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Unexplained Gamma-Ray Signal Hints At Unknown Feature Beyond Our Galaxy

"We found a much stronger signal, and in a different part of the sky, than the one we were looking for."

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Dr. Alfredo Carpineti

author

Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

Alfredo (he/him) has a PhD in Astrophysics on galaxy evolution and a Master's in Quantum Fields and Fundamental Forces.

Senior Staff Writer & Space Correspondent

Edited by Holly Large
Holly Large - Editorial Assistant

Holly Large

Editorial Assistant

Holly is a graduate medical biochemist with an enthusiasm for making science interesting, fun and accessible.

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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.

The most likely region where the excess of gamma rays comes from can be seen in purple.

Image credit: NASA’s Goddard Space Flight Center

Our solar system is not still in space; it moves around the galaxy at 370 kilometers per second (230 miles per second). This means that part of the universe looks like it is coming towards us, while the opposite side moves away from us. This effect is seen, for example, in the Cosmic Microwave Background (CMB), the light echo of the Big Bang. Researchers expected to see a similar effect if they were to look in the gamma-ray sky – but they discovered something very different.

The CMB is incredibly uniform, so the difference amounts to measuring about 0.12 percent hotter towards the constellation Leo (more microwaves than average) than on the other side of the Sky. The gamma-ray background comes from unresolved energetic events from across the universe, so it was expected that it would look roughly similar, though the difference enhanced given the fact that gamma rays are a billion times more energetic than regular light.

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But this dipole, as it is technically called, looks nothing like that of the CMB; it is in a different place and much more intense than expected. While not matching the CMB dipole signal, it does match what has been observed for the most energetic cosmic rays.

"It is a completely serendipitous discovery," said Alexander Kashlinsky, a cosmologist at the University of Maryland and NASA's Goddard Space Flight Center, in a statement. "We found a much stronger signal, and in a different part of the sky, than the one we were looking for."

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
This gif superimposes the Fermi map onto the cosmic ray map, illustrating the similarity of the dipole directions.
Image credit: Kashlinsky et al., 2024 and the Pierre Auger Collaboration


"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 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."

Ultrahigh-energy cosmic rays carry even more energy than gamma rays – a billion times more, in fact – and scientists do not know where they come from. However, there is an excess of these particles from the same region where there is an excess of gamma rays, and so researchers think that these two phenomena are related.

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The study is published in The Astrophysical Journal Letters and was recently presented at the 243rd meeting of the American Astronomical Society in New Orleans.


ARTICLE POSTED IN

spaceSpace and PhysicsspaceAstronomy
  • tag
  • cosmic rays,

  • Astronomy,

  • gamma ray,

  • Ultrahigh-energy cosmic ray

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