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NASA's Fermi Gamma-ray Space Telescope
watches the sky for gamma rays, the highest-energy form of light.
These detections help scientists learn more
about the most powerful events in the cosmos.
However, a recent absence of gamma ray detection
may have been just as informative.
Cosmic rays are small particles, like protons
and helium nuclei, traveling at nearly the speed of light.
It takes a lot of energy to accelerate them to that speed,
so scientists assume they're driven by powerful events
like exploding stars called supernovae.
Because cosmic rays are charged particles,
they interact with magnetic fields as they travel.
These interactions mean they don't follow a straight line from their sources,
and so scientists can't trace where they came from.
But when cosmic rays smash into other particles, they produce gamma rays.
And gamma rays do travel to us straight from their sources.
Fermi has even detected such gamma rays from supernova remnants,
which are thousands of years old.
If supernovae and their remnants really are a key source of cosmic rays,
then calculations tell astronomers how many gamma rays Fermi should detect.
But so far, the telescope hasn't
seen enough gamma rays from these sources.
Scientists had suspected this was because supernovae were too far away,
or observations began too late, well after peak production.
In May 2023, Fermi observed the most luminous nearby
supernova seen since the mission launched 15 years ago.
It captured data from the first few weeks
of the explosion,
when scientists anticipated
the greatest production of cosmic rays.
But Fermi didn't see any gamma rays
from the explosion.
Scientists aren't yet sure what this means
for the link between cosmic rays and supernovae.
There's still a lot of work left to do.
But Fermi's non-detection has added a very important new piece
to this high-energy puzzle.
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NASA