A gamma-ray burst is like a shot from a cosmic cannon: a brief but dazzling flash that outshines an entire galaxy for a few seconds. But when observed in the hardest gamma-ray band (photons with energies above a gigaelectronvolt), the signal often gets lost in the noise, like a whisper in a noisy hall. Astronomers came up with a clever trick: stack 330 such whispers together to hear the collective chorus.
And that's what they did: they took data from the Fermi-LAT space telescope over many years and aligned 330 gamma-ray bursts. The combined signal showed up clearly, up to energies of 100 GeV. It's like focusing the light from many dim flashlights into one spot: individually invisible, but together they form a bright patch. They even managed to construct light curves and spectra: that is, to see how the brightness and color of the emission changed over time.
Then it gets more interesting. The 220 bursts that were individually visible fit well into the standard picture: after the explosion, expanding matter emits via two mechanisms — ordinary synchrotron radiation (electrons spiraling in a magnetic field) and inverse Compton scattering (the same electrons boosting photons to enormous energies). But the 110 faint bursts, which are not individually visible, do not fit this picture. They emit more gamma rays than expected. It seems they have some additional energy source — possibly, the central engine continues to inject energy into the expanding matter for a long time.
What does this mean? Gamma-ray bursts are the most powerful explosions in the universe, and it is still not fully understood how they accelerate particles to such monstrous energies. The new result shows that faint bursts might work somewhat differently than bright ones. It's like discovering that compact cars have a hidden turbocharger. Now astronomers need to figure out what this "turbo" is and why it only kicks in for faint bursts.
The energy of a single 100 GeV photon is a hundred billion times greater than the energy of a visible-light photon. Such a photon is born when an electron is accelerated to nearly the speed of light and collides with another photon.
In Dan Simmons's novel "Hyperion," gamma-ray bursts are a weapon of the far future, capable of sterilizing entire planets. Real bursts, fortunately, occur too far away to harm us.