Evrim Yazgin
Cosmos science journalist
A galaxy sitting in the background of the Lizard constellation is emitting a narrow jet of ionised matter near the speed of light. But the radiation spectrum of the object has astronomers scratching their heads.
The object in question is BL Lacertae, first discovered in 1929. It was initially thought to be a star. Further observations revealed it is a galaxy about 900 million light-years from Earth.
The object is an example of a class of galaxies called “blazars” which emit narrow beams of ionised matter. Astronomers can identify blazars when their jets are oriented toward Earth. They are active, star-producing galaxies.
It is presumed that the near-lightspeed jets of matter are produced by complex processes within these dynamic galaxies when matter falls into the central supermassive black hole.
“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,” explains Dr. Alicja Wierzcholska of the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN). “Emission from blazars is observed at various electromagnetic wavelengths, ranging from radio to very high-energy gamma rays.”
Wierzcholska is first author of a new analysis of BL Lacertae, published in the journal Astronomy & Astrophysics, which has raised new questions about the nature of blazars.
“Why did we focus on this particular one?” she asks. “Because of its activity in recent years and some interesting features of the radiation emitted by it, which we had already noticed during earlier observation sessions.”
Blazars come in 2 main varieties: flat spectrum radio quasars and BL Lacertae objects (BL Lacs).
BL Lacs are divided further depending on the energy spectrum of the blazar. These spectra when plotted on a graph resemble volcanos: they have 2 peaks separated by an arched depression. High energies relate to high frequencies of light, while low energies relate to low frequency spectra.
If the spectral ‘volcano’ is on the high-energy side, the BL Lacertae object is classified as HBL (High-frequency peaked BL Lac), if towards the low-energy side it’s a LBL (Low-frequency peaked BL Lac), and objects in the middle are referred to as IBLs (Intermediate BL Lacs).
Astrophysicists believe that the 2 peaks on either side of the “volcano” in the blazar’s spectrum are due to separate physical phenomena involving different populations of particles in the jet.
Low-end peaks are likely related to radiation released by electrons. There is no consensus on the other peak, though, with some suggested causes being clusters of quarks or electrons interacting with low-energy photons.
The new observations of blazar BL Lacertae throw up even more questions
“Blazar BL Lacertae has so far been considered a representative of the intermediate class, the IBL,” Wierzcholska says.
“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 behaviour, the physical basis of which we are not yet able to explain.”
But BL Lacertae wasn’t done with the surprises. The X-ray activity of the blazar turned out to be a record in the entire history of its observations.
The researchers say further research into blazar BL Lacertae and other blazars is needed to explain these unexpected results.
