A fairly common object in the Universe are supermassive black holes that create powerful jets of high-energy particles from which light of extraordinary brightness emanates. When such a beam points directly at Earth, a system forming is called a blazar.

The study of blazars is carried out with the help of NASA's IXPE (Imaging X-ray Polarimetry Explorer) instrument, launched in December 2021. In particular, scientists are trying to understand why particles in the jet move with high speeds and energies. As explained in NASA, the IXPE instrument measures a special property of X-ray light called polarization and has to do with the organization of electromagnetic waves at X-ray frequencies. An international team of astrophysicists recently published new IXPE data on a blazar called Markarian 421, which turned out to be somewhat unusual.

The object is located in the constellation of the Big Dipper. Its distance from Earth is about 400 million light years. It turned out that in the part of the particle stream where the acceleration occurs, the magnetic field has a spiral structure.

"Markarian 421 is an old friend of high-energy astronomers," explained Laura Di Gesu, lead author of the paper published in the journal Nature Astronomy. Astrophysicists have long considered blazars an interesting object to study, but it has exceeded scientists' hopes. The object demonstrated how X-ray polarimetry "enriches our ability to study the complex magnetic field geometry and particle acceleration in different regions of relativistic jets."

Particle streams like the one emanating from Markarian 421 can span millions of light-years in length. They are extremely bright, because  the particles approach the speed of light, increase in energy and behave in strange ways, just as Einstein suggested. The beams from the blazars have an effect similar to changing the volume of an ambulance siren. The closer it gets, the louder it gets. Thus, the light from the beam directed at us also seems brighter. That's the reason why the blazar can outshine all the stars in the galaxy it's in.

Astrophysicists have been studying blazars for decades, but still don't fully understand all the physical processes going on in them. X-ray polarimetry IXPE, which measures the average direction of the electric field of light waves, allows us to study them at a new level. In particular, the physical geometry and the source of their emission.

Until now, research models of blazars have portrayed their helical structure as similar to the way human DNA is organized. But scientists didn't expect the spiral structure to contain regions of particles accelerated from the shocks.

IXPE discovered the strange still-variable polarization angle during three long observations of Markarian 421 in May and June 2022. "We expected that the polarization direction might change, but we thought large rotations would be rare based on previous optical observations of many blazars. Therefore, we planned several observations of the blazar, the first of which showed a constant polarization of 15%," said Herman Marshall, a research physicist at the Massachusetts Institute of Technology in Cambridge and co-author of the study

He also shared that an initial analysis of the polarization data from IXPE showed that it dropped to zero between the first and second observations. "Then we realized that the polarization was actually about the same, but its direction literally did a U-turn, turning almost 180 degrees in two days," Marshall explained. The third observation, which began a day later, showed that the direction of polarization continued to rotate at the same rate, surprising the scientists.

Simultaneous optical, infrared and radio measurements that showed no change in stability or structure at all were even more surprising. No change could be detected even when polarized X-rays were deflected. This means that the shock wave could propagate along spiral magnetic fields within the particle stream.

The concept of a shock wave accelerating particles in the beam is consistent with theories about Markarian 501, which is another blazar being monitored by IXPE. A detailed study about this object came out in late 2022.

Now Di Gesu, Marshall and their colleagues want to study Markarian 421 and other objects like it more deeply to learn more about the  oscillations and their frequency.

Earlier OBOZREVATELreported on white holes, which may be the antipodes of black holes, but are still controversial among scientists.

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