After nearly two decades of observations, astronomers have discovered that a distant blazar behaves in ways that leading theories struggle to explain.
Instead of solving the mystery of these powerful cosmic objects, the unusually long study has raised new questions about what is happening around their supermassive black holes.
Blazars are active galaxies with enormous jets of energetic matter shooting from regions near their central black holes.
When one of these jets happens to point almost directly toward Earth, the galaxy appears as an extremely bright, compact source.
These objects can produce radiation across almost the entire electromagnetic spectrum, from radio waves and visible light to X-rays and extremely energetic gamma rays.
Their brightness can also change dramatically, making them difficult to understand.
Researchers from the Institute of Nuclear Physics of the Polish Academy of Sciences and Heidelberg University studied one particular blazar, PKS 2155-304, located about 1.5 billion light-years away.
Their analysis, published in the Journal of High Energy Astrophysics, examined almost 20 years of observations from NASA’s Neil Gehrels Swift Observatory and Fermi Gamma-ray Space Telescope. Together, the spacecraft provided measurements ranging from visible and ultraviolet light to X-rays and gamma rays.
Most blazar studies are based on observations lasting only days or weeks, sometimes separated by months or years. That can provide only snapshots of objects whose behavior changes constantly.
The much longer dataset revealed a more complicated picture.
One popular explanation assumes that much of a blazar’s radiation comes from a single region within its jet and is produced by the same population of energetic electrons. If this were true, changes at different wavelengths should often be connected.
But over the full 20-year period, researchers found no clear long-term relationship between changes in different parts of the spectrum.
Another surprise involved X-rays. Blazars commonly become relatively brighter in higher-energy X-rays when they flare. PKS 2155-304 sometimes showed this pattern during short periods, but it disappeared when scientists examined the full two decades of observations.
Even individual outbursts appeared to behave differently, suggesting that different physical processes may dominate at different times.
The researchers also discovered something unusual in observations from 2012.
A blazar’s overall energy spectrum typically contains two broad peaks separated by a dip. Scientists have a good explanation for the lower-energy peak: fast-moving electrons interacting with magnetic fields produce radiation through a process known as synchrotron emission.
The origin of the higher-energy peak is less certain. It could be produced when energetic electrons transfer energy to lower-energy photons. Another possibility involves heavier particles known as hadrons, a group that includes protons and neutrons.
In two observations from 2012, PKS 2155-304 showed an additional unexpected dip in its spectrum, even though the blazar was not experiencing a major flare.
The researchers believe this feature indicates that another physical process briefly became important. One possibility is a process involving hadrons.
That possibility is particularly intriguing because such interactions could also produce neutrinos—tiny particles that rarely interact with ordinary matter and can travel enormous cosmic distances almost undisturbed.
Scientists are still trying to determine the sources of high-energy neutrinos detected from deep space. Previous observations have already linked some neutrinos with another blazar, TXS 0506+056.
The new study does not prove that PKS 2155-304 produces neutrinos. Instead, it demonstrates how much scientists may be missing when they observe highly variable blazars only for short periods.
After 20 years of watching the same object, the picture has not become simpler. It has revealed that the engines powering some of the universe’s brightest objects may be considerably more complicated than current models suggest.
Source: KSR.
