X-ray polarimetry is pushing the frontiers of space exploration.

The IXPE with the deployed coilable boom during testing. The boom provides a focal length of 4 meters and positions each of the mirror module assemblies above its respective X-ray detector unit. Credit: Ball Aerospace

For millennia, humanity has been fascinated by the secrets hidden in the vastness of space. The optical telescope, invented in the 17th century, allowed people to see stars as tiny glittering dots in the night sky. We can now launch telescopes into space to gain a better look at cosmic objects and even study them at wavelengths outside the visible spectrum, thanks to scientific advancements during the following four centuries. One such mission into space is NASA's Imaging X-ray Polarimetry Explorer (IXPE), which was launched on December 9, 2021.

The IXPE is a space-based observatory that was created in partnership with the Italian Space Agency (ASI). It has three identical telescopes at its focal point, each with an imaging X-ray detector sensitive to light polarization. With them, IXPE may investigate some of our universe's brightest cosmic X-ray sources, such as pulsars, black holes, and neutron stars. IXPE will begin its two-year baseline mission by investigating dozens of X-ray sources in the first year, followed by more thorough studies of the selected targets in the second year.

This multi-national project was conceived in 2017 and became a reality in 2021 thanks to the collaboration of multiple space agencies to complete various components of the mission. The optics and detectors of IXPE, as well as the mission's scientific aims, are described in-depth in a recent article published in the Journal of Astronomical Telescopes, Instruments, and Systems.

IXPE was launched into an equatorial orbit at a height of 600 kilometers by a reusable Falcon 9 rocket from the Kennedy Space Center. This orbit was intended to decrease the charged-particle background, extend the explorer's lifespan, and make data downloads to primary and backup ground stations more convenient (Kenya and Singapore, respectively). To stay on course in orbit, the observatory utilizes a system of 12 sun sensors, a three-axis magnetometer, and two-star trackers.

A mirror module assembly is included in each telescope of the IXPE observatory (MMA). X-rays are focused on polarization-sensitive detection units by the MMA (DUs). By giving time information and polarization sensitivity data, the DUs aid in energy and location determination. The detector service unit (DSU) receives the data gathered by the DUs and analyzes it before sending it to the ground. After launch, a lightweight, coilable boom is extended to achieve proper focal length and line the MMAs with the DUs. Onboard, there is also a tip-tilt-rotate mechanism that may be used to align the mirrors with the detectors.

IXPE commenced its baseline mission, delivering high-quality polarization data from diverse sources, once the early phases of alignment and calibration were completed. In February, the first imaging data was released. The IXPE team predicts that the most spectacular early photos will come from shell-type supernova remnants (a supernova that emits most of its radiation from a shell of shocked material). They anticipate IXPE will be able to scan active galaxies, such as the Milky Way's galactic core, as well as "blazars," which are galaxies that release intense jets of ionized matter and radiation. This will allow us to push the boundaries of observation even further by investigating new sources of specific interest in order to get physical insights.

The astrophysics community has been looking forward to this capability, according to Megan Eckart, deputy editor for JATIS. IXPE provides a new window on the X-ray sky by giving orders of magnitude more sensitivity than prior X-ray polarimeters in space.

IXPE is a technological and scientific marvel that will offer the first information on X-ray polarization for numerous celestial sources. IXPE, with its cutting-edge telescopes and detectors, has the potential to broaden the breadth of our understanding of the cosmos.

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