Two decades of blazar observations, and the mysteries keep piling up

For over two decades, astronomers have been puzzled by the behavior of blazars, a type of active galaxy that emits powerful jets of luminous matter towards Earth. Polish-German researchers led by Dr. Alicja Wierzcholska have now conducted the most extensive long-term analysis of one such blazar, PKS 2155-304, located one and a half billion light-years away.

Their study, published in the Journal of High Energy Astrophysics, reveals that the current understanding of these celestial objects is incomplete and may require new theoretical models.

Blazars are characterized by their extreme variability and the wide range of energies emitted, from radio waves to gamma rays. This variability has long been observed, but the Polish team's extensive data collection provides new insights. They found that the popular theory, which assumes a single electron population emitting radiation in a specific jet zone, does not fully explain the complex behavior of PKS 2155-304 over long periods.

For instance, the analysis showed that changes in brightness across different energy ranges did not exhibit the expected correlations, as intuitively assumed. Instead, fluctuations in one range seemed unrelated to changes in others, suggesting different physical mechanisms at play during each outburst. This finding challenges the notion that a single electron population is responsible for the blazar's radiation across the entire spectrum.

Moreover, the researchers discovered an intriguing "double-peak" feature in the energy spectrum of PKS 2155-304, with no clear explanation for the high-energy peak. The low-energy peak can be attributed to electrons emitting synchrotron radiation, but the origin of the high-energy peak remains uncertain. It could be due to electron-photon collisions resulting in inverse Compton scattering, or it might involve more exotic phenomena, such as hadronic interactions involving quarks.

These new findings underscore the need for further observations and theoretical advancements to fully comprehend the nature of blazars and the processes occurring within them. The current observational campaigns, which typically span a few months to a dozen days, are insufficient to capture the full complexity of these variable objects.

As the Polish-German team demonstrates, a longer-term, comprehensive study reveals a more intricate and dynamic picture of blazars, hinting at the existence of previously unknown physical phenomena.