The universe is teeming with powerful supermassive black holes that spawn powerful jets of high-energy particles, creating sources of extreme brightness in the vastness of space. When one of those jets points directly at Earth, scientists call the black hole system a blazar.
To understand why particles in the jet move with great speeds and energies, scientists turn to NASA’s IXPE (Imaging X-ray Polarimetry Explorer), which launched in December 2021. This advanced tool measures a unique property of X-ray light, known as polarization, which pertains to the arrangement of electromagnetic waves at X-ray frequencies.
Fresh Insights From Markarian 421
This week, an international team of astrophysicists published fresh findings from IXPE concerning a blazar called Markarian 421. Located in the Ursa Major constellation and roughly 400 million light-years from Earth, this blazar has stunned scientists with evidence of a helical structure in the magnetic field where particles are accelerated.
“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 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.”
The new study detailing the IXPE team’s findings at Markarian 421 is available in the latest edition of Nature Astronomy.
The Phenomenon of Blazar Jets
Blazar jets, such as the one emitted from Markarian 421, can span millions of light-years in length. They are especially bright because as particles approach the speed of light, they give off a tremendous amount of energy and behave in weird ways that Einstein predicted. Blazar jets are brighter still because, just like an ambulance siren sounds louder as it approaches, light pointed toward us also appears brighter. This explains why blazars can outshine all of the stars within their host galaxies.
The Conundrum of Blazar Jet Dynamics
In spite of decades of research, the physical processes shaping the dynamics and emissions of blazar jets remain elusive to scientists. However, the novel X-ray polarimetry of IXPE – which measures the average direction of the electric field of light waves – offers an unrivaled perspective on these objects, their physical geometry, and the origin of their emissions.
Researchers’ models typically depict the powerful jet outflow with a spiraling helix structure, akin to the organization of human DNA. But scientists did not expect that the helix structure would contain regions of particles being accelerated by shocks.
IXPE discovered surprising variability in the polarization angle during three extended observations of Markarian 421 in May and June 2022.
“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, 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%.”
Remarkably, he added, initial analysis of the polarization data from IXPE appeared to show it dropped to zero between the first and second observations.
“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,” Marshall said. “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.”
Shockwave Propagation and Future Observations
Stranger still was that concurrent optical, infrared, and radio measurements showed no change in stability or structure at all – even when the polarized X-ray emissions deviated. This suggests a shockwave might be propagating along spiraling magnetic fields inside the jet.
The concept of a shockwave accelerating the jet’s particles is consistent with theories about Markarian 501, a second blazar observed by IXPE that led to a published study in late 2022. But its cousin Markarian 421 shows more clearcut evidence of a helical magnetic field contributing to the shock.
Di Gesu, Marshall, and their colleagues are eager to conduct further observations of Markarian 421 and other blazars to learn more about these jet fluctuations and how frequently they occur.
“Thanks to IXPE, it’s an exciting time for studies of astrophysical jets,” Di Gesu said.
Reference: “Discovery of X-ray polarization angle rotation in the jet from blazar Mrk 421” by Laura Di Gesu, Herman L. Marshall, Steven R. Ehlert, Dawoon E. Kim, Immacolata Donnarumma, Fabrizio Tavecchio, Ioannis Liodakis, Sebastian Kiehlmann, Iván Agudo, Svetlana G. Jorstad, Fabio Muleri, Alan P. Marscher, Simonetta Puccetti, Riccardo Middei, Matteo Perri, Luigi Pacciani, Michela Negro, Roger W. Romani, Alessandro Di Marco, Dmitry Blinov, Ioakeim G. Bourbah, Evangelos Kontopodis, Nikos Mandarakas, Stylianos Romanopoulos, Raphael Skalidis, Anna Vervelaki, Carolina Casadio, Juan Escudero, Ioannis Myserlis, Mark A. Gurwell, Ramprasad Rao, Garrett K. Keating, Pouya M. Kouch, Elina Lindfors, Francisco José Aceituno, Maria I. Bernardos, Giacomo Bonnoli, Víctor Casanova, Maya García-Comas, Beatriz Agís-González, César Husillos, Alessandro Marchini, Alfredo Sota, Ryo Imazawa, Mahito Sasada, Yasushi Fukazawa, Koji S. Kawabata, Makoto Uemura, Tsunefumi Mizuno, Tatsuya Nakaoka, Hiroshi Akitaya, Sergey S. Savchenko, Andrey A. Vasilyev, José L. Gómez, Lucio A. Antonelli, Thibault Barnouin, Raffaella Bonino, Elisabetta Cavazzuti, Luigi Costamante, Chien-Ting Chen, Nicolò Cibrario, Alessandra De Rosa, Federico Di Pierro, Manel Errando, Philip Kaaret, Vladimir Karas, Henric Krawczynski, Lindsey Lisalda, Grzegorz Madejski, Christian Malacaria, Frédéric Marin, Andrea Marinucci, Francesco Massaro, Giorgio Matt, Ikuyuki Mitsuishi, Stephen L. O’Dell, Alessandro Paggi, Abel L. Peirson, Pierre-Olivier Petrucci, Brian D. Ramsey, Allyn F. Tennant, Kinwah Wu, Matteo Bachetti, Luca Baldini, Wayne H. Baumgartner, Ronaldo Bellazzini, Stefano Bianchi, Stephen D. Bongiorno, Alessandro Brez, Niccolò Bucciantini, Fiamma Capitanio, Simone Castellano, Stefano Ciprini, Enrico Costa, Ettore Del Monte, Niccolò Di Lalla, Victor Doroshenko, Michal Dovčiak, Teruaki Enoto, Yuri Evangelista, Sergio Fabiani, Riccardo Ferrazzoli, Javier A. Garcia, Shuichi Gunji, Kiyoshi Hayashida, Jeremy Heyl, Wataru Iwakiri, Fabian Kislat, Takao Kitaguchi, Jeffery J. Kolodziejczak, Fabio La Monaca, Luca Latronico, Simone Maldera, Alberto Manfreda, C.-Y. Ng, Nicola Omodei, Chiara Oppedisano, Alessandro Papitto, George G. Pavlov, Melissa Pesce-Rollins, Maura Pilia, Andrea Possenti, Juri Poutanen, John Rankin, Ajay Ratheesh, Oliver J. Roberts, Carmelo Sgrò, Patrick Slane, Paolo Soffitta, Gloria Spandre, Douglas A. Swartz, Toru Tamagawa, Roberto Taverna, Yuzuru Tawara, Nicholas E. Thomas, Francesco Tombesi, Alessio Trois, Sergey S. Tsygankov, Roberto Turolla, Jacco Vink, Martin C. Weisskopf, Fei Xie and Silvia Zane, 17 July 2023, Nature Astronomy.
IXPE is a collaboration between NASA and the Italian Space Agency with partners and science collaborators in 12 countries. IXPE is led by NASA’s Marshall Space Flight Center in Huntsville, Alabama. Ball Aerospace, headquartered in Broomfield, Colorado, manages spacecraft operations together with the University of Colorado’s Laboratory for Atmospheric and Space Physics in Boulder. IXPE’s observations of Markarian 421 were complemented with data gathered by partner observatories across the United States and in France, Japan, Spain, and Crete.