On Aug. 15, the CTAO LST Collaboration and the MAGIC Collaboration released findings from observations of OP 313, the most distant very-high-energy blazar ever recorded. The paper, published in Astronomy & Astrophysics, presents in-depth observations conducted with the prototype Large-Sized Telescope (LST-1) and the MAGIC telescopes at the Roque de los Muchachos Observatory in La Palma, Spain.
This study follows the discovery of the source at very high energies by the LST-1 in December 2023, announced at the time via an Astronomer’s Telegram (ATel). The joint observations captured a flux of very-high-energy photons originating from a distance of roughly 8 billion light-years.
By analyzing these gamma rays, scientists obtained information about the diffuse extragalactic background light (EBL) and particle acceleration processes at work within the engine of the distant galaxy.
A flare from cosmic noon
Blazars are exceptionally bright active galactic nuclei—galaxies powered by a central supermassive black hole. OP 313, in particular, is classified as a flat-spectrum radio quasar, a type of blazar that ranks among the brightest and most powerful emitters in the universe. Around 11 billion years ago, the universe experienced a period of peak activity known as “cosmic noon,” characterized by an intense rate of star and galaxy formation.
As this burst slowed and galaxies began to mature, the universe transitioned into a quieter phase that continues today. It was at the beginning of this era that OP 313 emitted the powerful flare of very-high-energy gamma rays detected by the LST-1 and MAGIC telescopes.
As these highly energetic gamma rays traveled across the cosmos for 8 billion years (reaching us from a redshift of z = 0.997), they interacted with the EBL, a persistent radiation field from energy emitted by cosmic objects throughout the universe’s history. This interaction attenuates the gamma-ray signal through a process known as “pair production.”
When these gamma rays collide with the EBL, their energy transforms into pairs of particles, specifically an electron and a positron. As a result, the original gamma-ray flux of the cosmic source is reduced over the vast distance it travels, making it challenging to detect. Doing so requires exceptionally sensitive instruments.
Electrons behind the signal
By analyzing the joint data set from the LST-1 and MAGIC telescopes, alongside lower-energy data from other facilities, the paper’s authors obtained stringent constraints on the EBL density and characterized the flux variability. They determined that the intense gamma-ray emission was driven by a dense population of relativistic electrons.
In this so-called “leptonic scenario,” electrons were accelerated to near-light speed within a massive jet of plasma launched by OP 313’s central supermassive black hole.
As they collide with lower-energy light surrounding the black hole, the electrons transfer part of their immense energy to the photons, boosting them into very-high-energy gamma rays. Ultimately, these findings mark a major step forward in understanding the internal engines of flat-spectrum radio quasars.
LST-1’s early performance test
The LST-1 is the prototype for the Large-Sized Telescopes (LSTs), currently undergoing commissioning at the CTAO-North site in La Palma, Spain. Discovering the most distant very-high-energy blazar during this testing phase is clear proof of the telescope’s performance and promising future.
On Oct. 15 this year, the complete LST subarray, featuring three additional telescopes, will be inaugurated in La Palma by the LST Collaboration. Tasked with driving the CTAO’s low-energy sensitivity down to 20 GeV, the LSTs’ future observations will be capable of extending the gamma-ray horizon, allowing researchers to observe extreme radiation at distances never reached before.
Publication details
K. Abe et al, Detection of the distant quasar OP 313 with the first Large-Sized Telescope of CTAO, Astronomy & Astrophysics (2026). DOI: 10.1051/0004-6361/202558646
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Cherenkov Telescope Array Observatory
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LST-1 and MAGIC telescopes shatter distance record for the very high-energy blazar OP 313 (2026, August 24)
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