Twenty Years of Watching One Galaxy, and It Made Less Sense
Blazars are galaxies with a jet aimed almost directly at us, and they flicker across every part of the spectrum at once. Almost everything we think we know about them comes from short observing campaigns just a handful of days, every year or two, in one narrow band of energy. A Polish and German team has now done the opposite, following a single blazar for nearly twenty years with two orbiting…
Studying blazars, extragalactic objects that appear as brilliant points in the sky due to jets of ionized matter pointed directly at Earth, has proven challenging. These cosmic phenomena exhibit constant variations across the electromagnetic spectrum, with X-rays and optical light sometimes behaving independently. Observing blazars requires continuous, multi-wavelength coverage, which no single instrument can provide.
Consequently, astronomers have adopted a campaign approach, monitoring these objects intermittently over several months or years, focusing on specific energy bands. A recent study utilizing the Imaging X-ray Polarimetry Explorer (IXPE) suggested that the acceleration of particles within these jets occurs through a shock wave. However, the team from the Institute of Nuclear Physics in Cracow, led by Alicja Wierzcholska, has made a significant stride by analyzing a particularly long-term dataset of PKS 2155-304, a blazar located 1.5 billion light-years away in the constellation Piscis Austrinus.
This dataset, spanning nearly two decades, was collected from NASA's Swift observatory, which observed optical, ultraviolet, and X-ray emissions, alongside the Fermi Gamma-ray Space Telescope for gamma-ray data. The accepted theory posits that radiation emanates from a singular region within the jet, fueled by a distinct population of electrons.
Under this conventional view, both optical and X-ray brightness should demonstrate a synchronized rise and fall, albeit with potential delays. Nevertheless, Wierzcholska's observations over the course of twenty years failed to reveal any long-term correlation between these variables. Flares in X-rays consistently exhibit the emission of higher-energy photons compared to lower-energy ones.
An individual examination of PKS 2155-304 outbursts confirms this pattern, yet the specifics fluctuate from flare to flare, and the trend remains inconsistent over an extended period. Each outburst appears to be triggered by a unique stimulus. Interestingly, a peculiar occurrence transpired during two observations conducted in 2012, revealing an unexpected dip in the spectrum.
Notably, this dip materialized during periods when the blazar was not actively flaring. Wierzcholska's research team posits that hadronic processes, involving the acceleration of protons rather than electrons, are the most plausible explanation for this anomaly. This finding holds particular significance as hadronic processes are instrumental in the generation of neutrinos.
Numerous high-energy neutrinos have been detected reaching Earth from unidentified sources, with TXS 0506+056 emerging as a noteworthy candidate. The primary link between this blazar and the neutrino event transpired in September 2017, when Fermi Gamma-ray Space Telescope identified PKS 2155-304 as the most active it had been in the past decade.
Written by urgent.news from Universe Today's reporting — not their text. Machine-written — may contain errors; check the original before relying on it.
