In some distant galaxies, a narrow stream of matter is directed almost exactly toward Earth. From afar, it appears as a single bright point. Previously, such objects were monitored for only a few days at a time. Astronomers have now compiled nearly twenty years of data and found that the processes occurring in these objects are more complex than previously thought.
A Point of Light in the Southern Sky
A blazar is an active galaxy whose jet of ionized matter from the vicinity of a supermassive black hole is directed almost along our line of sight. From this angle, the entire system appears as a single point and shines across the entire spectrum — from radio waves to gamma rays.
The object studied was PKS 2155-304 in the constellation Piscis Austrinus. It is located one and a half billion light-years away, and its brightness fluctuates constantly and inconsistently across different wavelength bands. X-ray emission may behave in one way, while optical emission changes completely differently, even within a single observing session.
Data from Two Orbital Observatories
The analysis covered nearly twenty years of observations instead of just a few days. Optical, ultraviolet, and X-ray measurements came from the Neil Gehrels Swift Observatory, while gamma-ray data were obtained from the Fermi space telescope.
The study was carried out by Alicja Wierzcholska of the Institute of Nuclear Physics of the Polish Academy of Sciences in Kraków together with Michael Zacharias of Heidelberg University. The results were published in the peer-reviewed Journal of High Energy Astrophysics, and an overview was published by Universe Today.
Missing Connection Between Wavelength Bands
The most popular models assume that all radiation originates in a single region of the jet and is produced by one population of electrons. In that case, changes in optical and X-ray emission should occur in a coordinated manner, at most with a slight delay.
Over two decades, no such connection was found at all. It appears only over short time intervals. Previous ideas about the nature of such sources were based precisely on these intervals.
Each Outburst Follows Its Own Scenario
During a rapid increase in X-ray brightness, the hard, high-energy part of the spectrum is usually expected to brighten more strongly. For this object, that rule was not confirmed over the twenty-year period, although it can be seen during individual events.
The distribution curves have different slopes from one outburst to another. This indicates somewhat different physical processes in each case.
In July 2006, one of the fastest flares ever observed in blazars was recorded from the same source, when the flux of very-high-energy gamma rays doubled in approximately three minutes. The event was detected by the ground-based Cherenkov telescopes of the High Energy Stereoscopic System (H.E.S.S.). It remains a benchmark example of extreme variability.
A Proton Signature in the Spectrum
The emission graph of such objects shows two peaks in brightness. The first, at lower energies, arises from synchrotron radiation produced by electrons. The cause of the second remains unclear. It may result from collisions between electrons and low-energy photons, during which the photons gain additional energy, or from processes involving protons and neutrons.
In two measurements made in 2012, another dip appeared in the graph that had not been seen before. No flare occurred at that time, meaning that some additional mechanism began operating between those two observing sessions. Alicja Wierzcholska believes that protons, rather than electrons, were most likely involved.
This is important because neutrinos can be produced under such conditions. These particles constantly arrive at Earth from space, but identifying a specific source is almost never possible. The only reliable case is associated with the blazar TXS 0506+056, from which neutrinos were detected during a powerful outburst in 2017.
