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Balloon-Borne Telescope XL-Calibur Takes Flight to Unravel Black Hole Secrets

Scientists from Washington University in St. Louis launched a balloon-borne telescope to learn more about astrophysical black holes and neutron stars, some of the universe's most extreme objects.

Balloon-Borne Telescope XL-Calibur Takes Flight to Unravel Black Hole Secrets

A stadium-sized scientific balloon carried the XL-Calibur instrument aloft. Image Credit: Nicole Rodriguez Cavero

On July 9th, 2024, the Swedish Space Corporation launched the XL-Calibur device from the Esrange Space Center, which is located north of the Arctic Circle close to Kiruna, Sweden.

We are excited to measure polarization of the black hole X-ray binary Cyg X-1 to determine how matter swirls around a black hole before it falls in, liberating enormous amounts of energy in the process. We hope that our results will have some impact on the measurement of black hole spin.

Henric Krawczynski, Wilfred R. and Ann Lee Konneker Distinguished Professor, Washington University in St. Louis

Krawczynski is the principal investigator for XL-Calibur.

Esrange, situated in the expansive and uninhabited region of northernmost Sweden, is an ideal site for NASA's 2024 scientific ballooning campaign.

The location of the launch range and the stratospheric winds allow for excellent flight conditions to gather many days of scientific data as the balloons traverse from Sweden to northern Canada.

Andrew Hamilton, Acting Director, NASA’s Balloon Program Office, Washington University in St. Louis

Scientists from the United States, Sweden, and Japan collaborated on XL-Calibur.

In addition to components constructed at Washington University (WashU), the instrument incorporates a mirror that was a flight spare from the Japanese space agency's Hitomi mission, a gondola and pointing control mechanism developed at NASA’s Wallops Flight Facility, and a shielding device developed at the KTH Royal Institute of Technology in Sweden.

This device is being flown on a long-duration balloon as part of NASA’s scientific ballooning program. Data from XL-Calibur will be publicly available and hosted by NASA’s High Energy Astrophysics Science Archive and Research Center.

During this flight, XL-Calibur scientists aim to study the accretion of matter by the black hole Cygnus X-1. They will also gather data to test the mechanisms by which pulsars accelerate particles. The data collected by XL-Calibur can be utilized independently or in conjunction with data from the space-borne Imaging X-Ray Polarimetry Explorer (IXPE) satellite.

We now have IXPE results from 2-8 keV. Extending those results to 15-80 keV will allow for deeper tests of the models advanced to explain the IXPE results.

Henric Krawczynski, Wilfred R. and Ann Lee Konneker Distinguished Professor, Washington University in St. Louis

Krawczynski is also a member of the IXPE science team and a faculty fellow of the McDonnell Center for the Space Sciences at WashU.

The 3,500-pound XL-Calibur device is mounted on a gondola carried by a stadium-sized scientific balloon, lifting it into the stratosphere. The instrument will collect measurements at approximately 125,000 feet (38,100 meters) above the Earth, positioning it above 99.97% of the atmosphere.

This marks the second flight for XL-Calibur. Washington University (WashU) received $1.5 million in funding from NASA for this flight, with additional support from Japan and Sweden.

Of course, we would love to be in the air for as long as possible, to get as much data as possible! We had been aiming for at least 4 to 5 days of data-taking flight, but it really depends on the stratospheric winds, at any given point in time,” said Ephraim Gau, Graduate Student in Physics in Arts & Sciences who also works on XL-Calibur and is stationed at Esrange for the launch.

Gau said, “Regardless, this has the potential to be one of the most scientifically successful flights of XL-Calibur or its predecessors because of how well it complements recent results from IXPE. I would especially like to credit the many previous graduate students and post-docs including Lindsey Lisalda, Andrew West, and Quin Abarr who spent years working to make this experiment a reality.”


At a screen time of about 09:00:20 in this launch video, the inflated balloon begins to rise. By 09:01:15, the balloon tugs at XL-Calibur, which is being held aloft by a crane. At 09:01:45, XL–Calibur is released and begins its ascent. Video Credit: Swedish Space Corporation

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