Interestingly, the gamma-ray signal detected by NASA's Fermi Gamma-ray Space Telescope has a similar orientation to another unexplained feature produced by some of the most energetic cosmic particles ever detected. and are found to be approximately the same size.
This artist's concept shows the entire sky in gamma rays, with a magenta circle indicating the uncertainty in the direction in which more high-energy gamma rays appear to be arriving than average. In this view, the plane of the Milky Way crosses the center of the map. The circle encloses the region that contains these gamma ray sources with a probability of 68% (inside) and 95%. Image credit: NASA's Goddard Space Flight Center.
“It was a completely serendipitous discovery. We found a much stronger signal in a different part of the sky than what we were looking for,” said the University of Maryland and NASA's Goddard Space Flight Center in Space. said academic Dr. Alexander Kashlinsky.
Dr. Kasilinsky and his colleagues were looking for gamma-ray signatures associated with the cosmic microwave background (CMB), the oldest light in the universe.
This light occurred when the hot, expanding universe cooled enough to form the first atoms, and this event released a burst of light that could penetrate the universe for the first time.
Stretched out by the subsequent expansion of the universe over the past 13 billion years, this light was first detected in 1965 in the form of faint microwave waves across the sky.
In the 1970s, astronomers noticed that the CMB had a so-called dipole structure, which was later measured with high precision by NASA's COBE mission.
The CMB has more microwaves than average in the direction toward Leo and is about 0.12% hotter, and in the opposite direction it is cooler by the same amount with fewer microwaves than average.
To study small temperature changes within the CMB, this signal must be removed.
Astronomers generally believe that this pattern is the result of our solar system's motion relative to the CMB at about 370 km per second (230 miles per second).
This movement causes a dipole signal in the light coming from astrophysical sources, but so far only the CMB has been accurately measured.
By looking for patterns in other forms of light, astronomers can confirm or refute the idea that the dipole is entirely due to the motion of the solar system.
“Such measurements are important because the discrepancy in the size and orientation of the CMB dipole allows us to extend the possibility of going back to the very beginning of the universe, when the universe was less than a trillionth of a second old. “Because we can get a glimpse of certain physical processes,” said Professor Fernando Atrio Barrandera from the University of Salamanca.
Astronomers reasoned this by summing up years of data from Fermi's Large Area Telescope (LAT).
Due to the effects of relativity, gamma-ray dipoles should be amplified five times more than currently detected CMBs.
The authors integrated 13 years of Fermi LAT observations of gamma rays above about 3 billion electron volts (GeV). For comparison, visible light has an energy of about 2 to 3 electron volts.
They removed all resolved and identified sources and removed the central plane of the Milky Way to analyze the extragalactic gamma-ray background.
“We have discovered a gamma-ray dipole, but its peak is located in the southern sky, far from the CMB, and its magnitude is 10 times larger than expected from our motion.” said astrophysicist Dr. Chris Schroeder. Catholic University of America.
“Although this is not what we were looking for, we think it may be related to similar features reported for the highest-energy cosmic rays.”
Cosmic rays are accelerated charged particles, primarily protons and atomic nuclei. The rarest and most energetic particles, called UHECRs (Ultra High Energy Cosmic Rays), carry more than a billion times the energy of 3 GeV gamma rays, and their origin remains one of the greatest mysteries in astrophysics.
Since 2017, the Pierre Auger Observatory in Argentina has report Dipole in the direction of arrival of UHECR.
Because cosmic rays are electrically charged, they are deflected by galaxies' magnetic fields by different amounts depending on their energy, but the peak of the UHECR dipole is at a position in the sky similar to that found by researchers with gamma rays.
And both have surprisingly similar sizes. About 7% more gamma rays or particles than average come from one direction, and correspondingly less gamma rays or particles come from the opposite direction.
“The two phenomena are probably related, and an as-yet-unidentified source may be producing both gamma rays and very high-energy particles,” the scientists said.
“To solve this cosmic puzzle, we must either locate these mysterious sources or propose alternative explanations for both features.”
of findings Published in Astrophysics Journal Letter.
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A. Kashirinsky other. 2024. Exploration of dipoles in the diffuse gamma-ray background. APJL 961, L1; doi: 10.3847/2041-8213/acfedd
