A supermassive black hole, blazar BL Lacertae, has finally revealed how these cosmic features generate X-rays, thanks to new international research using NASA’s Imaging X-ray Polarimetry Explorer (IXPE).
Blazar BL Lacertae’s jets point directly toward Earth, pouring forth from the supermassive black hole surrounded by a brilliant halo, which presents an optimal condition for the scientific study of its X-rays. The new work proves that electron and photon interaction drives these X-ray emissions, after detailed polarity research.
Blazar BL Lacertae
A bright accretion disk encircles the supermassive black hole, with the swirling mass of material spiraling toward the celestial object’s event horizon. While accretion pulls matter inward, it also ejects streams of electrons outward at nearly the speed of light, surrounded by helical magnetic fields.
To observe this complex action and understand how blazar BL Lacertae generates X-rays, researchers combined IXPE observations with data from other radio and optical telescopes.
Astronomers initially mistook BL Lacertae for a variable star after its discovery in 1929. Decades of observations eventually led scientists to identify it as one of the first known blazars—a galaxy core firing a jet of ionized matter at almost light speed, a term coined in 1978.
Testing X-Ray Origin Hypotheses
Before this study, astrophysicists debated two competing explanations for how black holes generate X-rays. One theory proposed that protons interacting with the jet’s photons or magnetic fields were responsible. The other suggested that electron-photon interactions produced the X-rays.
Going into the project, scientists knew that measuring the X-rays’ polarization would determine which hypothesis was correct. Higher polarization would point to the proton model, while lower polarization would support the electron model. Only IXPE had the precision to collect these measurements, gathered over seven days in late November 2023.
“This was one of the biggest mysteries about supermassive black hole jets,” said lead author Iván Agudo, an astronomer at the Instituto de Astrofísica de Andalucía – CSIC in Spain. “And IXPE, with the help of a number of supporting ground-based telescopes, finally provided us with the tools to solve it.”
Electron Evidence
After analyzing the IXPE data, scientists confirmed the electron hypothesis through evidence of Compton scattering. The Compton effect occurs when photons gain or lose energy after interacting with particles, typically electrons. The electrons in BL Lacertae’s jets had enough energy to scatter infrared photons into the X-ray range.
During this observation period, BL Lacertae reached a high optical polarization of 47.5%, while the X-rays measured less than 7.6% polarization. This discrepancy indicated that electrons, through the Compton effect, interacted with photons to generate the X-rays.
“This was not only the most polarized BL Lac has been in the past 30 years, this is the most polarized any blazar has ever been observed!” said co-author Ioannis Liodakis, an astrophysicist at the Institute of Astrophysics – FORTH in Greece.
IXPE Continues to Keep an Eye Out
Launched in 2021, IXPE is a collaboration between NASA and the Italian Space Agency, with support from a dozen countries. The mission to study X-ray polarization in black holes, quasars, pulsars, and other cosmic objects has a little more than a year and a half of planned activity remaining.
“IXPE has managed to solve another black hole mystery,” said Enrico Costa, astrophysicist at the Istituto di Astrofísica e Planetologia Spaziali of the Istituto Nazionale di Astrofísica in Rome. “IXPE’s polarized X-ray vision has solved several long-lasting mysteries, and this is one of the most important. In some other cases, IXPE results have challenged consolidated opinions and opened new enigmas, but this is how science works and, for sure, IXPE is doing very good science.”
“One thing we’ll want to do is try to find as many of these as possible,” Ehlert said. “Blazars change quite a bit with time and are full of surprises.”
The paper “High Optical to X-ray Polarization Ratio Reveals Compton Scattering in BL Lacertae’s Jet” is available in prepublication form and will see print in The Astrophysical Journal Letters.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.