The universe is full of powerful supermassive black holes that create 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 look to NASA’s IXPE (Imaging X-ray Polarimetry Explorer), which launched in December 2021. IXPE
measures a special property of X-ray light called polarization, which has to do
with the organization of electromagnetic waves at X-ray frequencies.
This week, an international team of
astrophysicists published new findings from IXPE about a blazar called
Markarian 421. This blazar, located in the constellation Ursa Major, roughly
400 million light-years from Earth, surprised scientists with evidence that in
the part of the jet where particles are being accelerated, the magnetic field
has a helical structure.
This NASA illustration shows the structure of a black hole jet as inferred by recent observations of the 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. Helical magnetic fields are threaded through the jet. IXPE observations have shown that the X-rays must be generated in a shock originating within material spiraling around the helical magnetic fields. The inset shows the shock front itself. X-rays are generated in the white region nearest the shock front, whereas optical and radio emission must originate from more turbulent regions further away from the shock. Credits: NASA/Pablo Garcia
"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.
Jets like the one beaming out of Markarian
421 can extend 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 extra bright because, just like an ambulance siren sounds louder as it
approaches, light pointed toward us also appears brighter. That’s why blazars
can outshine all of the stars of the galaxies they inhabit.
Despite decades of study, scientists still
don’t fully grasp the physical processes that shape the dynamics and emission
of blazar jets. But IXPE’s groundbreaking X-ray polarimetry – which measures
the average direction of the electric field of light waves – gives them an unprecedented
view of these targets, their physical geometry, and where their emissions
originate.
Research models for the typical outflow of
the powerful jets typically depict a spiraling helix structure, similar to the
way human DNA is organized. But scientists did not expect that the helix
structure would contain regions of particles being accelerated by shocks.
IXPE found surprising variability in the
polarization angle during three prolonged 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.”
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 means
that a shockwave could be propagatating 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.
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.
Source: NASA’s IXPE Fires Up Astronomers With New Blazar Findings | NASA
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