• NASA's IXPE mission demonstrates that X-rays in blazar jets, specifically those of BL Lacertae, are generated primarily through Compton scattering involving electrons and photons.
• IXPE's polarization measurements disproved the alternative hypothesis that protons were the main source of X-ray emission in these jets.
• The optical polarization of BL Lacertae reached a record high of 47.5% during the IXPE observations, while X-ray polarization was significantly lower (less than 7.6%), confirming the electron-photon interaction model.

NASA's Imaging X-ray Polarimetry Explorer (IXPE), in conjunction with ground-based radio and optical telescopes, has successfully determined the mechanism behind X-ray generation in the jets of blazars, which are supermassive black holes with jets oriented toward Earth. Analyzing the blazar BL Lacertae, the IXPE mission revealed that the X-rays are produced through Compton scattering, where high-energy electrons interact with photons, boosting them to X-ray wavelengths. This finding resolves a long-standing debate about the nature of X-ray production in these extreme environments.

• What: IXPE observations reveal that X-rays in blazar jets are generated through Compton scattering, where high-energy electrons interact with photons, increasing their energy to X-ray wavelengths.
• Who: NASA's IXPE mission, led by NASA's Marshall Space Flight Center, and an international team of astronomers including Iván Agudo, Ioannis Liodakis, Steven Ehlert, and Enrico Costa.
• When: IXPE observed BL Lacertae in November 2023 for seven days. The findings were published in Astrophysical Journal Letters in 2025.
• Where: The observations focused on the blazar BL Lacertae, a supermassive black hole in the constellation Lacerta.

The IXPE mission's findings provide crucial insights into the processes occurring within black hole jets. By measuring the polarization of X-rays, IXPE was able to distinguish between two competing theories regarding X-ray generation. The low polarization of X-rays, compared to the high optical polarization observed simultaneously, strongly supports the Compton scattering model, where electrons interact with photons. This demonstrates the power of X-ray polarimetry in probing extreme astrophysical environments and resolving long-standing questions about black hole physics.

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.

This was not only the most polarized BL Lacertae has been in the past 30 years, this is the most polarized any blazar has ever been observed!

IXPE has managed to solve another black hole mystery. IXPE’s polarized X-ray vision has solved several long lasting mysteries, and this is one of the most important.

The IXPE mission has successfully resolved the question of how X-rays are generated in the jets of blazars, specifically BL Lacertae, demonstrating that Compton scattering involving electrons is the primary mechanism. This finding provides valuable insights into the extreme physical processes occurring near supermassive black holes and highlights the importance of X-ray polarimetry in advancing our understanding of these phenomena. Future research will focus on observing more blazars to further refine the understanding of jet formation and X-ray emission mechanisms.