Credits: NASA/Pablo Garcia

The universe is filled with powerful supermassive black holes that create powerful jets of high-energy particles, creating sources of extreme brightness in the vastness of space. When one of those jets is pointed directly at Earth, scientists call the black hole system a blazar.

To understand why the particles in the jet move with great speeds and energies, scientists look at NASA’s Imaging X-ray Polarimetry Explorer (IXPE), which launched in December 2021. IXPE measures a special property of X-ray light called polarization, which has to do with the organization of electromagnetic waves at X-ray frequencies.

This week, an international team of astrophysicists published new IXPE findings about a blazar called Markarian 421. This blazar, located in the constellation Ursa Major, approximately 400 million light-years from Earth, surprised scientists with evidence that in the part of the jet where the particles are being accelerated, the magnetic field has a helical structure.

This NASA illustration shows the structure of a black hole jet as inferred from recent observations of the Markarian 421 blazar by the Imaging X-ray Polarimetry Explorer (IXPE). The jet is powered by an accretion disk, shown at the bottom of the image, which orbits and falls into the black hole over time. Helical magnetic fields are threaded through the jet. The IXPE observations have shown that the X-rays must be generated by a shock originating within material spiraling around helical magnetic fields. The inset shows the shock front itself. X-rays are generated in the white region closest to the shock front, while optical and radio emissions must originate in more turbulent regions further from the shock.Credits: NASA/Pablo García

“Markarian 421 is an old friend for high-energy astronomers,” said Italian Space Agency astrophysicist Laura Di Gesu, lead author of the new paper. “We were sure that the blazar would be a valuable target for IXPE, but discoveries from it were beyond our reach.” better prospects, successfully demonstrating how X-ray polarimetry enriches our ability to probe the complex geometry of the magnetic field and particle acceleration in different regions of relativistic jets.”

The new study detailing the IXPE team’s findings on Markarian 421 is available in the latest issue of Nature Astronomy.

Jets like the one coming out of Markarian 421 can be millions of light-years long. They are especially bright because as the particles approach the speed of light, they give off an enormous amount of energy and behave in strange ways that Einstein predicted. Blazar jets are extra bright because, just as an ambulance siren gets louder the closer it gets, the light coming at us also seems brighter. That is why blazars can outshine all the stars in the galaxies they inhabit.

Despite decades of study, scientists still do not fully understand the physical processes that shape the dynamics and emission of blazar jets. But IXPE’s innovative X-ray polarimetry, which measures the average direction of the electric field of light waves, gives them an unprecedented view of these targets, their physical geometry, and where their emissions originate.

Research models for the typical outflow from powerful jets typically depict a spiral helix structure, similar to the way human DNA is organized. But the scientists did not expect the helical structure to contain regions of particles accelerated by shocks.

IXPE found surprising variability in the angle of polarization during three long observations of Markarian 421 in May and June 2022.

“We anticipated that the direction of polarization could change, but we thought that large rotations would be rare, based on previous optical observations of many blazars,” said Herman Marshall, a research physicist at the Massachusetts Institute of Technology in Cambridge and co-author of the study. paper. “So, we planned several observations of the blazar, and the first one showed a constant 15% polarization.”

Surprisingly, he added, initial analysis of the IXPE polarization data seemed to show that it dropped to zero between the first and second observations.

“Then we realized that the polarization was actually pretty much the same, but its direction literally did a U-turn, turning almost 180 degrees in two days,” Marshall said. “Then he surprised us again during the third observation, which began a day later, by observing that the direction of polarization continued to rotate at the same rate.”

Even stranger was that simultaneous optical, infrared, and radio measurements showed no change in stability or structure, even when the polarized X-ray emissions were deflected. This means that a shock wave could propagate along spiraling magnetic fields within the jet.

The concept of a shock wave accelerating jet particles is consistent with theories about Markarian 501, a second blazar observed by IXPE that led to a study published in late 2022. But its cousin Markarian 421 shows clearer evidence of a helical magnetic field contributing to the crash.

Di Gesu, Marshall, and their colleagues are eager to make more observations of Markarian 421 and other blazars to learn more about these fluctuations in the jets and how often they occur.

“Thanks to IXPE, it is an exciting time for astrophysical jet studies,” Di Gesu said.