BL Lacertae is an amazing object that changes its brightness. Its name hints that it should be a star. However, scientists have long ago discovered that this is actually an active galactic nucleus, although there is still debate about what type it belongs to.
An unclassifiable outbreak
Blazars are active galaxies that emit from their centers narrow jets of ionized matter directed toward Earth. Depending on the properties of the electromagnetic radiation emitted by jets, astronomers divide such objects into different, well-defined classes. However, with the BL Lacertae outbreak, things turned out to be not so simple.
Deep space has surprised explorers again. Until now, it seemed that blazars — active galaxies emitting jets of matter in our direction — could be divided into fairly distinct groups according to the electromagnetic radiation they generate. This hitherto understandable situation has now become very complicated.
In the journal Astronomy & Astrophysics, a Polish-German team of scientists from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Krakow and the University of Heidelberg (HU) reports recent observations of an outburst that, for unknown reasons, is not subject to the current classification.
The object now known as BL Lacertae was discovered in 1929. Astronomers originally thought it was one of many variable stars in our Galaxy. However, later observations led to a surprising discovery: something that looked like a star was actually located 900 million light-years away from us — so it certainly couldn’t have been a star.
Active galaxies
Of the hundreds of billions of galaxies visible in the visible Universe, some are active galaxies. These are galaxies whose nuclei emit large amounts of electromagnetic radiation, probably as a result of the complex processes that occur when matter falls into the central supermassive black hole.
In some galaxies, narrow jets of ionized matter ejected near the poles of a black hole over gigantic distances, in extreme cases even more than a million light years, are an impressive sign of activity. If the jet is pointing toward Earth, astronomers call the galaxy emitting it a blazar. BL Lacertae turned out to be just such an object.
“Blazars are interesting for many reasons, not least because the orientation of the jets and the enormous velocities of their particles, close to the speed of light, lead to a variety of effects described by the theory of relativity. Emission from blazars is observed at various electromagnetic wavelengths, ranging from radio to very high-energy gamma rays,” explains Dr. Alicja Wierzcholska (IFJ PAN).
Observations of blazar
Reported observations occurred between 2020 and 2023. They were conducted in orbit around Earth using instruments from the U.S. Neil Gehrels Swift Observatory satellite; only in the hard X-ray range, they were supplemented with data from the NuSTAR space telescope.
In addition to the X-ray range, which most interested the Polish-German researchers, the optical and ultraviolet regions of the spectrum were also recorded. This is because the electromagnetic radiation produced by blazars extends from the radio band through the optical, ultraviolet and X-ray regions to gamma rays of the highest energies.
Blazars are divided into radio quasars with a flat spectrum and objects of the BL Lacertae type, which are characterized by fainter emission lines and whose name derives precisely from the aforementioned emission source. However, the latter can also be divided into several types. Indeed, diagrams showing the full energy spectrum of blazars resemble volcanic cones: they have two peaks separated by an arc-shaped depression.
Demonstration of different types of blazars
If the spectral “volcano” is shifted toward higher energies, the Lacertae is classified as LBL (Low-frequency peak BL Lac), and objects with an intermediate shift are called IBL (Intermediate BL Lacs).
“BL Lacertae objects lend themselves quite unambiguously to being assigned to a specific type. Blazar BL Lacertae has so far been considered a representative of the intermediate class, the IBL. It was therefore, with no small degree of surprise, that we noticed that in the X-ray range it looked like an HBL at some phases of the observation period, at others like an LBL, and at other times, ‘politely’ gave the impression of an IBL-type object. As if this were not enough, these sorts of changes occurred very quickly. This is unusual behavior, the physical basis of which we are not yet able to explain”, said Dr. Wierzcholska and stressed that there were more surprises: the recorded X-ray activity of the blazar turned out to be a record for the entire history of its observations.
Physical explanation of the two peaks in the spectrum
It is now hypothesized that separate physical phenomena associated with different populations of particles in the jet are responsible for the existence of the two peaks in the blazar spectra. Many astrophysicists agree with the assumption that the low-energy peak is due to electrons and the synchrotron radiation they emit.
There is no consensus on the second peak. Perhaps, it is also a consequence of the behavior of electrons. For example, their collisions with low-energy photons, resulting in an increase in photon energy (this is known as inverse Compton scattering).
However, other hypotheses have been proposed, such as those involving hadrons (i.e., clusters of quarks such as protons or neutrons). But in order to explain the behavior of the BL Lacertae outbreak, something more would have to be pointed out: not only the physical processes responsible for the formation of the two peaks, but above all the mechanism responsible for their rapid switching. It can be safely said that before this happens, many theoretical astrophysicists will spend many sleepless nights.
According to phys.org
