Researchers from Penn State have characterized over a hundred previously unclassified high-energy cosmic emissions as new blazars, which are distant and active galaxies with a central supermassive black hole that produces powerful jets. These newly identified blazars, though dim compared to more typical ones, have provided the researchers with the opportunity to test a controversial theory on blazar emissions. The findings have contributed to our knowledge of black hole growth and have implications for theories of general relativity and high-energy particle physics.
When matter outside a supermassive black hole’s event horizon is ejected in a jet and travels at near-light speeds, emissions are sent across the universe. If the jet is directed straight at Earth, the system is known as a blazar.
Stephen Kerby, a graduate student in astronomy and astrophysics at Penn State and the paper’s lead author, explained, “Blazars can be seen from much farther away than other black hole systems because the jet is pointed directly at us, similar to how a flashlight appears brightest when you’re looking directly at it.” The Astrophysical Journal has accepted the paper for publication, and the accepted version is available on the arXiv preprint server.
Blazars provide a unique opportunity to study supermassive black holes in the universe. The study of blazars is particularly exciting as they emit light across the entire electromagnetic spectrum, from low-energy wavelengths like radio and infrared to high-energy wavelengths such as X-rays and gamma rays. Typically, astronomers observe two broad peaks in the emissions, one at lower-energy wavelengths and another in gamma rays.
The “blazar sequence” theory predicts that brighter blazars will have a lower-energy, or redder, lower-energy peak, while the lower-energy peak for dimmer blazars will be bluer, or higher energy. To test this theory, the researchers employed new techniques to characterize 106 previously unclassified dim blazars.
While some of the most extreme blazars are discovered through gamma-ray emissions, they cannot be classified or understood without further multiwavelength observations. Abe Falcone, the lead of a high energy astrophysics group at Penn State, explained, “Blazar studies require multiwavelength observations to be fully understood, and some of the most exciting and extreme blazars are discovered by detecting their gamma-ray emission.”
Detecting and classifying dim blazars with lower-energy peaks is a challenge for current telescopes, whereas it is easier to find blazars with higher-energy peaks or brighter sources. The researchers sought to overcome this selection bias by exploring the blazar sequence at lower luminosities of both low-energy and high-energy peaked blazars.
The team, along with former associate research professor Amanpreet Kaur, initially identified potential blazars from a catalog of gamma-ray sources detected by the Fermi Large Area Telescope. They then searched for lower-energy emissions that may have come from the same source, using X-ray, UV, and optical observations from the Neil Gehrels Swift Observatory. The team also analyzed archival data of infrared and radio emissions to identify counterpart emissions for each blazar. The researchers were ultimately able to characterize the spectra of 106 new, dim blazars by cross-referencing this information.
Kerby explained that using the Swift telescope for observations allowed for pinpointing the positions of blazars with greater accuracy compared to relying solely on Fermi data. By combining emission data and utilizing two new technical approaches, the team was able to determine where in the electromagnetic spectrum the low-energy peak occurs for each blazar, which provides information on properties such as the strength of the jet’s magnetic field and the speed of charged particles.
To identify where the peak occurred for dim blazars, the researchers employed machine-learning and direct physical fitting approaches. While each approach has its advantages and disadvantages, both methods showed that the emissions from the sample of dim blazars generally peak in higher-energy blue light. This finding aligns with the blazar sequence and expands upon existing knowledge about this pattern.
However, the researchers noted that there are still approximately a thousand Fermi unassociated sources without an X-ray counterpart, many of which are likely blazars that are too dim to detect. The team plans to use the lessons learned from this study to make predictions about these blazars and further test the blazar sequence.
The new catalog of blazars is available for other astronomers to study in detail. Kerby emphasized the importance of expanding datasets to include dimmer sources, as it helps to make theories more comprehensive and less susceptible to unexpected biases.
Studying supermassive black holes also presents a unique opportunity to better understand physical theories in the universe. Falcone explained that these black holes and their surroundings offer cosmic laboratories that are more energetic than any particle accelerator on Earth. They enable researchers to study relativity theories, particle behavior at high energies, potential sources of cosmic rays, and the formation and evolution of supermassive black holes and their jets.
Looking ahead, the researchers are excited about the potential for new telescopes to explore even dimmer blazars in the future.
Source: Pennsylvania State University