Surprise gamma-ray discovery could shed light on cosmic mystery

Astronomers have discovered an unexpected and unexplained feature outside our Milky Way galaxy that's radiating high-energy light called gamma rays.

The team behind the discovery, including NASA and University of Maryland cosmologist Alexander Kashlinsky, found the gamma-ray signal while searching through 13 years of data from NASA's Fermi Telescope. 

"It is a completely serendipitous discovery," Kashlinsky said in a statement. "We found a much stronger signal, and in a different part of the sky, than the one we were looking for."

What makes this gamma-ray signal even stranger is the fact that it is located toward another unexplained feature in space, the source of some of the most energetic cosmic particles ever detected.

Related: Pulsar surprises astronomers with record-breaking gamma-rays

The team thinks the newfound signal is related to these high-energy particles, or cosmic rays, which are made up mostly 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 origins remain one of the biggest mysteries in astrophysics — a mystery that the discovery of this gamma-ray source deepens. 

Cosmic fossil hunt led to gamma ray suprise

This new mysterious gamma-ray feature may be akin to a peculiar characteristic 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 around 380,000 years after the Big Bang. Before this, the universe had been a hot, dense soup of free electrons and protons through which light could not travel. 

Around this time, however, the universe cooled enough to allow electrons and protons to join together to form primordial atoms. The sudden lack of free electrons meant that photons, particles of light, were no longer endlessly scattered by these negatively charged particles.

The universe effectively went from being opaque to transparent in an instant, allowing the first light to travel. The CMB is made up of these first free-travelling photons.

Related: What is the cosmic microwave background?

As the universe expanded in the subsequent almost 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 spotted by American radio astronomers Robert Wilson and Arno Penzias in May 1964 as microwave radiation in all directions of the sky over Earth. In the 1990s, however, this seeming uniformity was challenged when NASA's Cosmic Background Explorer (COBE) spacecraft discovered tiny variations in the CMB temperature.

COBE found that the CMB is 0.12% hotter and has more microwaves toward the direction of the constellation Leo and is 0.12% colder than average in the opposite direction, with fewer microwaves.

This pattern, or "dipole," in the CMB has been attributed to the motion of our solar system — 230 miles per second relative to the fossil radiation field. If this is the case, however, similar dipoles caused by the movement of the solar system should arise in all light from astrophysical sources far beyond the solar system, but this hasn't been seen to date.

Astronomers are hunting for this effect in other types of light so they can confirm the CMB dipole is the result of our movement. 

"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 team member Fernando Atrio-Barandela, a professor of theoretical physics at the University of Salamanca in Spain.

One cosmic mystery or two?

The team turned to Fermi and its Large Area Telescope (LAT), which scans the entire sky over Earth several times a day to gather and collate many years of data. The researchers hoped that within the LAT data was buried a dipole emission pattern that could be detected in gamma rays.

Because of the effects of special relativity and the high-energy nature of gamma rays, such a dipole should be five times as prominent in this data as it is in the low-energy microwave light of the CMB. The team found something resembling this pattern, but not where they expected.

"We found a gamma-ray dipole, but its peak is located in the southern sky, far from the CMB's [peak], and its magnitude is 10 times greater than what we would expect from our motion," said team member Chris Shrader, an astrophysicist at the Catholic University of America. "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."

There is a corresponding dipole in the showers of high-energy charged particles that comprise UHECRs as they arrive at Earth, which was first spotted by the Pierre Auger Observatory in Argentina back in 2017. 

Even though these charged particles take deflections from the magnetic field of the Milky Way 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 in a  location similar to where Kashlinsky and colleagues found the gamma-ray source.

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 producing both phenomena. 

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 connected and if they represent one cosmic mystery to solve or two. 

The team's findings were presented at the 243rd meeting of the American Astronomical Society in New Orleans, Louisiana, by Kashlinsky and are discussed in a paper published on Wednesday (Jan. 10), in The Astrophysical Journal Letters.