When an international team of astronomers pointed the LST-1 telescope at the distant blazar OP 313 in December 2023, they witnessed something unprecedented: the farthest quasar ever detected emitting very high-energy gamma rays, firing particles across more than 8 billion light-years of space to reach us on Earth.
OP 313 is no ordinary cosmic object. Originally mistaken for a variable star when first identified in 1959, this blazar sits at the heart of a distant galaxy, powered by a supermassive black hole whose relativistic jets point almost directly at us. What makes this discovery particularly striking is that OP 313 had never been reliably detected at such extreme energies before—gamma rays above 100 GeV, a scale of radiation normally reserved for the universe's most violent phenomena.
The team, led by Kenshin Abe of Tokai University in Japan, used LST-1, the prototype of four large-sized telescopes now being installed at the Cherenkov Telescope Array Observatory on La Palma in Spain's Canary Islands. With its 23-meter diameter mirror dish and camera of 1,855 photomultipliers, LST-1 is exquisitely sensitive to faint gamma-ray signals from distant sources. During the December 2023 flare, OP 313 blazed at 0.3 Crab Units above 100 GeV—a measurement astronomers use to compare how bright distant objects appear against a standard cosmic benchmark. The telescope's data revealed something remarkable: the flare brightness was a factor of 50 times brighter than the average emissions previously detected by NASA's Fermi spacecraft.
What struck the researchers was not just the brightness but the steadiness of the observation. Night after night during the campaign, the gamma-ray signal remained consistent, with no significant variability—suggesting a relatively stable emission mechanism despite the tremendous energies involved. The spectrum itself showed a soft, gentle slope across frequencies, a signature pattern expected when radiation travels such vast distances and encounters the diffuse background light scattered throughout the universe over billions of years.
The team constructed a detailed model of OP 313's light output across the entire electromagnetic spectrum, from infrared through visible light to gamma rays. Their analysis revealed a two-zone system where particles spiral around the black hole, producing light through synchrotron radiation and inverse-Compton scattering—processes where high-energy electrons collide with lower-energy photons from the galaxy's dusty torus and broad line region, boosting them to gamma-ray energies. The modeling work suggests the active gamma-ray producing region sits remarkably close to the outer edge of the broad line region, a finding that sheds new light on the geometry of these extreme objects.
This detection opens a new window into the violent universe at the highest energies. OP 313 now stands as both a scientific milestone and a proof of concept: if LST-1 can reliably detect gamma rays from such distant, faint sources, the four large-sized telescopes working together as part of the full Cherenkov Telescope Array will unlock observations of an entire population of previously invisible distant blazars. In doing so, astronomers will gain unprecedented insight into how supermassive black holes behave across cosmic time, from the universe's youth to today.