Fermi Gamma-ray Space Telescope detects surprise gamma-ray feature beyond our galaxy

Astronomers analyzing 13 years of data from NASA’s Fermi Gamma-ray Space Telescope have discovered an unexpected and still unexplained feature outside of our galaxy.

“It’s a completely serendipitous finding,” said Alexander Kashlinsky, a cosmologist at the University of Maryland and NASA’s Goddard Space Flight Center in Greenbelt, who presented the research at the 243rd meeting of the American Astronomical Society in New Orleans. “We found a much stronger signal, and in a different part of the sky, than we were looking for.”

Intriguingly, the gamma-ray signal is in a similar direction and with almost identical amplitude to another unexplained feature, produced by some of the most energetic cosmic particles ever detected.

An article describing the results is published in Letters from the astrophysical journal.

The team was looking for a feature of gamma rays linked to the CMB (cosmic microwave background), the oldest light in the universe. Scientists say the CMB appeared when the hot, expanding universe cooled enough to form the first atoms, an event that released a burst of light that, for the first time, could permeate the cosmos. Stretched by the subsequent expansion of space over the past 13 billion years, this light was first detected as faint microwaves throughout the sky in 1965.

In the 1970s, astronomers realized that the CMB had a so-called dipolar structure, which was then measured with high precision by NASA’s COBE (Cosmic Background Explorer) mission. The CMB is about 0.12% warmer, with more microwaves than average, towards the constellation Leo, and colder by the same amount, with fewer microwaves than average, in the opposite direction.

In order to study the tiny temperature variations within the CMB, this signal must be removed. Astronomers generally consider this trend to be the result of our own solar system’s motion relative to the CMB at about 230 miles (370 kilometers) per second.

This motion will give rise to a dipole signal in the light from any astrophysical source, but so far the CMB is the only one that has been measured accurately. By looking for this trend in other forms of light, astronomers could confirm or challenge the idea that the dipole is entirely due to the motion of our solar system.

“Such a measurement is important because a disagreement with the size and direction of the CMB dipole could give us insight into the physical processes operating in the very beginning of the universe, potentially when it was less than a trillionth of a second ” said co-author Fernando Atrio-Barandela, professor of theoretical physics at the University of Salamanca in Spain.

The team estimated that by adding together several years of data from Fermi’s Large Area Telescope (LAT), which scans the entire sky several times a day, an associated dipole emission pattern could be detected in the gamma rays. Thanks to the effects of relativity, the gamma ray dipole is expected to be amplified up to five times compared to currently detected CMBs.

The scientists combined 13 years of Fermi LAT observations of gamma rays above about 3 billion electron volts (GeV); For comparison, visible light has energies between about 2 and 3 electron volts. They removed all resolved and identified sources and removed the central plane of our galaxy, the Milky Way, in order to analyze the extragalactic gamma ray background.

“We found a gamma-ray dipole, but its peak is located in the southern sky, far from that of the CMB, and its magnitude is 10 times greater than we would expect from our motion,” said co-author Chris Shrader , astrophysicist. at the Catholic University of America at Washington and Goddard. “Although this is not what we were looking for, we think it could be related to a similar feature reported for higher energy cosmic rays.”

Cosmic rays are accelerated charged particles, mainly protons and atomic nuclei. The rarest and most energetic particles, called UHECR (ultra-high energy cosmic rays), carry more than a billion times the energy of 3 GeV gamma rays, and their origins remain one of the greatest mysteries of astrophysics.

Since 2017, the Pierre Auger Observatory in Argentina has reported a dipole in the direction of arrival of UHECRs. Being electrically charged, cosmic rays are deflected by the galaxy’s magnetic field in different proportions depending on their energies, but the UHECR dipole peaks in a location in the sky similar to what Kashlinsky’s team finds in gamma rays . And both have strikingly similar magnitudes: about 7% more gamma rays or particles than average coming from one direction and proportionately smaller amounts arriving from the opposite direction.

Scientists believe it is likely that the two phenomena are linked: as yet unidentified sources produce both gamma rays and very high energy particles. To solve this cosmic conundrum, astronomers must either locate these mysterious sources or come up with alternative explanations for the two features.