IXPE X-ray observatory completes commissioning, eyes Cassiopeia A for calibration

by Justin Davenport

The pioneering Imaging X-ray Polarimeter Explorer (IXPE) telescope, the first observatory in space to perform detailed polarimetry of some of the highest-energy objects the universe has to offer, has ended its highly-successful commissioning phase just over 30 days after lifting off from the Kennedy Space Center on December 9, 2021, on a SpaceX Falcon 9 rocket.

As commissioning comes to an end and project leads turn their attention to the calibration of IXPE’s first-of-their-kind instruments with the Cassiopeia A supernova remnant, NASASpaceflight spoke with Principal Investigator Dr. Martin Weisskopf about the status of the mission and how the first-of-its-kind telescope will be calibrated.

IXPE itself is a joint project between NASA’s Marshall Space Flight Center and the Italian Space Agency (ASI). Placed into a 600 km (+/- 15 km) circular equatorial orbit inclined just 0.2 degrees to the Equator, the 330-kilogram IXPE deployed its solar arrays very shortly after separating from its launch vehicle while mission controllers prepared for the month-long commissioning phase.

The spacecraft completed the most critical step of this process on December 15 when the all-important boom containing the spacecraft’s three X-ray telescopes deployed successfully.

Without that deployment, the IXPE mission would have ended before it could have begun as the telescopes need to be mounted away from the spacecraft’s main body to achieve the correct focal length for their observations.

As Dr. Weisskopf stated regarding deployment, “For those of us in the space game, moving parts are always frightening.” Past missions, notably the Galileo space probe mission to orbit Jupiter, have been affected by deployment failures.

With the commissioning phase for IXPE ending on Monday, January 10, Dr. Weisskopf described the entire process as having “gone incredibly smoothly,” — with the only issue of note being a “safe mode” event triggered by an unexpected aspect solution from the dual-head star cameras, which are used for navigation and tracking.

As Dr. Weisskopf explained, “One of the things [the cameras] do is they set a flag if the next X aspect solution, which occurs every second, is not the same as the expected one. And if that happens six times in a row, it sets another flag. And that other flag should not be tripped to go into safe mode. It’s just for information, but in this software, it was tripped on.”

A software update to avoid this issue has been implemented. Otherwise, commissioning went very well, with only occasional minor tweaks to procedures written before the launch — which is quite common.

With commissioning complete, IXPE now must be calibrated. To do that, the telescope’s first observation will be of supernova remnant Cassiopeia A beginning on Monday, January 10 — once commissioning ends.

IXPE’s first target, Cassiopeia A — seen here in false color from the Hubble and Spitzer Space Telescopes as well as the Chandra X-ray Observatory. (Credit: NASA/JPL-Caltech)

Coincidentally, Cassiopeia A was also the Chandra X-ray Observatory’s first target, and Dr. Weisskopf works on both projects.

The IXPE observations of Cassiopeia A are scheduled to last 21 days, with only minor interruptions of a few minutes every orbit due to the observatory’s passage near the South Atlantic Anomaly (SAA).

The SAA is a region where the Van Allen radiation belts dip down to an altitude of approximately 190 km above Earth sea level, resulting in an influx of charged particles and higher radiation levels.

As Dr. Weisskopf explained, “When our detectors see [the influx of charged particles from the SAA], they go crazy. We can’t take data than because it [would just be] swamped by that signal. There’s also an impact on the lifetime of the detector if we just bombarded them with X-ray particles. We just don’t want to do that”.

After the 21 days of planned observations of Cassiopeia A, 30 days of compilation and review will be conducted before the data is made public via the NASA HEASARC (High Energy Astrophysics Science Archive Research Center) website, located here. After three months, data from observations will be archived a week after the observation is made — with IXPE planned to conduct an average of one study a week.

After Cassiopeia A, other planned targets include magnetars — neutron stars with particularly intense magnetic fields — as well as black holes, galactic nuclei, quasars, pulsars, and blazars, along with targets of opportunity during the two-year primary mission.

Of particular interest to Dr. Weisskopf are the forthcoming studies of the polarization of magnetars — given his uncle wrote one of the first papers on magnetar magnetic fields in the 1930s.

Another particular target of note for Dr. Weisskopf is the Crab Nebula supernova remnant as he has written more papers about the nebula and its pulsar than any other object in his career.

As he describes the plans for observing this famous supernova remnant, “I want to look at the polarization as a function of the pulse phase. I tried to do that with OSO-8, but we didn’t have enough photons in order to do anything but upper limits. With IXPE, we’ll be able to nail this with high precision because IXPE is about 100 times more sensitive than OSO was”.

The OSO-8 mission, launched in 1975, was a pioneer in observing polarized X-rays from objects, light that vibrates in a particular direction that can provide insight into the physics of an object and the environment in which it formed.

Artist impression of the IXPE spacecraft. (Credit: NASA)

Until now, other X-ray observatories such as Chandra could not detect whether X-rays were polarized.

The observations IXPE will make require the spacecraft to collect a large number of photons, which is the reason certain observations, like Cassiopeia A, will take up to 21 days. As Dr. Weisskopf stated, “We need those photons. Most of your signal is background. If it’s 5% polarized, that means 95% of the flux isn’t polarized.. and that’s just not helping”.

Although brighter extragalactic objects like quasars and blazars will be observed, IXPE won’t have the sensitivity required to go deeper in time and further out in space like the Chandra X-ray Observatory, Hubble Space Telescope, and the James Webb Space Telescope can.

As a Small Explorer mission, IXPE is under strict cost limits and cannot afford the larger telescopes that would have allowed this capability.

The $188 million (US) mission is made possible by the Gas Pixel Detectors (GPDs) made in Italy and provided by the Italian Space Agency. These detectors are the heart of the spacecraft’s capabilities and use a dimethyl ether-helium gas mixture to trap photons that enter through a window made of beryllium.

The Italian Space Agency also provided the primary ground station IXPE communicates with: Malindi, Kenya, on the Indian Ocean. In addition, a Finnish company (Oxford Instrument Technology OY) assisted with GPD assembly while Nagoya University in Japan provided the thermal shields.

Collaboration between NASA, the Italian Space Agency, and Colorado-based spacecraft contractor Ball Aerospace helped the mission proceed despite difficulties ranging from government furloughs to the COVID-19 pandemic.

Overall, IXPE is one of several astronomical space missions, large and small, that will paint a more complete picture of how the universe works.

Additionally, IXPE was one of two major international observatories launched in December 2021, with the NASA/ESA/CSA James Webb Space Telescope following IXPE on December 25 on an Arianespace Ariane 5 rocket from French Guiana.

Webb successfully completed its never-before-attempted unfolding process on Saturday, January 8, with a five-and-a-half-month commissioning and cool-down phase to follow.

(Lead image: Artist’s rendering of IXPE in Earth orbit. Credit: NASA)

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