XL-Calibur telescope launched to study black holes

Scientists from Washington University in St. Louis have launched a balloon-borne telescope to unlock the secrets of astrophysical black holes and neutron stars, some of the most extreme objects in the universe.

The device known as XL-Calibur was launched from the Swedish Space Corporation’s Esrange Space Center, situated north of the Arctic Circle near Kiruna, Sweden on July 9.

“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,” said Henric Krawczynski, the Wilfred R. and Ann Lee Konneker Distinguished Professor of Physics in Arts & Sciences, who is the principal investigator for XL-Calibur. “We hope that our results will have some impact on the measurement of black hole spin.”

Esrange, located in a vast unpopulated area in the northernmost part of Sweden, is an ideal location 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,” said Andrew Hamilton, acting director of NASA’s Balloon Program Office, in a statement.

XL-Calibur is a collaboration among scientists from the United States, Japan and Sweden.

In addition to parts built at WashU, the instrument uses 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 KTH in Sweden. The device is flying on a long-duration balloon as part of NASA’s scientific ballooning program, and XL-Calibur data will be publicly delivered and hosted by NASA’s High Energy Astrophysics Science Archive and Research Center.

During this flight, XL-Calibur scientists plan to study how the black hole Cygnus X-1 accretes matter. They will also collect data to test how pulsars accelerate particles. The XL-Calibur data can be used on its own or together 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,” said Krawczynski, who 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 that lifts it into the stratosphere. The instrument will collect measurements at about 125,000 feet (38,100 meters) in the air, above 99.97% of the Earth’s atmosphere. This is the second flight for XL-Calibur.

“Of course we would love to be in the air for as long as possible, to get as much data as possible,” said Ephraim Gau, a graduate student in physics in Arts & Sciences who works on XL-Calibur and is stationed at Esrange for the launch. “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.

“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,” Gau said. “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.”