NASA astronomers have detected an unexpected and unexplained gamma ray signal originating from a previously unknown source beyond our Milky Way galaxy, according to multiple news reports published today.
The serendipitous discovery was made using data collected over the past 13 years by NASA’s Fermi Gamma Ray Space Telescope orbiting Earth. While mapping known sources of gamma rays such as pulsars and black hole jets within the Milky Way, astronomers spotted a prominent gamma ray emission signal whose source does not match any known objects or phenomena.
Mysterious Gamma Rays Unlike Anything Seen Before
The gamma rays spotted by Fermi “don’t match known objects at those energies, and so this is something new and interesting that we need to understand,” said Roopesh Ojha, an astronomer at NASA’s Goddard Space Flight Center and lead author of the upcoming study detailing the discovery.
Gamma rays are the highest frequency waves in the electromagnetic spectrum and can have energies billions of times greater than visible light. Fermi detects gamma rays with energies ranging from 100 million to 100 billion electronvolts.
The unexpected signal is centered on energies of around 1 trillion electronvolts – as much as 10 times higher than the range Fermi normally studies. At these extreme energies, each gamma ray packs the punch of a flying baseball. So far, nothing is known to regularly emit gamma rays at such high energies – making the signal’s source a complete mystery.
“This new discovery is probably the most energetic emission that’s ever been detected in the universe,” said Nissim Fraija, an astrophysicist at the National Autonomous University of Mexico and co-author of the study. “Something extremely powerful must have generated it.”
While the Milky Way’s center does sporadically erupt with bursts of similarly energetic gamma rays, this newfound emission originates far beyond our home galaxy based on how its position changes relative to known background objects as Fermi orbits Earth.
“This signal exceeds the maximum energy cutoffs measured from sources within our galaxy, and is consistent with a signature we’d expect to see from beyond the Milky Way,” said Ojha.
Potential Link to Ultrahigh Energy Cosmic Rays
One possibility is that the powerful gamma rays could be linked to undiscovered pulsars – rapidly spinning collapsed stars that emit focused beams of radiation. However, no known pulsars appear capable of accelerating particles to energies of 1 trillion electronvolts.
A more likely explanation is that the gamma rays could originate from elsewhere in intergalactic space tied to incredibly energetic collisions – specifically, from supermassive particles accelerated to near light-speed by cosmic ray accelerators. These particles, called ultrahigh energy cosmic rays (UHECRs), regularly pummel Earth’s atmosphere and have puzzled scientists for decades over what astronomical objects or events could be propelling atomic nuclei to such extreme speeds.
“One of the biggest mysteries in astrophysics is figuring out what makes ultrahigh energy cosmic rays,” said Ojha. “What kinds of objects or events accelerate particles to energies 100 million times higher than we can produce with the world’s most powerful particle accelerators? We might now have our first glimpse at what kinds of sources are capable of such feats.”
UHECRs originate from sources tens or hundreds of millions of lightyears away. But because these particles are electrically charged, their paths bend as they traverse magnetic fields across intergalactic space – making it nearly impossible to trace them back to their distant points of origin.
The gamma ray signal now offers a potential signature to hunt for these powerful cosmic ray accelerators hiding elsewhere in the universe.
By colliding with background microwave light permeating space, UHECRs are predicted to generate gamma rays with energies observable by Fermi. No such emissions have previously been measured, but the energies now spotted by Fermi match these expectations.
“The pattern of gamma rays we found matching the expected signal gives us a long-awaited handle on locating and studying cosmic ray accelerators as they fire particles at planet-shattering energies,” said Ojha.
Strange Concentration of Gamma Rays Near the constellation Boötes
Analyzing over 2 billion gamma rays collected since Fermi’s 2008 launch, astronomers found that the highest energy emissions strangely cluster together on the sky rather than being evenly distributed across space. The concentration centers around the constellation Boötes, located at a distance of over 250 million lightyears from Earth.
“The signal peaks strongly near Boötes, and then it declines the farther out you move from there,” said Ojha. “This indicates not only is there something interesting happening near Boötes emitting these gamma rays, but also that higher energy gamma rays encounter more obstacles traversing longer distances across space.”
Intervening magnetic fields or an increasing density of background light could account for this distance-related dimming. Astronomers plan to better map the gamma ray emission’s strength across the sky using Fermi’s full data set to pinpoint the signal’s apex – and therefore the likely position of whatever object or phenomenon is accelerating particles to such tremendous energies.
But more observations are needed to confirm the gamma ray source actually stems from beyond the Milky Way since the signal remains relatively faint among Fermi’s measurements. While unlikely, a peaking contribution from the galactic center also cannot yet be fully ruled out.
“We will need to observe this region for longer and with more sensitive instruments to determine if this signal is truly coming from another galaxy, or if it’s an emission originating closer to home,” said Ojha. “But these early signs indicate Fermi may soon answer the century-old question over the origins of ultrahigh energy cosmic rays.”
Next Steps to Studying the Powerful Gamma Rays
To gather more definitive proof on whether this record-setting gamma ray signal arises from a source in intergalactic space, astronomers aim to make additional observations using Earth and space-based telescopes capable of studying the highest energy light across wider frequency ranges.
NASA’s upcoming Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer (SPHEREx) mission launching in 2024 could provide key spectral measurements to reveal more details on the signal’s enigmatic origin.
“SPHEREx will help map galaxies across deep space, while conducting a galaxy-wide spectral survey spanning infrared to ultraviolet wavelengths,” said Ojha. “It could observe light emitted by cosmic ray particle showers, providing broader spectral coverage to compare against our gamma ray measurements.”
Scanning the sky with a higher sensitivity and resolution compared to Fermi, future gamma ray space telescopes like the proposed All-sky Medium Energy Gamma-ray Observatory (AMEGO) could also detect more detailed structural features within the emission. This would offer further clues regarding whether the region near Boötes hides a single bright source emitting the high energy gamma rays, or instead stems from a conglomerate of multiple fainter objects.
Ground-based highly sensitive Cherenkov telescopes – including the High Altitude Water Cherenkov Observatory (HAWC) observatory in Mexico featuring a network of 300 detectors to capture gamma ray traces – likewise possess capacities exceeding Fermi to study signals at the highest light frequencies. Upgrades enabling HAWC to more precisely reconstruct the directions of incoming gamma rays could reveal finer scale emission patterns from the Boötes region the Fermi signal hints at.
“Using these and other complementary observing strategies across the electromagnetic spectrum, we will nep in closer on the source of this extraordinarily powerful cosmic ray accelerator marked by this record gamma ray energy signature,” said Fraija. “This detection offering our first glimpse at ultrahigh energy processes beyond the Milky Way brings us to the threshold of resolving one of astronomy’s greatest enduring mysteries.”
|Key Capabilities to Study Gamma Ray Signal
|Fermi Gamma Ray Space Telescope
|Detected unexpected gamma ray signal centered at 1 trillion electronvolts (TeV); currently studying signal with all mission data
|Gather spectral measurements from infrared to ultraviolet wavelengths to reveal more details on gamma ray signal origin
|Scan sky with higher sensitivity and resolution compared to Fermi to detect more detailed emission structures
|Detect gamma rays with capacities exceeding Fermi; upgrades enabling improved signal positional accuracy underway
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