Fluctuations in extragalactic gamma rays reveal two source classes but no dark matter
Gamma rays are particles of light, or photons, with the highest energy in the universe, invisible to the human eye. The most common emitters of gamma rays are blazars: supermassive black holes at the centres of galaxies. In smaller numbers, gammy rays are also produced by a certain kind of stars called pulsars and in huge stellar explosions such as supernovae.
In 2008 NASA launched the Fermi satellite to map the gamma-ray universe with extreme accuracy. The Large Area Telescope, mounted on the Fermi satellite, has been taking data ever since. It continuously scans the whole sky every three hours. The majority of the detected gamma rays is produced in our own Galaxy (the Milky Way), but the Fermi telescope also managed to detect more than 3000 extragalactic sources (according to the latest count performed in January 2016). However, these individual sources are not enough to explain the total amount of gamma-ray photons coming from outside our Galaxy. In fact, about 75% of them are unaccounted for.
Isotropic gamma-ray background
As far back as the late 1960’s, orbiting observatories have found a diffuse background of gamma rays streaming from all directions in the universe. If you had gamma-ray vision, and looked at the sky, there would be no place that would be dark.
The source of this so-called isotropic gamma-ray background is hitherto unknown. This radiation could be produced by unresolved blazars, or other astronomical sources too faint to be detected with the Fermi telescope. Parts of the gamma-ray background might also hold the fingerprint of the illustrious dark matter particle, a so-far undetected particle held responsible for the missing 80% of the matter in our universe. If two dark matter particles collide, they can annihilate and produce a signature of gamma-ray photons.
“The analysis and interpretation of fluctuations of the diffuse gamma-ray background is a new research area in high-energy astrophysics,” explains Eiichiro Komatsu at the Max Planck Institute for Astrophysics, who developed the necessary analysis tools for fluctuations in this radiation. He was also part of the team that for the first time reported fluctuations in the gamma ray background in 2012. For this latest analysis, the researchers used 81 months of data gathered by the Fermi Large Area Telescope – much more data and with a larger energy range than in previous studies.
The scientists were able to distinguish two different contributions to the gamma-ray background. One class of gamma-ray sources is needed to explain the fluctuations at low energies (below 1 GeV), and another type of sources is needed to generate the fluctuations at higher energy – the signatures of these two contributions is markedly different.
The gamma rays in the high-energy ranges – from a few GeV up – are likely originating from unresolved blazars, the researchers suggest in their paper. Further investigation of these potential sources is currently under way. However, it seems much harder to pinpoint a source for the fluctuations with energies below 1 GeV. None of the known gamma-ray emitters have a behaviour that is consistent with the new data.
Constraints on dark matter
So far, the Fermi telescope has not detected any conclusive indication of gamma-ray emission originating from dark-matter particles. Also this latest study showed no indication of a signal associated with dark matter. “Our measurement complements other search campaigns that used gamma rays to look for dark matter,” says lead author Mattia Fornasa from the University of Amsterdam. “It confirms that there is little room left for dark matter induced gamma-ray emission in the isotropic gamma-ray background.”
The precision of the fluctuation measurement has improved markedly since the first result in 2012. “I am glad to see that our measurements provide significant new insights into the origin of the gamma-ray background,” says Komatsu.
“My original motivation to do this analysis in 2006 was to find evidence for gamma-rays from dark matter particles. Well, we have not found gamma-rays from dark matter yet,” Komatsu concedes, “but I am still excited about our measurements leading to a new understanding of populations of astrophysical gamma-ray sources such as blazars. I have not given up hope on finding gamma-rays from dark matter yet though, and we have some new ideas on how to improve our method.”