06 May 2025IXPE explores Black Hole’s Jet

The Imaging X-ray Polarimetry Explorer (IXPE), a NASA mission in collaboration with the Space Italian Agency (ASI), has resolved a fundamental question concerning the origin of X-rays in blazar jets. These supermassive black holes, whose relativistic jets are directed towards the Earth, are still the source of many enigmas. By pointing IXPE, but also optical and radio telescopes, at the BL Lacertae blazar, a team of researchers set out to measure the polarization of the light emitted across the entire spectrum in order to solve the problem of high-energy light generation. Measuring the multi-wavelength polarization of blazars means determining which interaction between particles (protons, electrons, photons) is responsible for the polarization signature observed in X-rays.

Credits: IXPE/NASA

Two competing theories existed until then: one involving high-energy protons spinning in the jet’s magnetic field (or protons interacting with jet photons), the other electrons moving at high speeds and colliding with photons. Proton-related processes would produce highly polarized X-rays, while electron-photon interactions, notably Compton scattering, would result in weaker polarization of X-rays compared to the polarization of optical and radio photons.

IXPE observations, carried out in November 2023, showed that while BL Lac‘s optical light reached a record polarization of 47.5% – the highest ever recorded for a blazar – the X-rays were much less polarized, with an upper limit of just 7.6%. This disparity confirmed that X-rays are produced by electrons scattering photons (probably infrared) towards X-ray energies by Compton scattering. This discovery rules out the hypothesis of proton-induced X-ray emission mechanisms.

Credits: NASA, Pablo Garcia

These results are important not only for BL Lac, one of the first blazars discovered, but also for a more general understanding of the physical processes at work in black hole jets. IXPE’s unique ability to measure X-ray polarization has now solved several major astrophysical puzzles, and future observations will aim to identify similar behavior in other blazars, whose emissions are known to vary considerably over time.

More information :
Article : High optical to X-ray polarization ratio reveals Compton scattering in BL Lacertae’s jet, ApJ Letters, 2025.
Science contact : Frédéric Marin – frederic.marin@astro.unistra.fr
NASA press release : https://www.nasa.gov/missions/ixpe/nasas-ixpe-reveals-x-ray-generating-particles-in-black-hole-jets/
Astronomy Picture Of the Day, May 9 2025.