Unidentified Cosmic Phenomena: Fermi’s Gamma-Ray Discovery Shocks Astronomers

Fermi Mission Detects Surprising Gamma-Ray Feature

This artist’s concept shows the entire sky in gamma rays with magenta circles illustrating the uncertainty in the direction from which more high-energy gamma rays than average seem to be arriving. In this view, the plane of our galaxy runs across the middle of the map. The circles enclose regions with a 68% (inner) and a 95% chance of containing the origin of these gamma rays. Credit: NASA’s Goddard Space Flight Center

NASA’s Fermi Telescope data revealed a surprising gamma-ray signal outside our galaxy, potentially linked to the most energetic cosmic particles, and suggesting new, unidentified cosmic phenomena.

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

“It is a completely serendipitous discovery,” 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 the one we were looking for.”

Gamma-Ray Signal and Cosmic Mystery

Intriguingly, the gamma-ray signal is found in a similar direction and with a nearly identical magnitude as another unexplained feature, one produced by some of the most energetic cosmic particles ever detected.

A paper describing the findings was published on January 10 in The Astrophysical Journal Letters.

The scientists combined 13 years of Fermi Large Area Telescope observations of gamma rays above about 3 billion electron volts (GeV), removed all discrete sources and stripped out the central plane of our Milky Way galaxy in order to analyze the extragalactic gamma-ray background. Analysis of the resulting data revealed a part of the sky where more high-energy gamma rays are arriving than average. The direction is not precisely known. The circles show regions where there is a 68% and a 95% chance of containing the origin of these gamma rays for one analysis approach. Credit: NASA’s Goddard Space Flight Center

Investigating the Cosmic Microwave Background

The team was searching for a gamma-ray feature related to the CMB (cosmic microwave background), the oldest light in the universe. Scientists say the CMB originated when the hot, expanding universe had 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 in the form of faint microwaves all over the sky in 1965.

Dipole Structure in the CMB

In the 1970s, astronomers realized that the CMB had a so-called dipole structure, which was later measured at high precision by NASA’s COBE (Cosmic Background Explorer) mission. The CMB is about 0.12% hotter, with more microwaves than average, toward 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 regard the pattern as a result of the motion of our own solar system relative to the CMB at about 230 miles (370 kilometers) per second.

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

“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.

The team was searching for a gamma-ray signal related to our solar system’s motion of about 230 miles (370 kilometers) per second relative to the CMB, which is widely regarded as being responsible for the dipole emission it displays. What they found instead was a gamma-ray signal 10 times stronger than expected from our galaxy’s motion and located far from the CMB dipole. Credit: NASA’s Goddard Space Flight Center

Gamma-Ray Dipole Surprises Researchers

The team reasoned that by adding together many years of data from Fermi’s LAT (Large Area Telescope), which scans the entire sky many times a day, a related dipole emission pattern could be detected in gamma rays. Thanks to the effects of relativity, the gamma-ray dipole should be amplified by as much as five times over the currently detected CMB’s.

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 stripped out the central plane of our Milky Way galaxy 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 the CMB’s, and its magnitude is 10 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 at 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.”

Cosmic rays are accelerated charged particles – mostly protons and atomic nuclei. The rarest and most energetic particles, called UHECRs (ultrahigh-energy cosmic rays), carry more than a billion times the energy of 3 GeV gamma rays, and their origins remain one of the biggest mysteries in astrophysics.

Top: An all-sky map of extragalactic gamma rays in which the central plane of our galaxy, shown in dark blue where data has been removed, runs across the middle. The red dot and circles indicate the approximate direction from which more gamma rays than average seem to be arriving. Bottom: A similar all-sky map showing the distribution of ultrahigh-energy cosmic rays detected by the Pierre Auger Observatory in Argentina. Red indicates directions from which greater than average numbers of particles arrive, blue indicates directions with fewer than average. This video superposes the Fermi map onto the cosmic ray map, illustrating the similarity of the dipole directions. Credit: Kashlinsky et al. 2024 and the Pierre Auger Collaboration

Linking Gamma Rays and Cosmic Rays

Since 2017, the Pierre Auger Observatory in Argentina has reported a dipole in the arrival direction of UHECRs. Being electrically charged, cosmic rays are diverted by the galaxy’s magnetic field by different amounts depending on their energies, but the UHECR dipole peaks in a sky location 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 correspondingly smaller amounts arriving from the opposite direction.

The scientists think it’s likely the two phenomena are linked – that as yet unidentified sources are producing both the gamma rays and the ultrahigh-energy particles. To solve this cosmic conundrum, astronomers must either locate these mysterious sources or propose alternative explanations for both features.

Reference: “Probing the Dipole of the Diffuse Gamma-Ray Background” by A. Kashlinsky, F. Atrio-Barandela and C. S. Shrader, 10 January 2024, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/acfedd

The Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership managed by Goddard. Fermi was developed in collaboration with the U.S. Department of Energy, with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden, and the United States.

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