Astronomers have discovered an unexpected and unexplained phenomenon outside our Milky Way Galaxy that emits high-energy light called gamma rays.
The team behind the discovery, including NASA and cosmologist Alexander Kashlinsky of the University of Maryland, found the gamma-ray signal while sifting through 13 years of data from NASA’s Fermi telescope.
“It is a completely accidental discovery,” Kashlinsky said in a statement. “We found a much stronger signal, and in a different part of the sky, than what we were looking for.”
What makes this gamma-ray signal even stranger is the fact that it is located near another unexplained feature in space, the source of some of the most energetic cosmic particles ever detected.
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The team believes the new signal is related to these high-energy particles, or cosmic rays, which consist mainly of protons, neutrons and atomic nuclei.
These ultra-high-energy cosmic rays (UHECRs) carry more than a billion times the energy of gamma rays, and their origin remains one of the greatest mysteries in astrophysics – a mystery that deepens the discovery of this source of gamma rays.
Cosmic fossil hunting led to gamma ray surprise
This new mysterious feature of gamma rays may resemble a peculiar feature of the cosmic microwave background (CMB).
The CMB represents the oldest light in the universe and is a cosmic fossil left over from an event that occurred about 380,000 years after the Big Bang. Before then, the universe had been a hot, dense soup of free electrons and protons through which light could not travel.
However, around this time the universe cooled enough for electrons and protons to fuse to form primordial atoms. Due to the sudden lack of free electrons, photons, light particles, were no longer endlessly scattered by these negatively charged particles.
The universe went from opaque to transparent in an instant, allowing the first light to travel. The CMB consists of these first freely traveling photons.
Related: What is the cosmic microwave background?
As the universe expanded over the next nearly 13.8 billion years, these photons lost energy and now have a uniform temperature of a chilling minus 454 degrees Fahrenheit (minus 270 degrees Celsius).
The CMB was first noticed in May 1964 by American radio astronomers Robert Wilson and Arno Penzias as microwave radiation in all directions of the sky above the Earth. However, in the 1990s, this apparent uniformity was called into question when NASA’s Cosmic Background Explorer (COBE) spacecraft detected small variations in the CMB temperature.
COBE found that the CMB is 0.12% hotter and has more microwaves toward the constellation Leo, and 0.12% colder than average in the opposite direction, with fewer microwaves.
This pattern, or ‘dipole’, in the CMB is attributed to the motion of our solar system: 370 kilometers per second relative to the fossil radiation field. However, if this is the case, similar dipoles caused by the motion of the Solar System should arise in all light from astrophysical sources far beyond the Solar System, but this has not yet been observed.
Astronomers are looking for this effect in other types of light so they can confirm that the CMB dipole is the result of our motion.
‘Such a measurement is important because a disagreement over the size and direction of the CMB dipole could give us a glimpse into the physical processes that took place in the very early universe, possibly back to the time when it was less than a trillionth of a second old. said team member Fernando Atrio-Barandela, professor of theoretical physics at the University of Salamanca in Spain.
One cosmic mystery or two?
The team turned to Fermi and his Large Area Telescope (LAT), which scans the entire sky above Earth several times a day to collect and bring together many years of data. The researchers hoped that hidden within the LAT data was a dipole emission pattern that could be detected in gamma rays.
Due to the effects of special relativity and the high-energy nature of gamma rays, such a dipole should be five times as prominent in these data as in the low-energy microwave light from the CMB. The team found something similar to this pattern, but not where they expected.
“We found a gamma-ray dipole, but its peak is in the southern sky, far from the CMBs [peak]and its magnitude is ten times larger than what we would expect from our motion,” said team member Chris Shrader, an astrophysicist at the Catholic University of America. “While it’s not what we were looking for, we suspect it’s related to a similar feature reported for the highest energy cosmic rays.”
There is a corresponding dipole in the shower of high-energy charged particles that make up UHECRs when they arrive on Earth, which was first noticed by the Pierre Auger Observatory in Argentina in 2017.
Although these charged particles undergo deflections from the Milky Way’s magnetic field and other magnetic fields as they travel toward Earth, and the strength of this deflection depends on the energy of the particle and its charge, the UHECR dipole still peaks at a location similar to where Kashlinsky and colleagues found the source of gamma radiation.
The team theorizes that, because of this correlation in location, the mysterious gamma rays and the UHECRs are likely linked, especially considering that unidentified sources are causing both phenomena.
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Astronomers now want to investigate the locations of these emissions to determine the source, or perhaps sources, of this ultra-high energy light and these ultra-high energy particles to see if they are indeed linked and if they represent one cosmic mystery to be solved or two .
The team’s findings were presented by Kashlinsky at the 243rd meeting of the American Astronomical Society in New Orleans, Louisiana, and are discussed in a paper published Wednesday (Jan. 10) in The Astrophysical Journal Letters.