NASA’s Imaging X-ray Polarimetry Explorer (IXPE) Surprises Astronomers with Blazar Discovery

by Klaus Müller
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Blazar Discovery

A remarkable new discovery has left astronomers astonished as NASA’s Imaging X-ray Polarimetry Explorer (IXPE) unveils fresh insights into the enigmatic world of blazars. Blazars are black hole systems that emit powerful jets of high-energy particles, creating dazzling displays across the cosmos when directed towards Earth.

To unravel the mysteries behind these jets and the extraordinary speeds and energies of the particles within them, scientists have turned to IXPE. Launched in December 2021, this cutting-edge tool measures the unique property of X-ray light known as polarization, providing valuable information about the arrangement of electromagnetic waves at X-ray frequencies.

The latest findings from IXPE, published by an international team of astrophysicists, focus on a blazar named Markarian 421. Positioned approximately 400 million light-years away in the Ursa Major constellation, this blazar has astounded scientists with evidence of a helical structure within its magnetic field, where particles undergo acceleration.

Laura Di Gesu, lead author of the study and astrophysicist at the Italian Space Agency, expresses her excitement about the results: “Markarian 421 is a familiar subject for high-energy astronomers. We had high expectations for IXPE’s observations of this blazar, but the discoveries exceeded our wildest imaginations. They demonstrate how X-ray polarimetry enhances our ability to investigate the intricate magnetic field geometry and particle acceleration in various regions of relativistic jets.”

The comprehensive study detailing the remarkable findings of the IXPE team regarding Markarian 421 can be found in the latest edition of Nature Astronomy.

Blazar jets, like the one emitted from Markarian 421, stretch across millions of light-years in length. These jets exhibit exceptional brightness because as particles approach the speed of light, they emit tremendous amounts of energy and exhibit peculiar behaviors predicted by Einstein’s theories. Moreover, the apparent brightness of blazars is further intensified due to the Doppler effect, similar to how an ambulance siren sounds louder as it approaches. This phenomenon explains why blazars can outshine all the stars in their host galaxies.

Despite decades of research, scientists have yet to fully comprehend the physical processes influencing the dynamics and emissions of blazar jets. However, the novel X-ray polarimetry capabilities of IXPE, which measure the average direction of the electric field in X-ray waves, offer an unparalleled perspective on these cosmic phenomena, shedding light on their physical geometry and the origins of their emissions.

The team’s models typically depict the powerful jet outflows with a helical spiral structure reminiscent of human DNA organization. However, the unexpected discovery from IXPE reveals that regions within this helix structure are associated with particle acceleration caused by shocks.

During three extended observations of Markarian 421 in May and June 2022, IXPE uncovered surprising variability in the polarization angle. “We expected some polarization direction changes, but we anticipated significant rotations to be rare based on previous optical observations of numerous blazars,” explains Herman Marshall, a co-author of the study and research physicist at the Massachusetts Institute of Technology. “However, the initial analysis of the polarization data from IXPE revealed a seemingly drastic drop to zero between the first and second observations.”

Marshall adds that upon closer examination, they realized the polarization remained relatively constant but had undergone a complete U-turn in direction, rotating nearly 180 degrees in just two days. Astonishingly, the third observation, conducted a day later, continued to observe polarization direction rotation at the same rate.

Interestingly, while the polarized X-ray emissions deviated, concurrent measurements in the optical, infrared, and radio wavelengths displayed no changes in stability or structure. This discrepancy suggests that a shockwave may be propagating along the spiral magnetic fields within the jet.

This concept of a shockwave accelerating the particles in the jet aligns with theories proposed for another blazar observed by IXPE, Markarian 501, which led to a published study in late 2022. However, Markarian 421 exhibits clearer evidence of a helical magnetic field contributing to the shock.

Laura Di Gesu, Herman Marshall, and their colleagues eagerly anticipate further observations of Markarian 421 and other blazars to gain deeper insights into these fluctuations within the jets and their frequency of occurrence.

Di Gesu concludes, “Thanks to IXPE, we are living in an exciting era of astrophysical jet studies.”

Frequently Asked Questions (FAQs) about Blazar Discovery

What is IXPE and what does it measure?

IXPE stands for Imaging X-ray Polarimetry Explorer. It is a NASA mission that measures a unique property of X-ray light called polarization, which provides insights into the arrangement of electromagnetic waves at X-ray frequencies.

What is a blazar?

A blazar is a black hole system that emits powerful jets of high-energy particles. These jets create sources of extreme brightness in space when they are pointed directly at Earth.

What did the IXPE mission discover about Markarian 421?

The IXPE mission discovered a helical structure in the magnetic field of the blazar Markarian 421, where particles undergo acceleration. This finding provides valuable insights into the complex magnetic field geometry and particle acceleration in relativistic jets.

How do blazar jets produce extreme brightness?

Blazar jets span millions of light-years in length and appear extremely bright because the particles within them emit a tremendous amount of energy as they approach the speed of light. The Doppler effect also contributes to their brightness, making blazars outshine all the stars within their host galaxies.

What is the significance of the polarization angle rotations observed by IXPE?

The unexpected polarization angle rotations observed by IXPE provide new information about the dynamics and emissions of blazar jets. The presence of these rotations suggests the propagation of shockwaves along the spiral magnetic fields within the jets, shedding light on the mechanisms behind particle acceleration in these cosmic phenomena.

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