NASA’s Fermi Gamma-ray Space Telescope has detected a surprising signal of high-energy radiation coming from a previously unseen structure beyond our home galaxy, the Milky Way. The discovery, announced on January 11th, 2024, has left astronomers puzzled as they scramble to understand the origin and nature of these gamma rays.
An Unexpected Discovery
The gamma-ray signal was first noticed by Dan Hooper, an astrophysicist at Fermilab. Hooper was searching through over 11 years of data collected by Fermi’s Large Area Telescope (LAT) when he spotted a concentration of gamma rays in a region of sky that does not match any known celestial source.
“This structure popped out at me as something we didn’t expect,” said Hooper. “It has all the hallmarks of being a real object, but what’s causing the gamma rays is a mystery.”
Gamma rays are the highest frequency waves of light in the electromagnetic spectrum and typically signify some extremely violent cosmic events involving extreme gravitational or magnetic fields. Potential sources include the collisions of black holes, dense neutron star remnants, powerful stellar explosions, and jets of high-energy particles from supermassive black holes at the centers of galaxies.
An Enduring Mystery
Since reporting his discovery at a recent American Astronomical Society meeting, Hooper has been working with other Fermi scientists to conduct follow-up investigations. So far they have confirmed that the signal is statistically significant and not an artifact or error.
“We’ve done many cross-checks between the main Fermi telescope, our complex analysis system, and another NASA gamma-ray observatory called Swift,” said Elizabeth Ferrara, a member of the Fermi LAT team. “They all confirm an extended region of gamma-ray emission that is unlike any identified object.”
The structure spans over 1000 light years across and lies over 8,000 light years away from Earth, placing it well beyond the edge of the Milky Way which has a diameter of about 100,000 light years. But the source of the gamma rays remains unknown. No optical, infrared or radio signatures match the location, leaving astronomers clueless as to what physical structures or forces could be emitting such powerful rays.
“It’s rare to see something new that stands out at gamma-ray energies,” said Ferrara. “These rays arise from the most violent and energetic phenomena, but because this structure seems to be unique, we’re struggling to grasp what could cause it.”
Clues to a Deeper Mystery
One provocative observation is that the newly spotted gamma-ray concentration overlaps with what astronomers call the ‘isotropic diffuse background.’ This refers to a general haze of gamma rays coming from all directions that has puzzled scientists for years.
Some theorists propose it originates from the cumulative emissions of distant galaxies too faint to detect individually. But the intensity of these diffuse rays exceeds predictions, prompting ideas that more exotic sources like dark matter particle annihilations may contribute.
“This new source seems to be enhancing the isotropic diffuse background in its vicinity,” Ferrara noted. “An exciting possibility is that both this structure and the general gamma-ray haze arise from a hypothesized population of ultrahigh-energy cosmic rays permeating all space.”
Cosmic rays are charged subatomic particles accelerated to extreme velocities by cataclysmic forces like supernova shockwaves. Those with the very highest energies can pierce thousands of light years of space relatively unimpeded by interstellar magnetic fields.
Some theorists calculate that these ultrahigh-energy cosmic rays inevitably produce gamma rays when they collide with the sparse intergalactic medium. The particles interact with background low-energy photons left over from the Big Bang, boosting them to gamma-ray energies in a process called inverse Compton scattering. The result could be the isotropic diffuse background detected by Fermi and other telescopes.
If this model is correct, then the newfound structure could be revealing the actual source of some portion of these highest energy cosmic rays, thought to originate many millions of light years beyond the Milky Way.
“What’s exciting about this result is that it could be giving us a view of cosmic ray accelerators in action,” says Ferrara.
|3KHz – 300GHz
|1GHz – 300GHz
|300GHz – 400THz
|400THz – 800THz
|800THz – 30PHz
|30PHz – 30EHz
|Inner shell electron transition
|Nuclear transition, particle annihilation
Table showing different types of electromagnetic radiation and their origins
An Emerging New Vista
The revelation of this unexplained source of gamma rays beyond the galaxy opens up an unseen landscape teeming with extremely energetic activity. As Fermi continues gathering data over years to come, astronomers expect the signal’s shape and spectrum to come into clearer focus. This will enable more detailed models about its possible origins.
Follow-up studies by other instruments like the upcoming Cherenkov Telescope Array may also provide critical clues. By imaging the gamma-ray signal’s interaction with Earth’s atmosphere, the spatial structure can be mapped even more precisely.
NASA mission scientists plan to target the region with the Hubble Space Telescope as well, hoping to identify any associated galaxies hiding optical light emissions. This will help estimate the kinds of stellar processes that may be underlying the high-energy ray production.
“We were lucky to have spotted this unexpected source with Fermi because the instrument surveys the entire sky, capturing the highest-energy light from across the universe,” said Ferrara. “It reminds us that the cosmos is still full of surprises and that there are almost certainly amazing discoveries still ahead.”
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