• NASA's IXPE mission has confirmed that electrons, via Compton scattering, are the source of X-rays in blazar jets.
• Observations of the blazar BL Lacertae showed significantly lower polarization in X-rays compared to optical light, supporting the electron-based theory.
• The findings resolve a decades-old debate about the origin of X-rays in extreme black hole environments, advancing our understanding of high-energy astrophysical processes.

NASA's Imaging X-ray Polarimetry Explorer (IXPE) has provided definitive evidence that X-rays emitted from the jets of supermassive black holes, specifically blazars like BL Lacertae, originate from electrons. This conclusion was reached through observations of the blazar and measurements of X-ray polarization, which were then compared with simultaneous radio and optical polarization measurements from ground-based telescopes. The data supports the theory that Compton scattering, the interaction between electrons and photons, is the primary mechanism behind X-ray production, resolving a long-standing mystery in astrophysics.

• What: IXPE observations of blazar BL Lacertae confirm that X-rays are generated by electrons interacting with photons via Compton scattering in the black hole's jets.
• Who: NASA's Imaging X-ray Polarimetry Explorer (IXPE), Iván Agudo (lead author of the study), Steven Ehlert (IXPE project scientist).
• When: Observations of BL Lacertae occurred for seven days in late November 2023. IXPE was launched on December 9, 2021.
• Where: Blazar BL Lacertae, a supermassive black hole with jets directed towards Earth.

The IXPE mission's findings represent a significant advancement in understanding the high-energy processes occurring in the vicinity of supermassive black holes. The confirmation that electrons are responsible for X-ray production in blazar jets, through Compton scattering, resolves a long-standing debate in the field. The key to this discovery was IXPE's unique ability to measure X-ray polarization, coupled with coordinated observations from ground-based telescopes. The dramatic difference between optical and X-ray polarization levels provided crucial evidence supporting the electron-based model. This research has implications for our understanding of particle acceleration and radiation mechanisms in extreme astrophysical environments.

This was one of the biggest mysteries about supermassive black hole jets, and IXPE, with the help of a number of supporting ground-based telescopes, finally provided us with the tools to solve it.

The fact that optical polarization was so much higher than in the X-rays can only be explained by Compton scattering.

IXPE's groundbreaking research has definitively identified electrons as the source of X-ray emissions in the jets of supermassive black holes, specifically in blazars such as BL Lacertae. By leveraging its unique X-ray polarization measurement capabilities and coordinating with ground-based telescopes, IXPE provided critical evidence supporting the Compton scattering mechanism. This discovery not only resolves a long-standing mystery but also paves the way for further investigations into the complex physical processes governing these extreme cosmic environments. Future research will likely focus on further characterizing the properties of these jets and exploring the role of magnetic fields in particle acceleration and radiation.