Radiation that travelled through space for longer than the Sun and Earth have existed has now been captured by the LST-1 and MAGIC telescopes in the Canary Islands. It was emitted by the active galactic nucleus OP 313, powered by a massive black hole. No one has yet observed a more distant active galactic nucleus in the ultra-high-energy range.
But the record is not the most important part. Photons, the particles that make up light and other kinds of radiation, passed on their way to us through the accumulated light of all the stars and galaxies that have ever shone in the Universe, and some of them were lost in the process.
It is precisely from how many went missing that scientists were able to measure more accurately how much of this light the Universe contains. They also described how the black hole at the heart of the distant galaxy accelerates particles to nearly the speed of light.
“We consider this event to be very significant because it sets a new record for the distance of observed objects at very high energies, using a new-generation telescope that is still in commissioning,” says Jakub Juryšek from the Department of Astroparticle Physics at the Institute of Physics of the Czech Academy of Sciences. “In this study, as part of the LST and MAGIC collaborations, we jointly analyzed data from various observatories across a wide range of wavelengths, which were acquired nearly simultaneously. Thanks to this, we were able not only to shed light on the mechanisms of radiative processes in the extreme environments of such objects, but also to set strict limits on the density of photons filling the universe, using a method that is completely independent of other measurements at similar wavelengths.”
The Institute of Physics plays a significant role in the LST collaboration. Scientists from the Department of Astroparticle Physics are responsible for planning observations, rapid data analysis, and the physical interpretation of the data, particularly for sources within our Galaxy. They also participate in the actual data collection directly at La Palma. Colleagues from the Joint Laboratory of Optics are responsible for quality control of the LST telescope mirrors, their installation, and proper focusing during observations.
A galaxy powered by a black hole
The source of the record-breaking flare, the object OP 313, belongs to the so-called active galactic nuclei. This is the term astronomers use for exceptionally bright galactic nuclei powered by a supermassive black hole at their centre. OP 313 furthermore belongs to a subclass known as flat spectrum radio quasars, which rank among the very brightest and most powerful emitters of radiation in the Universe.
Around 11 billion years ago, the Universe was at its most active in its entire history. This period is known as the "Cosmic Noon," and stars and galaxies were forming at a record rate. Then the pace slowed and the Universe settled down. It was at the beginning of this quieter phase, which continues to this day, that OP 313 emitted a powerful flare of very high-energy gamma radiation. And it is this flare that the LST-1 and MAGIC telescopes have now captured.
The results were published by the international CTAO LST and MAGIC collaborations in the journal Astronomy & Astrophysics. The LST-1 telescope first detected the source back in December 2023, while still in its commissioning phase. The new study builds on that observation.
What the photons' journey through the universe revealed
The space between galaxies is filled with faint radiation. It has accumulated there over billions of years from the light of all the stars and galaxies that have ever shone. Astronomers call it the extragalactic background light (EBL). When gamma radiation passes through it, it gradually fades.
Whenever a gamma-ray photon collides with a photon of this accumulated light, both are annihilated and transform into a pair of particles, an electron and a positron. And the longer the journey the radiation makes, the more photons are lost along the way.
That is exactly why a record-distant blazar is such a valuable source of information. From how much its signal weakened over eight billion years, scientists significantly refined the estimate of the density of this accumulated light, and they also described how the source's brightness varied over time. When they then combined the data from the LST-1 and MAGIC telescopes with measurements from other observatories, they also learned what powered the mighty flare.
The black hole at the centre of OP 313 launches a jet of plasma in which it accelerates a dense swarm of electrons to nearly the speed of light. These electrons then collide with the surrounding light, transferring part of their energy to the photons with each collision.
The photons thereby gain very high energies, and it is precisely this radiation that the LST-1 and MAGIC telescopes captured. Astronomers call this mechanism the leptonic scenario, and its description is a significant step towards understanding what goes on inside similar galaxies.
Success while still in commissioning
The LST-1 is the prototype of the Large-Sized Telescopes (LSTs), currently being commissioned at the CTAO-North site in La Palma. Remarkably, it discovered the record-distant blazar while still in its commissioning phase before it even began observing at full capacity, which testifies to its outstanding performance and promising future.
And that future is fast approaching. On October 15 of this year, the LST team will officially launch the full set of four of these telescopes into test operation. Because the array will also be sensitive to lower-energy radiation, the CTAO observatory will see fainter and more distant sources than any instrument before it. Scientists will thus peer into regions of the Universe that no one has ever glimpsed.
Photo: Mireia Nievas Rosillo