NASA's IXPE mission reveals unusual structure of energetic black hole jet 'blazar'

Study of Markarian 421

According to NASA, blazar jets may span millions of light-years in length. The brightness is caused by the acceleration of particles at the speed of light, which emits a massive quantity of energy in the process. 

Blazars are so bright that they can outshine all of the stars in their host galaxy. 

Scientists have been investigating blazar jets for over a decade, but the physical mechanisms that contribute to their dynamics have been difficult to understand.  

Markarian 421 is situated in the constellation Ursa Major, 400 million light-years from Earth.

"Markarian 421 is an old friend for high-energy astronomers,” said Italian Space Agency astrophysicist Laura Di Gesu, lead author of the new paper, in an official release. 

This blazar has piqued the interest of scientists since the magnetic field structure seems to be helical in a part of a jet, where the particles are accelerated.

“We were sure the blazar would be a worthwhile target for IXPE, but its discoveries were beyond our best expectations, successfully demonstrating how X-ray polarimetry enriches our ability to probe the complex magnetic field geometry and particle acceleration in different regions of relativistic jets,” added Di Gesu.

Polarized X-rays are reported to be useful in studying such magnetic fields. 

Unusual spiraling structure 

According to NASA, some models have previously proposed that typical strong jets may have a "spiraling helix structure" – similar to the structure of human DNA. 

What is surprising is that scientists did not anticipate this structure displaying "regions of particles being accelerated by shocks." This suggests that a shockwave could be propagating spiraling magnetic fields inside the jet.

During three comprehensive observations in May and June 2022, IXPE discovered unexpected fluctuation in the "polarization angle" of Markarian 421. When the pathways of electrons flying at near-light speed are bent by a magnetic field, X-rays can be polarized. 

“We had anticipated that the polarization direction might change but we thought 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 a co-author of the paper. 

“So, we planned several observations of the blazar, with the first showing a constant polarization of 15%. Then we recognized that the polarization was actually about the same but its direction literally pulled a U-turn, rotating nearly 180 degrees in two days. It then surprised us again during the third observation, which started a day later, to observe the direction of polarization continuing to rotate at the same rate,” explained Marshall.

Up next, the team intends to undertake further observations of Markarian 421 and other blazars in order to understand more about these jet fluctuations.

The results have been published in the journal Nature Astronomy.

Study abstract:

The magnetic-field conditions in astrophysical relativistic jets can be probed by multiwavelength polarimetry, which has been recently extended to X-rays. For example, one can track how the magnetic field changes in the flow of the radiating particles by observing rotations of the electric vector position angle Ψ. Here we report the discovery of a ΨX rotation in the X-ray band in the blazar Markarian 421 at an average flux state. Across the 5 days of Imaging X-ray Polarimetry Explorer observations on 4–6 and 7–9 June 2022, ΨX rotated in total by ≥360°. Over the two respective date ranges, we find constant, within uncertainties, rotation rates (80 ± 9° per day and 91 ± 8° per day) and polarization degrees (ΠX = 10% ± 1%). Simulations of a random walk of the polarization vector indicate that it is unlikely that such rotation(s) are produced by a stochastic process. The X-ray-emitting site does not completely overlap the radio, infrared and optical emission sites, as no similar rotation of Ψ was observed in quasi-simultaneous data at longer wavelengths. We propose that the observed rotation was caused by a helical magnetic structure in the jet, illuminated in the X-rays by a localized shock propagating along this helix. The optically emitting region probably lies in a sheath surrounding an inner spine where the X-ray radiation is released.