Astronomers analyzed space telescope observations for 13 years to look for gamma-ray bursts that overlap with similar features of radiation in other bands. So they discovered an unexpected and, most importantly, still incomprehensible source of gamma radiation in a completely different part of the sky.

Residual radiation is the background thermal radiation of the universe that appears in the early stages of its development, during the period of primary recombination, when matter cools sufficiently and the first atoms are formed. Astronomers first detected this light in the form of weak microwaves in 1965.

In the 1970s it turned out that this radiation was not only disordered, but also had a bipolar structure: slightly hotter in one direction than in the other. It was later possible to measure it more precisely with the help of the COBE observatory, which operated from 1989 to 1993. It turns out that the remaining radiation in the direction of the constellation Leo is on average about 0.12% warmer and stronger. This is believed to be the reason for the movement of our solar system relative to the background.

This difference prevents the study of weaker changes in background radiation, so observational data are often stripped of it. The problem is that microwaves are not only the only background radiation in the universe, but also the only radiation that can be measured with sufficient accuracy today. And to check hypotheses about the cause of the dipole, you need to see the whole picture.

Therefore, a group of scientists decided to select all data on gamma radiation with an energy of more than three gigaelectron volts from observations made by the LAT gamma-ray telescope of the Fermi space observatory over 13 years. For comparison: the energy of visible light is between two and three electron volts.

The researchers then “cleaned” this radiation data from the Milky Way disk and from known sources of gamma rays. Then scientists saw an unexpected drop in density. The results of the study were published at: Astrophysics Journal Letters .

“This is a completely accidental discovery. We found a much stronger signal than we were looking for, and in another part of the sky.” commented one of the study’s authors, Oleksandr Kashlynskyi, a cosmologist from the University of Maryland (USA).

The source of gamma radiation was in the southern hemisphere, while the peak of residual radiation was in the north. Instead, it coincided with the source of ultra-high-energy cosmic rays (UHECRs), whose energy is billions of times higher than three gigaelectron volts, the minimum at which the authors of the new paper examined the Fermi data.

It is noteworthy that not only the direction coincides, but also the difference in intensity – 7% more gamma rays and particles “come” from this region of the sky than on average. The same decrease is observed in the opposite direction. Fluctuations in the amount of cosmic ultra-high energies were discovered recently, in 2017. Scientists logically assumed that these events are interrelated and have a common source. Maybe these are neutron stars or active galactic nuclei.