Astronomers have analysed 13 years of data from NASA’s Fermi Gamma-ray Space Telescope to reveal a surprise gamma-ray feature outside of our galaxy.
The gamma-ray signal found by the team is in a similar direction and has a nearly identical magnitude as another unexplained feature that is produced by some of the most energetic cosmic particles ever detected.
Alexander Kashlinsky, a cosmologist at the University of Maryland and NASA’s Goddard Space Flight Center in Greenbelt, said: “It is a completely serendipitous discovery.
“We found a much stronger signal, and in a different part of the sky, than the one we were looking for.”
The paper describing the findings was published in The Astrophysical Journal Letters.
Searching for a gamma-ray feature related to the cosmic microwave background
The team was looking for a gamma-ray feature related to the cosmic microwave background (CMB) – the oldest light in the Universe.
Scientists argue that the CMB developed when the Universe had cooled enough to form the first atoms. This event released a burst of light that permeated the cosmos for the first time.
The light was first detected in 1965 in the form of faint microwaves over the sky.
Dipole structure
In the 1970s, astronomers found that the CMB had a dipole structure which was measured at high precision by NASA’s COBE mission. The CMB is around 0.12% hotter, with more microwaves than average, toward the constellation Leo. In the opposite direction, it is colder by the same amount, with fewer microwaves than average.
To study the temperature variations within the CMB, the signal must be removed. Astronomers regard the pattern as a result of the motion of our solar system relative to the CMB at approximately 230 miles per second.
The motion will give rise to a dipole signal in the light coming from any astrophysical source.
However, the CMB is the only one that has been precisely measured. Astronomers have looked for the pattern in other forms of light to confirm or challenge the idea that the dipole is caused by the motion of our solar system.
“Such a measurement is important because a disagreement with the size and direction of the CMB dipole could provide us with a glimpse into physical processes operating in the very early Universe, potentially back to when it was less than a trillionth of a second old,” said co-author Fernando Atrio-Barandela, a professor of theoretical physics at the University of Salamanca in Spain.
Finding a gamma-ray dipole
The team added together years of data from Fermi’s LAT to find a related dipole emission pattern in the gamma-rays. Due to the effects of relativity, the gamma-ray dipole should be amplified by as much as five times over the currently detected CMBs.
The astronomers combined 13 years of observations of gamma rays above three billion electron volts. They removed all resolved and identified sources and stripped out the central plane of our galaxy to analyse the gamma-ray background.
“We found a gamma-ray dipole, but its peak is located in the southern sky, far from the CMB’s, and its magnitude is ten times greater than what we would expect from our motion,” said co-author Chris Shrader, an astrophysicist at the Catholic University of America in Washington and Goddard.
“While it is not what we were looking for, we suspect it may be related to a similar feature reported for the highest-energy cosmic rays.”
Linking the two phenomena
Cosmic rays are accelerated charged particles. The rarest and most energetic particles, called ultrahigh-energy cosmic rays, carry more than a billion times the energy of three GeV gamma rays. Their origins are one of the biggest mysteries in astrophysics.
The Pierre Auger Observatory in Argentina has reported a dipole in the arrival direction of UHECRs since 2017. Being electrically charged, cosmic rays are diverted by the galaxy’s magnetic field by different amounts depending on their energies. The team found that the UHECR dipole peaks in a sky location similar to what the team found in gamma rays.
Both have similar magnitudes – about 7% more gamma rays or particles than average coming from one direction and similar amounts arriving from the opposite direction.
The team believe that the phenomena are linked, as unidentified sources are producing both the gamma rays and the ultrahigh-energy particles. The astronomers must now either locate the mysterious sources or propose alternative explanations for both features.
