The blazar BL Lacertae, a supermassive black hole surrounded by a bright disk and jets oriented toward Earth, provided scientists with a unique opportunity to answer a longstanding question: How are X-rays generated in extreme environments like this?
NASA’s IXPE (Imaging X-ray Polarimetry Explorer) collaborated with radio and optical telescopes to find answers. The
results (preprint
available here), to be published in the journal
Astrophysical Journal Letters, show that interactions between fast-moving
electrons and particles of light, called photons, must lead to this X-ray
emission.
This artist’s concept depicts the central region of
the blazar BL Lacertae, a supermassive black hole surrounded by a bright disk
and a jet oriented toward Earth. The galaxy’s central black hole is surrounded
by swirls of orange in various shades representing the accretion disk of
material falling toward the black hole. While black holes are known for pulling
in material, this accretion process can result in the ejection of jets of
electrons at nearly the speed of light. The jet of matter is represented by the
cone of light that starts at the center of the black hole and widens out as it
reaches the bottom of the image. It is streaked with lines of white, pink and
purple which represent helix-shaped magnetic fields. We can observe these jets
in many wavelengths of light including radio, optical, and X-ray. NASA’s
Imaging X-ray Polarimetry Explorer (IXPE) recently collaborated with radio and
optical telescopes to observe this jet and determine how the X-rays are
generated in these types of celestial environments.
NASA/Pablo Garcia
Scientists had two competing possible explanations for the X-rays, one
involving protons and one involving electrons. Each of these mechanisms would
have a different signature in the polarization of X-ray light. Polarization is
a property of light that describes the average direction of the electromagnetic
waves that make up light.
If the X-rays in a black hole’s
jets are highly polarized, that would mean that the X-rays are produced by
protons gyrating in the magnetic field of the jet or protons interacting with
jet’s photons. If the X-rays have a lower polarization degree, it would suggest
that electron-photons interactions lead to X-ray production.
IXPE, which launched Dec. 9, 2021,
is the only satellite flying today that can make such a polarization
measurement.
“This was one of the biggest
mysteries about supermassive black hole jets” said Iván Agudo, lead author of
the study and astronomer at the Instituto de Astrofísica de Andalucía – CSIC in
Spain. “And IXPE, with the help of a number of supporting ground-based
telescopes, finally provided us with the tools to solve it.”
Astronomers found that electrons
must be the culprits through a process called Compton Scattering. Compton
scattering (or the Compton effect) happens when a photon loses or gains energy
after interacting with a charged particle, usually an electron. Within jets
from supermassive black holes, electrons move near the speed of light. IXPE
helped scientists learn that, in the case of a blazar jet, the electrons have
enough energy to scatter photons of infrared light up to X-ray
wavelengths.
BL Lacertae (BL Lac for short) is
one of the first blazars ever discovered, originally thought to be a variable
star in the Lacerta constellation. IXPE observed BL Lac at the end of November
2023 for seven days along with several ground-based telescopes measuring
optical and radio polarization at the same time. While IXPE observed BL Lac in
the past, this observation was special. Coincidentally, during the X-ray
polarization observations, the optical polarization of BL Lac reached a high
number: 47.5%.
“This was not only the most
polarized BL Lac has been in the past 30 years, this is the most polarized any
blazar has ever been observed!” said Ioannis Liodakis, one of the primary
authors of the study and astrophysicist at the Institute of Astrophysics –
FORTH in Greece.
IXPE found the X-rays were far less
polarized than the optical light. The team was not able to measure a strong
polarization signal and determined that the X-rays cannot be more polarized
than 7.6%. This proved that electrons interacting with photons, via the Compton
effect, must explain the X-rays.
“The fact that optical polarization was so much higher
than in the X-rays can only be explained by Compton scattering.
Steven Ehlert
Project Scientist for IXPE at Marshall Space Flight
Center
“The fact that optical polarization was
so much higher than in the X-rays can only be explained by Compton scattering”,
said Steven Ehlert, project scientist for IXPE and astronomer at the Marshall
Space Flight Center.
“IXPE has managed to solve another black
hole mystery” said Enrico Costa, astrophysicist in
Rome at the Istituto di Astrofísica e Planetologia Spaziali of the Istituto
Nazionale di Astrofísica. Costa is one of the scientists who conceived this
experiment and proposed it to NASA 10 years ago, under the leadership of Martin
Weisskopf, IXPE’s first principal investigator. “IXPE’s polarized X-ray vision
has solved several long lasting mysteries, and this is one of the most
important. In some other cases, IXPE results have challenged consolidated
opinions and opened new enigmas, but this is how science works and, for sure,
IXPE is doing very good science.”
What’s next for the blazar research?
“One thing we’ll want to do is try to
find as many of these as possible,” Ehlert said. “Blazars change quite a bit
with time and are full of surprises.”
More
about IXPE
IXPE, which continues to provide
unprecedented data enabling groundbreaking discoveries about celestial objects
across the universe, is a joint NASA and Italian Space Agency mission with
partners and science collaborators in 12 countries. IXPE is led by NASA’s
Marshall Space Flight Center in Huntsville, Alabama. BAE Systems, Inc.,
headquartered in Falls Church, Virginia, manages spacecraft operations together
with the University of Colorado’s Laboratory for Atmospheric and Space Physics
in Boulder. Learn more about IXPE’s ongoing mission here:
Source: NASA’s IXPE Reveals X-ray-Generating Particles in Black Hole Jets - NASA
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