Mysterious and extreme new type of cosmic objects 'defy classification'

Blazars are active galaxies that emit narrow jets of ionized matter aimed at Earth. Astronomers classify them based on the properties of their electromagnetic radiation. That is, until now.

However, BL Lacertae, a blazar in the Lacerta constellation, challenges this classification system.

Scientists from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow and the University of Heidelberg (HU) observed this object closely.

Their findings, published in Astronomy & Astrophysics, reveal that BL Lacertae does not fit into existing categories.

Understanding blazars – the basics

Blazars are some of the most extreme objects in the universe, blasting out energy with mind-bending intensity.

These cosmic powerhouses shoot out massive amounts of gamma rays, X-rays, and radio waves, lighting up the universe like a beacon.

Scientists study blazars because they help us understand how black holes interact with their surroundings and how galaxies evolve.

They also serve as natural particle accelerators, launching cosmic rays across vast distances. Some researchers even think blazars could be linked to mysterious high-energy neutrinos that hit Earth.

They’re like giant cosmic lighthouses – except instead of warning ships, they beam signals across billions of light-years.

However, as you’ll learn from this study, BL Lacertae threw them a curveball. Now, blazars are considered cosmic chameleon that escape classification.

A star that wasn’t a star

BL Lacertae was first discovered in 1929 and initially thought to be a variable star. Later observations proved otherwise.

The object was nearly 900 million light-years away, ruling out the possibility of it being a star.

Some galaxies have active nuclei that emit vast amounts of electromagnetic radiation. This activity stems from matter falling into their central supermassive black holes.

In certain cases, these galaxies also produce narrow jets of ionized matter extending millions of light-years. If such a jet points toward Earth, astronomers classify the galaxy as a blazar.

Unique blazar with strange behavior

“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,” explained Dr. Alicja Wierzcholska, an astrophysicist at IFJ PAN.

“Emission from blazars is observed at various electromagnetic wavelengths, ranging from radio to very high-energy gamma rays.”

Dr. Wierzcholska and her team analyzed the electromagnetic radiation from BL Lacertae, focusing on its recent activity. The goal was to understand some of the unusual properties of its emissions.

Observations challenge understanding

The team observed BL Lacertae from 2020 to 2023 using the Neil Gehrels Swift Observatory and NuSTAR space telescope.

Their primary focus was the X-ray spectrum, but they also collected optical and ultraviolet data. Blazars emit radiation across a wide range, from radio waves to high-energy gamma rays.

Astronomers classify blazars into two main types: flat-spectrum radio quasars and BL Lacertae objects (BL Lacs).

BL Lacs, named after BL Lacertae, have weak emission lines and fall into three groups based on their energy spectrum.

High-frequency peaked BL Lacs (HBL) have spectra shifted toward high energy. Low-frequency peaked BL Lacs (LBL) shift toward low energy. Intermediate BL Lacs (IBL) fall between HBL and LBL.

A blazar that changes its identity

“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,” said Dr. Wierzcholska.

“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.”

Adding to the mystery, BL Lacertae displayed record-breaking X-ray activity. The rapid fluctuations in its classification make it unique among blazars.

Theories and unanswered questions

Scientists believe that different physical processes contribute to the two peaks in a blazar’s spectrum.

Many agree that the low-energy peak results from synchrotron radiation emitted by electrons. However, the origin of the second peak remains debated.

Some researchers suggest that electron collisions with low-energy photons may cause the second peak.

This process, known as inverse Compton scattering, increases photon energy. Others propose that hadrons, such as protons or neutrons, might play a role.

However, these theories do not explain BL Lacertae’s rapid changes. A new model is needed to uncover the underlying mechanism behind its shifting classification.

A challenge for astrophysicists

Understanding BL Lacertae’s behavior will require extensive research and innovative thinking.

“One could venture to say that before this happens, many an astrophysicist-theorist will spend many a sleepless night,” said Dr. Wierzcholska.

As scientists continue their investigations, the BL Lacertae blazar serves as a reminder that the universe still holds many secrets waiting to be uncovered.

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The research relied on computational support from the Academic Computer Centre Cyfronet AGH. Funding came from a grant provided by the National Agency for Academic Exchange.

The study is published in the journal Astronomy & Astrophysics.

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