Researchers from Poland and Germany analysed observations of the blazar PKS 2155-304 collected between 2005 and 2024, finding that short-term observations fail to capture some of the processes taking place in its powerful jet.

The study, published in the Journal of High Energy Astrophysics (DOI: 10.1016/j.jheap.2026.100688), also identified an unexpected third bend in the blazar’s energy spectrum during a period when the object was not undergoing a flare.

Blazars are among the most extreme objects in the observable Universe. They are found at the centres of some active galaxies, where supermassive black holes accrete surrounding matter and accelerate some of it to enormous speeds before ejecting part of it in narrow streams of ionised matter known as jets.

When such a jet is directed towards Earth, the object is classified as a blazar.

BLAZAR’S IRREGULAR TIMETABLE

PKS 2155-304 is one of the brightest and most closely studied blazars in the southern sky. It is located about 1.5 billion light-years away in the constellation Pisces Austrinus.

In their study, Alicja Wierzcholska, PhD, a professor at the Institute of Nuclear Physics of the Polish Academy of Sciences, and Michael Zacharias, PhD, from the University of Heidelberg, analysed 20 years of continuous and intermittent observations of the object, covering the period from 2005 to 2024.

The data came from the American Neil Gehrels Swift Observatory, which observes in the optical, ultraviolet and X-ray ranges, and the Fermi Gamma-ray Space Telescope, which detects high-energy gamma radiation.

The long-term data set allowed the researchers to examine whether theoretical models describing radiation from blazar jets can explain their behaviour over much longer periods than those covered by typical observation campaigns.

The problem in understanding blazars is that their brightness can change unpredictably across different energy ranges.

‘The variability of blazars is their characteristic, long-known feature. Blazars can emit radiation not only in different ways during different observations, but even within the same observation, when changes in some energy ranges may look different than in others’, Wierzcholska said.

WAITING FOR THE BLAZAR

Scientists have traditionally studied blazars through relatively short observational campaigns. The researchers instead analysed two decades of intermittent and continuous observations to look for longer-term patterns.

The standard theoretical model assumes that radiation is produced in a single zone of the jet by one population of electrons.

Under that model, changes in the energy of the electrons should produce corresponding changes that can be detected in X-ray and optical observations.

The researchers found no such stable long-term correlations.

THE SPECTRE AND AN UNEXPECTED BEND

‘The basic conclusion from our analyses is the observation that the currently most popular theoretical models, assuming radiation emission within one jet zone by one population of electrons, can describe the variability of our blazar only in small time intervals. However, there is clearly something more complicated going on in this object that short observational campaigns are unable to capture’, Wierzcholska said.

The study confirmed that the energy spectrum of blazars typically has a two-hump shape.

The first hump is produced by electrons emitting synchrotron radiation, while the second can result from inverse Compton scattering or processes involving heavier particles such as protons.

However, the researchers identified an unexpected, statistically significant third bend in the spectrum of PKS 2155-304.

The feature appeared during a period when the blazar was not undergoing a flare.

‘The presence of a new bend in the blazar spectrum tells us that some additional physical mechanism must have been at work during the two observations - at a time when there was no flare! From various theoretical considerations, we can draw the conclusion that this mechanism was most likely hadronic in nature. This is extremely interesting, because theorists are increasingly arguing that neutrino production is possible in such situations’, Wierzcholska said.

NEUTRINO FACTORIES

Neutrinos have negligible mass, no electric charge and pass through ordinary matter with very little interaction. Every second, billions of them pass through every square centimetre of our bodies without leaving a trace.

The most energetic neutrinos arrive from the farthest reaches of space.

If hadronic reactions are responsible for the spectral feature identified by the researchers, blazars could be sources of high-energy cosmic neutrinos.

The findings indicate that the long-term behaviour of PKS 2155-304 is more complex than can be explained by models involving a single emission zone and one population of electrons.

The research also points to the importance of long-term, multi-band observations for studying blazars and their jets.

The research required significant computing power and was carried out with support from the Cyfronet AGH Academic Computer Centre. The Polish contribution was financed by a grant from the Polish National Agency for Academic Exchange.

PAP – Science in Poland, Ludwika Tomala

lt/ zan/

tr. RL