LST-1 and MAGIC Telescopes Shatter Distance Record for the Very High-Energy Blazar OP 313
24 Aug 2026
La Palma, Spain — On 11 August, the CTAO LST Collaboration and the MAGIC Collaboration released remarkable findings from observations of OP 313, the most distant very high-energy blazar ever recorded. The paper, published in the journal 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 comprehensive study follows the discovery of the source at very high energies by the LST-1 in December 2023, announced, at that time, via an Astronomer’s Telegram (ATel). The joint observations captured a flux of very high-energy photons originating from a distance of roughly eight billion light-years. By analysing these gamma rays, scientists were able to unveil crucial information about the diffuse extragalactic background light (EBL) and decode the complex particle acceleration processes at work within the engine of the distant galaxy.
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 specific kind of blazar that ranks among the most bright and powerful emitters in the Universe. Around 11 billion years ago, the Universe experienced a period of peak activity known as the “Cosmic noon,” characterised by an intense rate of star and galaxy formation. As this burst slowed down and galaxies began to mature, the Universe transitioned into a quieter phase, which is still ongoing today. It was during the beginning of this era that OP 313 emitted the powerful flare of very high-energy gamma rays that was detected by the LST-1 and MAGIC telescopes.
As these highly energetic gamma rays travelled across the cosmos for eight billion years (reaching us from a redshift of z = 0.997), they interacted with the EBL, a persistent field of radiation from all energies 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 a challenge to detect. To do so requires exceptionally sensitive instruments.
By analysing the joint dataset from the LST-1 and MAGIC telescopes, alongside lower-energy data from other facilities, the authors of the publication obtained stringent constraints on the EBL density and characterised 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.
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 outstanding performance and its promising future. On 15 October this year, the complete LST sub-array, 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 successfully extending the gamma-ray horizon, allowing researchers to observe extreme radiation at distances never reached before.
The CTAO LST Collaboration is an In-Kind Contributor (IKC) for the CTAO, in charge of building the Large-Sized Telescopes (LSTs). The collaboration is made up of more than 500 scientists and engineers from 25 institutions across 11 countries: Brazil, Bulgaria, Croatia, Czech Republic, France, Germany, Italy, Japan, Poland, Spain and Switzerland.
About the LST
The Large-Sized Telescopes (LSTs) are one of the three types of telescopes that the CTAO will use to cover its broad energy range, from 20 GeV to 300 TeV. When gamma rays interact with Earth’s atmosphere, they generate cascades of particles that produce Cherenkov light. Because lower-energy gamma rays create only small amounts of Cherenkov light, telescopes with large collection areas are needed to detect it. The LST, with its 23-meter diameter dish, will provide the CTAO’s unique sensitivity in the low-energy range between 20 GeV and 3 TeV.
Despite standing 45 meters tall and weighing 100 tonnes, each LST can reposition to any point in the sky within 20 seconds. Both this rapid repositioning and the low-energy threshold of the LSTs are critical for the CTAO’s studies of galactic transients, high-redshift active galactic nuclei, and gamma-ray bursts.
The CTAO LST Collaboration is responsible for designing and building these telescopes. It is made up of more than 500 scientists and engineers from 25 institutes across 11 countries: Brazil, Bulgaria, Croatia, Czech Republic, France, Germany, Italy, Japan, Poland, Spain and Switzerland.
About MAGIC
The MAGIC (Major Atmospheric Gamma Imaging Cherenkov) Florian Goebel telescopes are a system of two IACTs operating at the Observatory of Roque de Los Muchachos in stereo mode. The first MAGIC telescopes started operating in 2003, while the second one joined in 2009.
At the time of construction, with its 17-meter diameter dish and its 236 sq.m. active reflecting surface, MAGIC had the largest collection surface of any existing gamma-ray telescope worldwide.
The MAGIC telescope has revolutionised very-high-energy astronomy by bridging the gap between satellite and ground-based observations with a record-breaking sensitivity below 200 GeV. To date, the MAGIC collaboration has published more than 200 peer-reviewed papers, fundamentally reshaping our understanding of the extreme universe through a series of landmark discoveries.
The MAGIC Collaboration is made up of almost 400 scientists and engineers from 12 countries: Armenia, Brazil, Bulgaria, Croatia, Finland, Germany, India, Italy, Japan, Poland, Spain and Switzerland.
About the CTAO
The CTAO (Cherenkov Telescope Array Observatory; www.ctao.org) will be the world’s largest and most powerful observatory for gamma-ray astronomy. The CTAO’s unparalleled accuracy and broad energy range (20 GeV- 300 TeV) will help to address some of the most perplexing questions in astrophysics, falling under three major themes: understanding the origin and role of relativistic cosmic particles; probing extreme environments, such as black holes or neutron stars; and exploring frontiers in physics, searching for dark matter or deviations from Einstein’s theory of relativity. Additionally, the CTAO will play a key role in both multi-wavelength and multi-messenger fields in the coming decades thanks to its enhanced performance, which will allow it to provide fundamental gamma-ray information in the quest to probe the most extreme scenarios.
To cover its broad energy range, the CTAO will use three types of telescopes: the Large-Sized Telescopes (LST), the Medium-Sized Telescopes (MST) and the Small-Sized Telescopes (SST). More than 60 telescopes will be distributed between two telescope array sites: CTAO-North in the northern hemisphere at the Instituto de Astrofísica de Canarias’ (IAC’s) Roque de los Muchachos Observatory on La Palma (Spain), and CTAO-South in the southern hemisphere at the European Southern Observatory’s (ESO’s) Paranal Observatory in the Atacama Desert (Chile).
The CTAO is a Big Data project. The Observatory will generate hundreds of petabytes (PB) of data in a year (~12 PB after compression). Based on its commitment to Open Science, the CTAO will be the first gamma-ray observatory of its kind to operate as an open, proposal-driven observatory providing public access to its high-level science data and software products.