NASA’s Imaging X-ray Polarimetry Explorer (IXPE) has helped scientists uncover the mystery behind how X-rays are generated in powerful jets from supermassive black holes.
The study focused on a blazar called BL Lacertae, or BL Lac for short.
This blazar is a type of supermassive black hole with jets of energy shooting out from its center.
These jets are directed almost straight at Earth, making it an ideal target for researchers to study how X-rays are produced in such extreme environments.
For years, scientists debated whether the X-rays in these jets were created by protons or electrons. The answer depended on the polarization of the X-rays. Polarization describes the direction of light waves as they travel.
By measuring the polarization, scientists could figure out whether the X-rays came from protons moving in the jet’s magnetic field or from electrons interacting with photons—particles of light.
IXPE is the only satellite currently capable of measuring X-ray polarization. Launched on December 9, 2021, it has been crucial for studying high-energy cosmic events.
To solve this mystery, IXPE joined forces with radio and optical telescopes, observing BL Lac at the end of November 2023 for a week. During this time, astronomers also measured the blazar’s optical and radio polarization from the ground.
The results were surprising. While the optical polarization of BL Lac reached 47.5%—the highest ever recorded for any blazar in the past 30 years—the X-rays showed far less polarization.
In fact, the X-rays’ polarization was measured at no more than 7.6%, a major clue that ruled out protons as the source.
Instead, scientists concluded that the X-rays were being generated by electrons through a process called Compton Scattering.
Compton Scattering happens when photons—particles of light—collide with fast-moving electrons, causing the photons to gain energy. In the case of BL Lac, the electrons are moving at nearly the speed of light within the black hole’s jets. When these electrons interact with photons, the energy transfer causes the light to shift into the X-ray spectrum, explaining the emissions seen by IXPE.
Iván Agudo, the study’s lead author and an astronomer at the Instituto de Astrofísica de Andalucía—CSIC in Spain, expressed his excitement about finally solving this puzzle. “This was one of the biggest mysteries about supermassive black hole jets,” he said. “IXPE, with the help of a number of supporting ground-based telescopes, finally provided us with the tools to solve it.”
The findings not only answered a long-standing question but also opened new possibilities for studying blazars. “One thing we’ll want to do is try to find as many of these as possible,” said Steven Ehlert, project scientist for IXPE. “Blazars change quite a bit with time and are full of surprises.”
Enrico Costa, an astrophysicist at the Istituto di Astrofísica e Planetologia Spaziali in Rome, noted that IXPE’s work continues to challenge scientific assumptions while solving cosmic mysteries.
According to him, the satellite’s ability to measure X-ray polarization has been crucial in unraveling these secrets, proving once again that black holes still have plenty of surprises in store for us.
Source: NASA.
