The vastness of space holds many mysteries, and among its enigmatic inhabitants are supermassive black holes that wield immense power. When these black holes emit high-energy particle jets that directly align with Earth, astrophysicists label them as blazars, creating sources of dazzling brightness. Unlocking the secrets of these blazar jets has been a longstanding challenge for scientists, but a groundbreaking mission by NASA’s Imaging X-ray Polarimetry Explorer (IXPE) has recently taken us one step closer to unravelling the complexities of these cosmic phenomena.
IXPE, launched in December 2021, boasts a unique ability to measure polarization in X-ray light. This property reveals vital information about the organization of electromagnetic waves at X-ray frequencies, offering a novel lens through which we can investigate particle acceleration and magnetic field structures within blazar jets.
In a stunning revelation, an international team of astrophysicists has published their latest findings from IXPE, centred around a blazar called Markarian 421, located approximately 400 million light-years away in the constellation Ursa Major. What surprised the researchers was the evidence of a helical structure in the magnetic field within the part of the jet where particle acceleration occurs.
Lead author of the study, Laura Di Gesu from the Italian Space Agency, expressed her excitement about the unexpected discoveries. She noted that while Markarian 421 had been an anticipated target for IXPE, the results surpassed their best expectations, providing unprecedented insights into the complex geometry of magnetic fields and particle acceleration in relativistic jets.
Blazar jets, extending millions of light-years in length, shine with extraordinary brightness due to the tremendous energy emitted by particles approaching the speed of light, in line with Einstein’s predictions. The light directed toward Earth appears even brighter, akin to how an approaching ambulance siren sounds louder. As a result, blazars can outshine all the stars in the galaxies they inhabit.
Despite decades of research, the intricacies of the physical processes that shape blazar jets remain elusive. However, IXPE’s revolutionary X-ray polarimetry, measuring the average direction of light wave electric fields, offers an unprecedented view of these cosmic targets, shedding light on their physical structures and the origins of their emissions.
The team’s models for the typical outflow of powerful jets depict a spiraling helix structure, resembling the organization of human DNA. Nevertheless, they were taken aback by the presence of regions within the helix where particles are being accelerated by shocks.
IXPE’s observations of Markarian 421 in May and June 2022 revealed intriguing variability in the polarization angle. During the first observation, a constant polarization of 15% was expected, but the data showed a U-turn-like rotation of nearly 180 degrees between the first and second observations, continuing at the same rate in the third observation.
Moreover, concurrent measurements in optical, infrared, and radio wavelengths exhibited stability and structure, unaffected even when the polarized X-ray emissions deviated. This intriguingly suggests the possibility of a shockwave propagating along the jet’s spiralling magnetic fields.
The concept of shockwaves accelerating particles aligns with theories proposed for Markarian 501, another blazar observed by IXPE, leading to a published study in late 2022. However, Markarian 421 appears to present clearer evidence of a helical magnetic field contributing to the shock.
To deepen their understanding, Di Gesu, Herman Marshall from the Massachusetts Institute of Technology, and their colleagues are eager to conduct further observations of Markarian 421 and other blazars, seeking more insights into these jet fluctuations and their frequency.
With IXPE’s collaboration between NASA and the Italian Space Agency, spanning partners and science collaborators across 12 countries, we find ourselves at an exhilarating juncture for studying astrophysical jets. As we continue to uncover the mysteries of blazars, the enigmatic realm of black hole systems promises even more revelations, taking us on a quantum leap in our understanding of the cosmos.
