About 1.5 billion light-years from Earth, a galaxy called PKS 2155-304 is giving astronomers more questions than answers.
After studying nearly two decades of observations, researchers have found that this unusual object is far more complicated than standard theories suggest.
PKS 2155-304 is a blazar, a special type of active galaxy powered by a supermassive black hole. As material falls toward the black hole, enormous amounts of energy can be released.
Some active galaxies also produce narrow jets of extremely energetic particles that shoot outward from the region around the black hole.
When one of these jets happens to point roughly toward Earth, astronomers call the galaxy a blazar.
Because we are looking almost directly into the jet, blazars can appear extraordinarily bright and can produce radiation ranging from radio waves and visible light to X-rays and gamma rays.
The problem is that blazars are highly unpredictable. Their brightness can change dramatically, sometimes even during a single observation. Most studies have followed them for only days or weeks, making it difficult to understand their long-term behavior.
Researchers from Germany and Poland therefore examined PKS 2155-304 over almost 20 years. They combined observations from NASA’s Neil Gehrels Swift Observatory, which detects optical, ultraviolet and X-ray radiation, with gamma-ray observations from the Fermi Gamma-ray Space Telescope.
The long-term picture challenged a popular explanation of how blazars work. Many models assume that much of a blazar’s radiation comes from a single region in its jet containing one population of energetic electrons. Such models can explain observations over short periods, but they struggled to explain what happened over decades.
For example, if optical and X-ray radiation came from the same group of electrons, changes in the two types of radiation should be related. Yet the researchers found no clear long-term connection between them.
The blazar’s X-rays also behaved unexpectedly. During short periods, PKS 2155-304 showed relationships between brightness and X-ray energy that astronomers often see in blazars. But these patterns changed when observations covering many years were considered. This suggests that different outbursts may be driven by different physical processes.
Another surprise appeared in observations from 2012. Blazar spectra usually show two broad peaks representing different types of radiation. But researchers detected an additional dip in the spectrum of PKS 2155-304 even though the blazar was not experiencing a major outburst.
The team believes this unusual feature may point to processes involving hadrons, particles such as protons and neutrons. That possibility is particularly exciting because such processes could also produce neutrinos.
Neutrinos are tiny particles that rarely interact with matter, making them extremely difficult to detect. Scientists still do not fully understand where the highest-energy neutrinos from deep space originate. Blazars are among the leading suspects.
Rather than providing a simple explanation for PKS 2155-304, the 20-year study shows how much remains unknown.
It also demonstrates why long-term observations are essential: a blazar that looks understandable over a few days may tell a very different story when watched for decades.
