La Palma, Spain – On 15 December, the Large-Sized Telescope (LST) Collaboration announced through an Astronomer’s Telegram (ATel) the detection of the source OP 313 at very high energies with the LST-1. Although OP 313 was known at lower energies, it had never been detected above 100 GeV, making this the LST-1’s first scientific discovery. With these results, OP 313 becomes the most distant Active Galactic Nuclei (AGN) ever detected by a Cherenkov telescope, further showcasing the LST prototype’s exceptional performance while it is being commissioned on the CTAO-North site on the island of La Palma, Spain.

Read the ATel issued by the LST Collaboration.

OP 313 is what is known as a Flat Spectrum Radio Quasar or FSRQ, a type of AGN. These are very luminous objects found in the centres of some galaxies, where a supermassive black hole devours material from its surroundings, creating powerful accretion disks and jets of light and relativistic particles.

The LST-1 observed this source between December 10 and 14, after receiving an alert from the Fermi-LAT satellite that showed unusually high activity in the low-energy gamma-ray regime, confirmed also in the optical range with different instruments. With just four days of data, the LST Collaboration was able to detect the source above 100 Gigaelectronvolts (GeV), an energy level a billion times higher than the visible light humans can perceive.

Only nine quasars are known at very high energies, and 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 because the brightness of their accretion disk weakens the emission of gamma rays, but because they are further away. In this case, OP 313 is located at a redshift of 0.997 or ~8 billion light years away, making it the most distant AGN and the second most distant source ever detected at very high energies.

The more distant the source, the more difficult it is to observe at very high energies due to the so-called Extragalactic Background Light or EBL. The EBL is the collective light emitted by all objects outside the Milky Way that expands across multiple wavelengths, from visible, infrared and ultraviolet. The EBL interacts with very high-energy gamma rays, attenuating their flux and, thus, making their observation challenging. The characteristics of the LST-1, with an optimized sensitivity for the CTAO’s low energy range, between 20 and 150 GeV, where gamma rays are less affected by the EBL, enabled the LST Collaboration to extend the study of this source to tens of GeV for the first time.

The LST Collaboration will continue to observe this source with the LST-1 to expand the dataset and, thus, obtain a more precise analysis that allows scientists to improve their understanding of the EBL, study the magnetic fields within this type of source or delve into fundamental intergalactic physics.

About the LST

The Large-Sized Telescope (LST) is one of three types of telescope that will be built to cover CTAO’s full energy range (20 GeV to 300 TeV). The approved Alpha Configuration of the CTAO includes four LSTs arranged at the centre of the northern hemisphere array. An enhancement plan of such layout includes also two LSTs in the southern array, which are funded. These telescopes are optimized to cover the low-energy sensitivity between 20 and 150 GeV. Each LST is a giant 23 metre diameter telescope with a mirror area of about 400 square metres and a fine pixelized camera made of 1855 light sensors capable of detecting individual photons with high efficiency. Although the LST stands 45 metres tall and weighs around 100 tonnes, it is extremely nimble, with the ability to reposition within 20 seconds to capture brief, low-energy gamma-ray signals. Both the fast repositioning speed and the low energy threshold provided by the LSTs are critical for CTAO’s studies of transient gamma-ray sources in our own Galaxy and for the study of active galactic nuclei and gamma-ray bursts at high redshift. The prototype of the LST, the LST-1, is located at CTAO-North and is currently under commissioning. It is expected to become the first CTAO telescope once its commissioning is complete and it has been officially accepted.

About the LST Collaboration

The LST Collaboration is made up of over 400 scientists and engineers from 67 different institutions across twelve countries. The telescope operations and maintenance as well as the data-taking, analysis, and technical and scientific publications are only made possible with the collaborative effort of the entire LST Collaboration members from the following list of institutes:

About the CTAO

The Cherenkov Telescope Array Observatory (CTAO) will be the first open ground-based gamma-ray observatory and the world’s largest and most sensitive instrument for the exploration of the high-energy Universe. The CTAO’s unparalleled accuracy and broad energy range (20 GeV- 300 TeV) will provide novel insights into the most extreme and powerful events in the Cosmos, addressing questions in and beyond astrophysics falling under three major themes: Understanding the origin and role of relativistic cosmic particles, probing extreme environments (such as black holes and neutron stars) and exploring frontiers in physics (such as the nature of dark matter). To do so, 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’s (IAC’s) Roque de los Muchachos Observatory on La Palma (Spain), and CTAO-South in the southern hemisphere near the European Southern Observatory’s (ESO’s) Paranal Observatory in the Atacama Desert (Chile). The headquarters of the CTAO is hosted by the Istituto Nazionale di Astrofisica (INAF) in Bologna (Italy), and the Science Data Management Centre (SDMC) is hosted by the Deutsches Elektronen-Synchrotron (DESY) in Zeuthen (Germany). The CTAO will also be the first observatory of its kind to be open to the worldwide scientific communities as a resource for data from unique, high-energy astronomical observations.

The CTAO Central Organisation works in close cooperation with partners from around the world toward the development of the Observatory. Major partners include In-Kind Contribution teams, such as the Telescope teams that are developing essential hardware and software, in addition to the CTAC, an international group of researchers who have provided scientific guidance since the project’s inception.

The CTAO was promoted to a “Landmark” on the European Forum on Research Infrastructure (ESFRI) Roadmap 2018 , and was ranked as the main priority among the new ground-based infrastructures in the ASTRONET Roadmap 2022-2035.


Prof. Masahiro Teshima

LST Principle Investigator (PI)

(English, Japanese)

LST Outreach Team


Dr. Alba Fernández-Barral 

CTAO Chief Communication Officer


(English, Spanish and Italian)

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LST-1 Discovers the Most Distant AGN at Very High Energies - CTAO