LST-1 discovers a quasar with more than 100 gigaelectron volts – Enerzine

On December 15, a significant discovery was announced by the Large-Size Telescope (LST) collaboration. The object OP 313 (quasar) was discovered at very high energies, a first for LST-1. This discovery represents an important milestone in the study of active galactic nuclei (AGN) and flat spectrum quasars (FSRQ).

The LST Collaboration announced the discovery of the OP 313 source at very high energies using LST-1. Although OP 313 is known at lower energies, it has never been detected above 100 GeV. This discovery makes OP 313 the most distant AGN ever discovered by a Cherenkov telescope and demonstrates the exceptional performance of the LST prototype during its commissioning at the CTAO-Nord site on the island of La Palma, Spain.

OP 313 is a so-called flat spectrum quasar or FSRQ, a type of AGN. These are very luminous objects found at the centers of some galaxies, where a supermassive black hole devours surrounding matter, creating powerful accretion disks and jets of light and relativistic particles.

An extraordinary observation

LST-1 observed this source between December 10 and 14 after receiving an alert from the Fermi-LAT satellite indicating unusually high activity in the low-energy gamma-ray region. This activity has also been confirmed in the optical domain using various instruments. In just four days of data, the LST collaboration was able to detect the source above 100 gigaelectron volts (GeV), an energy level a billion times higher than the visible light that humans can perceive.

At very high energies, only nine quasars are known; OP 313 is now the tenth. In general, quasars are more difficult to detect at very high energies than other types of AGN. This is not only due to the brightness of their accretion disk, which weakens the emission of gamma rays, but also because they are further away. In this case, OP 313 is at a redshift of 0.997, or about 8 billion light-years away, making it the most distant AGN and the second most distant source ever discovered at very high energies. high.

The Large Telescope (LST)

The Large Telescope (LST) is one of three types of telescopes being built to cover the entire CTAO energy range (from 20 GeV to 300 TeV).

The approved CTAO Alpha configuration includes four LSTs located at the center of the Northern Hemisphere network. A plan to improve this configuration also includes two LSTs in the southern network being promoted. These telescopes are optimized to cover low energy sensitivities between 20 and 150 GeV.

Each LST is a giant telescope with a diameter of 23 meters, a mirror surface of around 400 square meters and a fine-pixel camera made up of 1,855 light sensors that can capture individual photons with high efficiency. Although the LST is 45 meters tall and weighs about 100 tons, it is extremely maneuverable and can reposition itself in 20 seconds to receive short, low-energy gamma-ray signals.

Representation of the Northern Hemisphere CTAO network. Photo credit: Gabriel Pérez Díaz, IAC

The repositioning speed and low energy threshold provided by LSTs are crucial for CTAO studies of transient gamma-ray sources in our galaxy, as well as for studying active galactic nuclei and long-shifted red-dip gamma-ray bursts. The LST prototype LST-1 is located at CTAO-North and is currently being commissioned. Once operational and officially accepted, it is expected to be CTAO’s first telescope.


The detection of OP 313 at very high energies by LST-1 marks an important step in the study of AGN and FSRQ. This discovery will allow scientists to improve their understanding of extragalactic background light (EBL), study magnetic fields within this type of source, or address fundamental intergalactic physics.

For better understanding

What is OP 313?

OP 313 is a flat-spectrum quasar, a type of active galactic nucleus. It lies about 8 billion light-years from Earth.

What is LST-1?

The LST-1 is a large prototype telescope currently in operation at the CTAO North site on the island of La Palma, Spain.

What does detection at very high energies mean?

Detection at very high energies refers to the detection of gamma rays at energies above 100 GeV. It is an important research area in astrophysics.

Why is this discovery important?

This discovery is important because it allows scientists to study very distant objects and better understand intergalactic physics.

What are the next steps?

The LST collaboration will continue to monitor this source with LST-1 to expand the data set and provide more detailed analysis.


Collaboration with Large-Size Telescopes (LST). (2023)

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