2024-01-15 00:38:20
Mysterious phenomenon: Astronomers have measured the diffuse gamma ray background of the cosmos – and discovered something surprising. Because the radiation shows an asymmetry that has not yet been explained. This so-called dipole is ten times stronger than it should be and its cause is unknown. However, the asymmetry is similar to an unequal distribution, which has also been measured in the most energetic charged particles of cosmic rays. However, what lies behind it is unknown in both cases.
The entire cosmos is filled with a faint, diffuse background of radiation and particles. Part of it consists of gamma radiation – high-energy scattered radiation that comes from distant quasars, supernovae or galaxies with strong star formation. However, exactly how the diffuse gamma ray background arises and what contribution the various sources make to it has only been partially clarified so far. This is also because not all features of this cosmic gamma-ray carpet have been mapped yet.
Asymmetrically instead of evenly distributed
Now astronomers have mapped one of these missing features of the gamma-ray background – its spatial symmetry. To do this, a team led by Alexander Kashlinsky from NASA’s Goddard Space Flight Center evaluated 13 years of data from the Fermi Gamma Ray Telescope. Radiation in the energy range between three and 100 gigaelectron volts was mapped. The astronomers removed point sources and interference effects from our own galaxy so that only the diffuse portion of the gamma-ray background remained.
The asymmetry (dipole) determined in the gamma ray background is ten times stronger than it should be. © NASA/Goddard Space Flight Center
The surprising result: Instead of the expected relatively even distribution, there was a clear asymmetry – known in technical language as a dipole. “After deducting all known sources, we find a dipole that is highly statistically significant and of extragalactic origin,” report Kashlinsky and his colleagues. Specifically, they determined that the background gamma radiation in one part of the southern sky is six to seven percent stronger than elsewhere.
Dipol does not fit either models or the CMB
“This is an unexpected and accidental discovery,” says Kashlinsky. “The signal we found is much stronger and in a different part of the sky than we expected.” Astronomers discovered a dipole structure in the much longer wavelength microwave background (CMB) decades ago. The microwave radiation in the northwestern part of the sky is a little hotter than the overall average. In a region in the southeast sky, however, it is a little colder.
The problem, however, is that the gamma ray asymmetry that has now been detected does not match this microwave dipole either in its distribution or in its extent. “The peak of our gamma dipole is far from that of the microwave background and it is ten times stronger than we would expect,” says co-author Chris Shrader of NASA. Even a distortion effect caused by our own motion can only explain a small part of the gamma ray asymmetry.
The newly detected dipole in the gamma-ray background (top) and the asymmetry in the high-energy charged particles (UHECR). © Kashlinsky et al. 2024; Pierre Auger Collaboration
Amazing agreement with cosmic extreme particles
But where does this mysterious dipole come from? So far, researchers have no answer. What is exciting, however, is that astronomers at the Pierre Auger Observatory in Argentina discovered a very similar, previously equally inexplicable dipole in the most energetic particles of cosmic rays in 2017. These are rare but extremely high-energy charged particles with more than one exa-electron volt, which are also known as UHECR (Ultrahigh-Energy Cosmic Rays).
Although the cosmic UHECR particles are billions of times more energetic than diffuse gamma rays, their asymmetry is similar: the dipole affects the same region of the sky and is also almost the same in amplitude, as the astronomers report. “This might indicate a common origin of the UHECR and the gamma-ray photons,” they explain. The only problem is that the source of the most energetic portion of the cosmic particles is still unknown.
Two phenomena, two asymmetries – and many open questions
This means that astronomers now have two cosmic phenomena that exhibit a previously inexplicable asymmetry. But in both cases, crucial insights into how and where their origin lies are missing. To solve this mystery, researchers must now either clarify the sources of UHECR and diffuse gamma radiation or develop theoretical models that shed light on possible physical mechanisms. This might then help in the search for the origin of the dipole. (The Astrophysical Journal Letters, 2024; doi: 10.3847/2041-8213/acfedd)
Quelle: NASA/Goddard Space Flight Center
January 15, 2024 – Nadja Podbregar
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