Regardless of extensive research and observation, blazars continue to keep many secrets, posing intricate questions about their nature and behaviors. The IXPE probe, a joint venture between NASA and the Italian Space Agency, launched in 2021, provides invaluable aid in this field of study. It offers exceptional sensitivity in observing these cosmic phenomena, particularly in measuring a unique characteristic of X-ray light known as polarization, which pertains to the arrangement of electromagnetic waves at X-ray frequencies.
Recently, a multinational team of researchers headed by Laura Di Gesu, an astrophysicist at the Italian Space Agency, divulged new IXPE data on a blazar named Markarian 421. This blazar, situated in the direction of the Ursa Major constellation and approximately 400 million light-years away from Earth, presented scientists with compelling evidence that the jet's section, where particles gain acceleration, possesses a helical magnetic field structure.
Jets emanating from objects like Markarian 421 can span millions of light-years and are strikingly luminous. As the particles race towards light speed, they discharge vast amounts of energy. Theories generally propose a helical spiral structure, reminiscent of human DNA, for these powerful jet outflows. However, the IXPE's recent observations of Markarian 421 in May and June 2022 revealed an unexpected fluctuation in the polarization angle, adding another layer to the cosmic mystery.
This NASA illustration shows the structure of a black hole jet as deduced from recent observations of blazar Markarian 421 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. The jet is traversed by helical magnetic fields. IXPE observations showed that the X-rays must be generated by a shock originating within the material spiraling around the helical magnetic fields. The inset shows the front of the shock itself. X-rays are generated in the white region closest to the shock front, while optical and radio emissions must come from more turbulent regions far from the shock. Credit: NASA/Pablo Garcia
"We noticed that the direction of the polarization literally did a U-turn, rotating almost 180 degrees in two days," explains Herman Marshall, a researcher at the Massachusetts Institute of Technology and co-author of the study. "We were then surprised again that during the third observation, which began a day later, the direction of the polarization continued to rotate at the same rate."
In addition, optical, infrared and radio measurements showed no change in stability or structure, even when the direction of the X-ray polarization was rotating rapidly, implying that a shock wave could be propagating along the spiral magnetic fields within the jet. Researchers are now eager to conduct further observations of Markarian 421 and other blazars to learn more about these jet fluctuations and their frequency.
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