This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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.
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.
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.
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