An international team launched the XL-Calibur telescope on a balloon in July 2024 to observe Cygnus X-1, providing the most accurate measurement yet of its hard X-ray polarization. These findings reveal details about the superheated material around the black hole. The mission also gathered data from the Crab pulsar, marking technical breakthroughs in X-ray astronomy.
Scientists from Washington University in St. Louis and international collaborators flew the XL-Calibur balloon-borne telescope across the Northern Hemisphere in July 2024, from Sweden to Canada. The instrument targeted Cygnus X-1, a black hole about 7,000 light-years away, to measure the polarization of its hard X-rays. Polarization describes the orientation of electromagnetic vibrations, offering clues about the shape and behavior of extremely hot gas and debris swirling at high speeds around the black hole.
The observations, detailed in a recent paper in The Astrophysical Journal, represent the most precise measurement of Cygnus X-1's hard X-ray polarization to date. Ephraim Gau, a graduate student at Washington University, explained the value of this approach: "If we try to find Cyg X-1 in the sky, we'd be looking for a really tiny point of X-ray light. Polarization is thus useful for learning about all the stuff happening around the black hole when we can't take normal pictures from Earth."
During the flight, XL-Calibur also collected data from the Crab pulsar and its surrounding wind nebula, a bright and stable X-ray source. Henric Krawczynski, the project's principal investigator and a professor at Washington University, noted: "The observations we made will be used by scientists to test increasingly realistic, state-of-the-art computer simulations of physical processes close to the black hole."
The mission achieved multiple technical milestones, validating the telescope's design. Mark Pearce, a collaborator from KTH Royal Institute of Technology in Sweden, said: "Our observations of Crab and Cyg X-1 clearly show that the XL-Calibur design is sound. I very much hope that we can now build on these successes with new balloon flights."
Looking forward, the team plans another launch from Antarctica in 2027 to study more black holes and neutron stars. Krawczynski added: "Combined with the data from NASA satellites such as IXPE, we may soon have enough information to solve longstanding questions about black hole physics in the next few years."
The effort involves institutions including Washington University, the University of New Hampshire, Osaka University, and NASA's Goddard Space Flight Center, supported by various NASA grants.