In new research, astronomers used NASA’s IXPE (Imaging X-ray Polarimetry Explorer) to observe a highly relativistic jet emanating from the blazar BL Lacertae, a supermassive black hole surrounded by a bright disk and jets oriented toward Earth.
This artist’s concept depicts the central region of the blazar BL Lacertae, a supermassive black hole surrounded by a bright disk and a jet oriented toward Earth. Image credit: NASA / Pablo Garcia.
Astrophysicists had two competing possible explanations for X-rays form highly relativistic jets, one involving protons and one involving electrons.
Each of these mechanisms would have a different signature in the polarization of X-ray light.
Polarization is a property of light that describes the average direction of the electromagnetic waves that make up light.
If the X-rays in a black hole’s jets are highly polarized, that would mean that the X-rays are produced by protons gyrating in the magnetic field of the jet or protons interacting with jet’s photons.
If the X-rays have a lower polarization degree, it would suggest that electron-photons interactions lead to X-ray production.
IXPE is the only satellite flying today that can make such a polarization measurement.
“This was one of the biggest mysteries about supermassive black hole jets” said Dr. Iván Agudo, an astronomer at the Instituto de Astrofísica de Andalucía – CSIC.
“And IXPE, with the help of a number of supporting ground-based telescopes, finally provided us with the tools to solve it.”
Astronomers found that electrons must be the culprits through a process called Compton scattering.
Also known as the Compton effect, this process happens when a photon loses or gains energy after interacting with a charged particle, usually an electron.
Within jets from supermassive black holes, electrons move near the speed of light.
IXPE helped the astronomers learn that, in the case of a blazar jet, the electrons have enough energy to scatter photons of infrared light up to X-ray wavelengths.
BL Lacertae is one of the first blazars ever discovered, originally thought to be a variable star in the constellation of Lacerta.
IXPE observed BL Lacertae in November 2023 for seven days along with several ground-based telescopes measuring optical and radio polarization at the same time.
Coincidentally, during the X-ray polarization observations, the optical polarization of BL Lacertae reached a high number: 47.5%.
“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,” said Dr. Ioannis Liodakis, an astrophysicist at the Institute of Astrophysics – FORTH.
The researchers found the X-rays were far less polarized than the optical light.
They were not able to measure a strong polarization signal and determined that the X-rays cannot be more polarized than 7.6%.
This proved that electrons interacting with photons, via the Compton effect, must explain the X-rays.
“The fact that optical polarization was so much higher than in the X-rays can only be explained by Compton scattering”, said IXPE project scientist Dr. Steven Ehlert, an astronomer at the Marshall Space Flight Center.
“IXPE has managed to solve another black hole mystery” said Dr. Enrico Costa, an astrophysicist at the Istituto di Astrofísica e Planetologia Spaziali of the Istituto Nazionale di Astrofísica.
“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.”
The findings appear in the Astrophysical Journal Letters.
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Ivan Agudo et al. 2025. High optical to X-ray polarization ratio reveals Compton scattering in BL Lacertae’s jet. ApJL, in press; doi: 10.3847/2041-8213/adc572
