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The LST-1 and MAGIC telescopes on La Palma stand in the darkness beneath a clear, star-studded night sky.

The LST-1 and MAGIC telescopes on the Canary Island of La Palma (Photo: Mireia Nievas Rosillo)

Gamma-ray Astronomy: New learnings from blazar OP 313

Located 8 billion light-years away, OP 313 is the most distant blazar known to produce light at very high energies. This discovery was achieved in 2023 by LST-1. Following this accomplishment, an in-depth observing campaign of OP 313 by the LST-1 and the two MAGIC telescopes revealed the particle acceleration processes at work in its engine. Moreover OP 313 provided more insights into the diffuse extragalactic background radiation. The research paper has now been published in the journal "Astronomy & Astrophysics".

Around eleven billion years ago, our universe was in a highly active phase: After a long period known as the “Dark Age,” a multitude of galaxies and stars started to form intensively. The interstellar medium was rich in cold gas, the building material for the rapid formation of stars—and supermassive black holes. 

What is a blazar?

According to current understanding, centers of galaxies host supermassive black holes, known as active galactic nuclei (AGN) when devouring matter—an extremely luminous phenomenon that releases immense amounts of intense radiation. A blazar, such as OP 313, is an AGN that emits a concentrated beam of plasma (known as a “jet”), that is directed straight toward Earth. OP 313 has yet another distinctive feature: as a “flat-spectrum radio quasar,” it ranks among the brightest and most powerful emitters in the universe. This makes OP 313 the source of the very high-energy gamma rays that were hauled into space 8 billion years ago and recorded by the LST-1 and MAGIC telescopes.

Very high-energy gamma rays are nothing more than light particles (photons) that reach energies of over 100 gigaelectronvolts. When a very high-energy gamma particle interacts with the ubiquitous, diffuse extragalactic background light (EBL) as it travels through the universe, its energy can be transformed into matter. “This produces an electron and a positron, also known as ‘pair production’”, says Axel Arbet-Engels, scientist at the Max Planck Institute for Physics and corresponding author of the paper. “This process in turn reduces the intensity of the gamma radiation. This means, that only a faint glow reaches Earth, where the telescopes are located–no matter, how intense the radiation may be at its source.”

Limits for the density of the Extragalactic Background Light

The LST and MAGIC telescopes are designed to meet these specific requirements. Their high sensitivity makes it possible to detect and study such a faint signal from the past. The Max Planck Institute for Physics made major contributions to both telescope generations, built to detect distant very-high energy gamma-rays. They actually measure Cherenkov light, a form of radiation produced when gamma particles interact with particles in Earth’s atmosphere. 

The authors of the publication analyzed the joint dataset from the LST-1 and MAGIC telescopes, along with lower-energy data from other facilities. From these measurements, the researchers obtained stringent limits on the maximum EBL density that our Universe can contain. Because the EBL consists of all the light emitted by stars and galaxies since the dawn of time, this measurement of prime importance to learn about the evolutionary history of the Universe. 

The engine that drives particle acceleration

On the blazar itself, they concluded that its remarkable gamma-ray emission was driven by a dense population of relativistic electrons. “These electrons were accelerated to near light speed within the jet”, explains Axel Arbet-Engels. “When interacting 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”. These findings mark a major step forward in understanding the internal engines of flat spectrum radio quasars. 

About the LST

The LST-1 is the prototype for the Large-Sized Telescopes (LSTs), currently undergoing commissioning Roque de los Muchachos Observatory 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. 

The LSTs are one of the three types of telescopes that the CTAO will use to cover its broad energy range, from 20 gigaelectronvolt (GeV) to 300 teraelectronvolt (TeV). The LST, with its 23-meter diameter dish, will provide unique sensitivity in the low-energy range between 20 GeV and 3 TeV. Despite standing 45 meters tall and weighing 100 tons, each LST can reposition to any point in the sky within 20 seconds. 

About MAGIC

The two MAGIC (Major Atmospheric Gamma Imaging Cherenkov) Florian Goebel telescopes are operated the Observatory of Roque de los Muchachos in stereo mode. The first MAGIC telescope 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 square-meter active reflecting surface MAGIC had the largest collection surface of any existing gamma-ray telescope worldwide. 

The MAGIC telescopes have revolutionized 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.

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