An international team of astronomers identified a mechanism that produces X-rays from the plasma jets of supermassive black holes.
New findings by an international team of astrophysicists, including eight researchers from the University of Turku (Finnish Centre for Astronomy with ESO [FINCA] and Department of Physics and Astronomy), based on data collected by NASA’s IXPE (Imaging X-ray Polarimetry Explorer) and several other instruments including the Nordic Optical Telescope (NOT) and the DIPOL polarimeter at the Sierra Nevada Observatory, has definitively identified the mechanism that generates the bright X-ray radiation associated with ultra-powerful plasma jets streaming away from certain supermassive black holes.
The origin of Blazar X-rays was an unsolved mystery
The most massive black holes live in distant galaxies feeding off abundant gas in the galactic centers. A small fraction of those supermassive black holes are being fed with enough infalling gas to create the most consistently bright objects in the sky composed of a gaseous disk and two collimated, fast, and very powerful streams of plasma emerging perpendicular to the disk. When the jets are oriented towards Earth, we call those systems blazars.
While the first blazars were discovered in the 1960s, the way that particles were accelerated within the jets and how high-energy X-rays were produced remained a mystery to this day. The X-rays could be emitted by either accelerated electrons or protons, themselves reaching their extremely high energies by uncertain acceleration mechanisms. IXPE has provided insight into many black hole mysteries from the smallest to the largest black holes, but the emission and acceleration mechanisms in jets were not yet conclusively found. In the most recent study published in the Astrophysical Journal Letters, IXPE astronomers find that X-rays in the jet must be made through Compton scattering, a mechanism that originates from the electrons within the jet.
“This was one of the biggest mysteries about black hole jets,” said Iván Agudo, lead author of the study and astronomer at the Instituto de Astrofisica de Andalucia. “And IXPE finally provided us with the tools to solve it.”
Compton scattering involves interaction of a photon with a charged particle, usually an electron. Specifically, when the photon hits an electron it can lose or gain energy after scattering. In the case of a blazar jet, the electrons have enough energy to scatter lower energy radio and infrared photons up to X-ray energies.
Low X-ray polarization revealed the mechanism of radiation generation
The IXPE satellite is a collaboration between NASA and the Italian Space Agency launched in December of 2021 that has opened a new window to the Universe in the form of X-ray polarization. Polarization is a property of light that describes the average direction of the electromagnetic waves that make up light. IXPE is the only satellite in the world capable of measuring the polarization of X-ray light. If the X-rays in the jets are highly polarized, it would mean that the X-rays are produced by either protons gyrating in the magnetic field of the jet or protons interacting with the jet's photons and other nearby protons. If the X-rays have a lower polarization degree, it would suggest that X-rays are produced by Compton scattering by electrons.
The latest study from IXPE used data from a blazar called BL Lacertae (BL Lac for short). This is one of the first blazars ever discovered, originally thought to be a variable star in the Lacerta constellation. IXPE observed BL Lac at the end of November 2023 for seven days along with several optical and radio telescopes located across the Earth also measuring BL Lac’s polarization at the same time.
This artist’s concept depicts the central region of the blazar BL Lacertae, a supermassive black hole surrounded by a bright disk and a jet oriented toward Earth. The galaxy’s central black hole is surrounded by swirls of orange in various shades representing the accretion disk of material falling toward the black hole. While black holes are known for pulling in material, this accretion process can result in the ejection of jets of electrons at nearly the speed of light. The jet of matter is represented by the cone of light that starts at the center of the black hole and widens out as it reaches the bottom of the image. It is streaked with lines of white, pink and purple which represent helix-shaped magnetic fields. We can observe these jets in many wavelengths of light including radio, optical, and X-ray. NASA’s Imaging X-ray Polarimetry Explorer (IXPE) recently collaborated with radio and optical telescopes to observe this jet and determine how the X-rays are generated in these types of celestial environments. Credit: Pablo Garcia, NASA Marshall Space Flight Center
While BL Lac has been observed by IXPE in the past, this observation was special. The polarisation of the X-rays was low, which pointed to Compton scattering, but at the same time, during the X-ray polarization observations, the optical polarization of BL Lac reached 47% as detected by the NOT.
“I could not believe my eyes! I had to triple check my analysis,” said University of Turku doctoral researcher Pouya Kouch, who first analyzed the NOT data. “It is mind boggling that photons from a few shock waves can outshine photons from hundreds of billions of stars combined!”
“This is not only the highest polarization ever recorded for BL Lac in the past 30 years, but it is also the highest polarization ever observed in any blazar!” said former FINCA Fellow Ioannis Liodakis, one of the primary authors of the study and astrophysicist at the Institute of Astrophysics – FORTH in Greece.
Such a high level of optical polarization is remarkable, but more remarkable is the fact that the X-rays were far less polarized. The team was not able to measure a strong polarization signal and determined that the X-rays cannot be polarized more than 7.6%.
This disparity between the optical and X-ray results provides the best testbed for theoretical models. The fact that optical polarization was so much higher than in the X-rays can only be explained by Compton scattering. This clearly points to electrons making X-rays in blazar jets.
Our precise NOT blazar polarization analyses owe much of their success to years of hardware and software contributions from FINCA and FINCA researchers, namely, Kari Nilsson, Elina Lindfors, and Vandad Fallah Ramazani.
“The agility of the data analysis pipeline for optical polarization, its accuracy and reliability were cornerstones of this project. To me, it was a great achievement to witness the evolution of the pipeline I created in 2016 over the years," said Vandad Fallah Ramazani.
“The IXPE-NOT synergy has been exceptionally successful, solving several black hole mysteries in just three years,” said Pouya Kouch who is the current PI of the NOT optical observations supporting IXPE. “We are continuing this collaboration and are excited to find out what other surprises blazars have in store for us.”
More about IXPE
IXPE, which continues to provide unprecedented data enabling groundbreaking discoveries about celestial objects across the universe, is a joint NASA and Italian Space Agency mission with partners and science collaborators in 12 countries. IXPE is led by NASA’s Marshall Space Flight Center in Huntsville, Alabama. Ball Aerospace, headquartered in Broomfield, Colorado, manages spacecraft operations together with the University of Colorado’s Laboratory for Atmospheric and Space Physics in Boulder.
Learn more about IXPE’s ongoing mission
More about Nordic Optical Telescope
The Nordic Optical Telescope (NOT) is a 2.56-m telescope located at the Spanish "Roque de los Muchachos" Observatory (ORM), La Palma, Canarias, Spain. The owners of the telescope and the organization behind the telescope are Aarhus University (AU), Denmark, and the University of Turku (UTU), Finland.
More about DIPOL
DIPOL-1 is a polarimeter designed and built at the University of Turku. The work was led by Dr. Vilppu Piirola. It is currently installed at the Sierra Nevada Observatory 90 cm T90 telescope (SNO, Granada, Spain).
More about FINCA
FINCA practices and co-ordinates Finnish high quality research in fields of astronomy with European Southern Observatory (ESO) and promotes technological development work related to ESO. It is also involved with developing the NOT.
