New discoveries about blazars come from Ixpe

This artist’s impression shows the structure of a jet black hole as inferred from recent observations of Blazar Markarian 421 using the X-ray Imaging Polarimetry Explorer (IXPE). The jet is powered by the accretion disk shown in the lower part of the image, which consists of material orbiting and falling into the black hole. The jet stream is permeated by a spiral magnetic field. Ixpe’s observations showed that the X-rays must have been generated in a shock contained within the material that orbited the spiral field lines. The inset shows the region from which the light we observe is actively emitting. The X-rays are generated in the white area closest to the front of the shock, while the optical and radio emissions must come from more disordered regions farther from the shock. Source: NASA/Pablo Garcia

The universe glows with energy, produced not only by stars, nebulae, and crowded galactic nurseries, but also by jets of free electrons emerging from some of the most powerful cosmic objects: the active galactic nucleus known as Blazars. This week, an international team of astrophysicists, using data from NASA’s X-ray Polarimetry Explorer (Ixpe), created in collaboration with the Italian Space Agency (ASI), published on natural astronomy New discoveries about the so-called blazar 421an active galactic nucleus and powerful source of gamma rays located in the constellation Ursa Major, approximately 400 million light-years from Earth.

said astrophysicist ASI Laura DiJesso, first author of the article. “We were sure that this Blazar would be an interesting target for Ixpe, but this discovery exceeded our best expectations. Indeed, it showed how X-ray polarimetry enriches our ability to investigate the complex geometry of magnetic field and particle acceleration in different regions of the relativistic jets. An exciting moment for studies of astrophysical jets.”

A blazar is an active galactic nucleus that is identified by the ejection of its powerful jet of free electrons towards terrestrial observers such as ground-based and space-based telescopes. The “fire” from these jets is very bright, and thus highly detectable due to the effects of relativistic space-time on the jet’s light.

Despite decades of study, scientists still do not fully understand the physical processes that drive the dynamics and emissions of relativistic jets ejected by blazars. But Ixpe’s pioneering ability to polarimetry X-rays — the ability to measure the average direction of the electric field of light waves — gives astronomers unprecedented insight into these objects, their geometry, and the origin of their emissions.

Theoretical models attempting to explain the properties of aircraft often depict a magnetic, spiral, spiral structure, similar to that found in the human DNA chain, but Ixpe found an unexpected discrepancy during three continuous observations of Markarian 421 in May and June 2022.

“We expected the polarization direction to change, but we thought major cycles would be rare, based on previous optical observations of many blazars,” he said. Herman Marshalla research physicist at the Massachusetts Institute of Technology in Cambridge and co-author of the study.

Thus Ixpe made three observations of Blazar Markarian 421, revealing a constant degree of polarization. Surprisingly, the behavior of the polarization angle was not symmetric: while the polarization angle remained constant in the first observation, in the second its orientation literally made a U-turn, turning almost 180 degrees in two days. Surprisingly, the direction of polarization continued to rotate at the same speed in the third observation.

“This remarkable discovery indicates that the plasma responsible for the X-ray emission follows the spiral structure of the magnetic field inside the jets, as proposed in 2008 by Alan Marcher and his team with radio wave observations,” he comments. Dawn Kima PhD student at Enav Rome and one of the authors of the study published on Natural astronomery. “Future observations of Markarian 421 will allow us to obtain physical information about the plane, such as the spatial dimension of the collimator region.”

Simultaneous optical, infrared, and radio measurements also showed no change in the direction of the light’s polarization, even when the direction of the X-ray’s polarization was rapidly rotating. This polarization rotation lends credence to a model in which the shock propagates along in-plane helical magnetic fields. The X-ray emission maps the spiral while the electrons that produce the optical light are in a different region of the jet, where the magnetic field does not produce a difference in polarization.

Ixpe will continue to monitor Markarian 421 and other blazars to learn more about jet fluctuations and how often they occur.

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