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The prototype LST-1 telescope has successfully tracked the pulsation of gamma rays from distant space beacons — blazars.
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At the heart of many galaxies lies an invisible beacon — a supermassive black hole. As it devours matter, it blasts a dazzling beam of gamma rays into space. If that beam points towards Earth, astronomers see a blazar — a bright but fickle point. These objects were discovered thanks to Maarten Schmidt, who in 1963 first described a quasar, their close cousin.
Each gamma-ray photon is like a bullet of monstrous energy: one such “projectile” carries billions of times more energy than a particle of visible light.
The LST-1 telescope spent two years catching the afterglow of these beams. It didn’t capture the gamma photons directly, but the faint blue flashes they sparked in the atmosphere. This allowed scientists to trace how the brightness of blazars changes: Markarian 501, for instance, would flare up tens of times brighter over days, then dim for months. Together with the orbiting Fermi telescope, they plotted spectra — graphs of radiation energy distribution — for different stages of activity.
Astonishingly, if the blazar’s beam were tilted by just one degree, Earth would remain in the dark — and we would never know these objects exist.
In the future, the CTAO telescope network will turn such monitoring into a permanent surveillance service for space beacons, helping us understand how black holes accelerate particles to fantastic energies.
Observing blazars lets us peer billions of years into the past: light from some of them has been traveling to Earth almost since the universe was born.