Astronomers have long believed that blazars could be reliably grouped based on their emission properties, but new observations have complicated this understanding. A Polish-German research collaboration, including scientists from the Henryk Niewodniczanski Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow and the University of Heidelberg (HU), has published findings in *Astronomy and Astrophysics* revealing the unusual behavior of BL Lacertae.
Initially identified in 1929 as a variable star within our galaxy, BL Lacertae was later determined to be an active galaxy approximately 900 million light-years away. Its classification as a blazar stems from the relativistic jet it emits, directed toward Earth. These jets, which accelerate particles to near-light speeds, produce emissions spanning the entire electromagnetic spectrum, from radio waves to high-energy gamma rays.
"Blazars present fascinating challenges for astrophysicists. The orientation of their jets and the extreme velocities of their particles lead to complex relativistic effects. Emissions from these objects extend from radio waves to high-energy gamma rays," stated Dr. Alicja Wierzcholska of IFJ PAN. "We focused on BL Lacertae due to its recent high activity and unique emission properties that had already piqued our interest in previous studies."
The team conducted their observations between 2020 and 2023 using instruments aboard the Neil Gehrels Swift Observatory, complemented in the hard X-ray spectrum by data from the NuSTAR space telescope. They examined optical and ultraviolet radiation in addition to X-ray emissions, as blazar radiation spans a broad spectrum of wavelengths.
Blazars are traditionally divided into two main types: flat spectrum radio quasars and BL Lacertae objects (BL Lacs). The latter category, named after BL Lacertae itself, exhibits weaker emission lines and can be further classified into three subtypes based on spectral energy distribution: high-frequency peaked BL Lacs (HBLs), low-frequency peaked BL Lacs (LBLs), and intermediate BL Lacs (IBLs).
"Typically, BL Lacertae-type blazars fit neatly into one of these categories. BL Lacertae itself has been considered an IBL. However, during our observations, we found that its classification varied depending on the phase of emission. At times, it resembled an HBL, while at others it behaved like an LBL. There were also intervals where it exhibited characteristics typical of an IBL," explained Dr. Wierzcholska. "This rapid and unpredictable variability is unprecedented and challenges our understanding of blazar classification."
Adding to the intrigue, the X-ray activity recorded for BL Lacertae during this period was the highest ever observed for this object. Current astrophysical models suggest that the two peaks in a blazar's spectral energy distribution arise from distinct physical processes involving different particle populations. The low-energy peak is widely attributed to synchrotron radiation from electrons, but the origin of the high-energy peak remains debated. One hypothesis suggests inverse Compton scattering, where electrons interact with low-energy photons to produce high-energy radiation. Another theory involves hadronic processes, wherein protons or other hadrons play a role in generating high-energy emissions.
To account for the rapid changes observed in BL Lacertae, researchers must not only refine existing models explaining these spectral peaks but also uncover the mechanism responsible for the sudden shifts between different classifications. This phenomenon presents a significant challenge for theoretical astrophysics.
"Understanding this behavior will require significant theoretical effort. Many astrophysicists are likely to spend long nights analyzing this puzzle before we reach a comprehensive explanation," Dr. Wierzcholska concluded.
Research Report:Exceptional X-ray activity in BL Lacertae
Related Links
The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences
Stellar Chemistry, The Universe And All Within It
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