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For a long time, astronomers have been intrigued by the blazar BL Lacertae, a gigantic black hole with jets facing Earth, and how X-rays are formed in such harsh conditions. NASA's Imaging X-ray Polarimetry Explorer (IXPE) may have solved the enigma. IXPE's results, obtained through collaboration with radio and optical telescopes and the use of X-ray polarisation measurements, suggest that the interaction of fast-moving electrons and photons may be the cause of X-ray emission under such settings.
Evidence for Compton Scattering
According to the IXPE findings, a high optical to X-ray polarization ratio suggests that Compton scattering might be the mechanism of X-ray creation. There are two distinct hypotheses for X-ray emission in blazar jets. One theory states that if the X-rays in black hole jets are strongly polarised, they are caused by photon interactions, whilst the other claims that low polarisation suggests X-ray creation by electron-photon interaction.
Scientists conducted a concentrated observation of BL Lac in November 2023, taking use of IXPE's unique X-ray polarisation measurement capability. During this time, BL Lac's optical polarization reached at 47.5%, the highest ever observed for any blazar. However, IXPE discovered that the X-ray polarization was substantially lower, with a maximum of 7.6%. This contrast supports Compton scattering and may rule out the photon-based theory.
Milestone for blazar studies
"This was one of the biggest mysteries about supermassive black hole jets," said Iván Agudo, main author of the study and astronomer at the Instituto de Astrofísica de Andalucía - CSIC, Spain. The discovery justifies IXPE's goal, which began in December 2021 to investigate X-ray polarization.
Astrophysicist Enrico Costa described it as one of IXPE's most noteworthy achievements. Yet, this is only the beginning. Project scientist Steven Ehlert emphasized the importance of monitoring additional blazars because their emissions change over time. Astronomers may now examine these strong cosmic jets more effectively thanks to IXPE.
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