created when incoming cosmic rays hit nitrogen gas molecules in the atmosphere.
Those collisions create a cascade of other collisions with atmospheric gases, resulting in “air showers” of particles detected by 523 table-like scintillation detectors spaced over 300 square miles of desert. These helped determine the energy and chemical makeup of the cosmic ray particles.