There are many mysterious and powerful phenomena in the universe, and the X-ray radiation they emit can reveal their nature and workings. However, to study these X-rays, we need a special instrument that can measure their polarization, which is the direction in which light is vibrating. Polarization can tell us what light is and how it is affected and altered.
This is the mission of the IXPE (Imaging X-ray Polar Coordinate Detector), an X-ray astronomical mission launched in 2021 that has revolutionized our understanding of cosmic phenomena such as flare variants and supernova remnants. On Dec. 9, astronomers and physicists commemorated the landmark X-ray science of the National Aeronautics and Space Administration's (NASA) IXPE mission two years ago.
IxPE's goals and technologies
The IXPE is a joint mission between NASA and the Italian Space Agency to study polarized X-ray light. It consists of three identical X-ray telescopes, each with a lens and a detector. The lens focuses the X-rays onto the detector, which measures the energy, position, and polarization of the X-rays. At the heart of the detector is a device called a gas pixel detector, which records the trajectory of electrons produced when X-rays collide with gas molecules, thereby estimating the polarization of X-rays.
Launched on December 9, 2021, IXPE orbits the Earth at an altitude of about 340 miles to study X-ray radiation from powerful cosmic phenomena thousands to billions of light-years from Earth, including remnants of supernovae such as quasars, flares, neutron stars, and streams of high-energy particles ejected from the vicinity of black holes at nearly the speed of light. These phenomena are the most extreme environments in the universe, and there are still many unknowns about their physical processes and energy conversions, and the observation of IXPE can help us uncover their secrets.
Outcomes and impact of IXPE
Over the past two years, IXPE has observed several important X-ray sources and made surprising discoveries. Here are some representative examples:
Flare variants: Flare variants are a special type of quasar that have a supermassive black hole surrounded by an accretion disk that emits powerful jets of particles from the poles. These jets of particles emit X-rays, but how are they accelerated to such high energies? Ixpe's data identified one of the flare variants, Makaria 501, as the culprit was a shock wave inside the jet. These shock waves create a strong magnetic field that accelerates the particles and produces polarized X-rays. This is the first direct observation of the X-ray polarization of flare variants, providing important clues to the understanding of their acceleration mechanisms.
Supernova remnants: Supernova remnants are shell-like structures made up of gas and dust left behind behind by stars** that expand and collide in the universe to form complex forms and structures. They are also particle accelerators in the universe and can produce high-energy X-rays. Ixpe observed three well-known supernova remnants, Cassiopeia A, Tycho, and SN 1006, and discovered their X-ray polarization signatures. These features can help us understand the origin and evolution of their magnetic fields, as well as their particle acceleration processes.
Black hole at the center of the Milky Way: There is a supermassive black hole at the center of the Milky Way called Sagittarius A*, which is usually quiet but can suddenly become active and emit intense X-ray flares. What causes these flares? Combined with data from other X-ray telescopes, Ixpe discovered the astonishing fact that Sagittarius A* had awakened once about 200 years ago, swallowing up large amounts of gas and other substances, producing a massive flare. The light of this flare has undergone complex scattering and reflection before reaching our eyes now. This is the first time that the historical activity of a black hole at the center of the Milky Way has been observed, providing a new perspective for exploring its nature and behavior.
Intense Radiation Pulses: Intense radiation pulses are rare and intense cosmic events that release a huge amount of energy in a matter of seconds, forming a beam of high-energy X-rays and gamma rays. Their mechanism is unclear and may have arisen as a result of the collision or merger of black holes or neutron stars. In October 2022, Ixpe captured a pulse of intense radiation, the brightest on record, and its glow was even visible to the naked eye. The data from Ixpe showed that the X-rays of this pulse were highly polarized, meaning they were produced in a strong magnetic field. This provides important clues to reveal the nature of the intense radiation pulses.
The future and outlook of IXPE
The IXPE mission is not over yet, and it will continue to observe more X-ray sources, including some new and unknown targets. It will also be open to the wider scientific community, inviting astronomers and physicists from around the world to propose and participate in research using IXPE. The data of IXPE will provide more information and inspiration for us to understand the mysteries of the universe, and will also bring more innovation and development to our science and technology.
IXPE is a mission to explore the X-ray eye of the universe, and it has already achieved impressive results and demonstrated the diversity and complexity of X-ray radiation in the universe. The IXPE observations have not only improved our understanding of known X-ray sources, but also uncovered some unexpected X-ray phenomena, such as strong radiation pulses and the historical activity of the black hole at the center of the Milky Way. The IXPE data also provides new opportunities for us to test and develop some physical theories, such as general relativity and quantum mechanics. There are many future challenges and opportunities for the IXPE mission, which will continue to explore the mysteries of the universe in X-rays and uncover the mysteries and beauty of the universe for us. The two-year outcome of the IXPE mission