In December 2023, a faint but extraordinary signal reached the LST-1 and MAGIC telescopes on the island of La Palma. They captured OP 313, the most distant blazar ever observed at very high gamma-ray energies. A study published in August 2026 says the detection offers a rare window into the universe’s distant past and advances research in high-energy astrophysics.
OP 313 is a type of active galactic nucleus powered by a supermassive black hole. Classified as a flat-spectrum radio quasar, it stands out for its intense luminosity and a jet of matter pointed almost directly at Earth, amplifying the radiation that reaches us. Its redshift, measured at z = 0.997, places it roughly 8 billion light-years away, making it the farthest blazar ever detected in this energy range. The previous record-holder, PKS 0346-27, sits just behind at z = 0.991.
Breakthrough Observation
The initial alert followed optical and high-energy signals that pointed to unusual activity from OP 313. LST-1 began targeted observations on December 10, 2023, and the MAGIC telescopes soon joined in. Over nine days, LST-1 collected about 15 hours of data, then gathered an additional five hours in January 2024. By then, the blazar’s activity had diminished, and its gamma-ray signal had fallen below detection thresholds. The campaign confirmed OP 313 as only the tenth flat-spectrum radio quasar ever identified in very high-energy gamma rays.
Tracing the Journey of Gamma Rays
The gamma rays detected by the Canary Islands telescopes did not arrive unscathed. During their 8-billion-year journey, these photons interacted with the extragalactic background light (EBL), a diffuse glow produced by all the stars and galaxies that have ever existed. Collisions with EBL photons could destroy gamma photons, converting them into electrons and positrons and gradually weakening the signal. By analyzing the loss of gamma photons at different energies, researchers set new constraints on EBL density, particularly in the 0.325 to 1.069 micrometer range. That range is impressive for reconstructing the history of cosmic light.
The study combined data from LST-1 and MAGIC with observations from Fermi-LAT and other observatories to refine these measurements. As researcher Mireia Nievas Rosillo noted, each distant detection “opens a new window for studying the extragalactic background light.”
What Powered the Flare?
During the observed flare, the Fermi-LAT satellite recorded a burst of high-energy emission from OP 313 about 50 times stronger than its usual output. The most plausible explanation is a dense population of electrons, accelerated to near light speed, streaming within a plasma jet from the vicinity of the black hole. These electrons collide with lower-energy photons, transferring energy through a process known as inverse Compton scattering. The models suggest a two-zone leptonic scenario, but the precise origin of the seed photons remains uncertain. They may come from the accretion disk or broad-line region, or from the dust torus. Multiple combinations can reproduce the observed spectrum, so further coordinated campaigns will be needed to clarify the mechanism.
Expanding the Gamma Horizon
The record-setting detection came while LST-1 was still in its commissioning phase. As the prototype for four planned large telescopes at the northern site of the Cherenkov Telescope Array Observatory (CTAO), LST-1 has a 23-meter mirror capable of capturing signals from energies as low as 20 gigaelectronvolts. The two MAGIC telescopes, each with a 17-meter mirror, provided stereoscopic confirmation and helped cover a broader energy range. Together, they enabled more strong and independent measurements.
The full array of four LST telescopes is scheduled to be inaugurated in La Palma on October 15, 2026. Once operational, the expanded network will boost sensitivity to distant extragalactic sources and rapidly changing phenomena. Gamma rays cannot be seen directly, unlike visible events such as the 2027 solar eclipse. Cherenkov telescopes instead detect brief flashes produced when gamma radiation interacts with Earth’s atmosphere, allowing astronomers to track fast flares and search for even older or fainter sources.
