Balloon Telescope XL-Calibur Reveals New Details About Matter Spiraling Into Black Holes

A team of international physicists has captured some of the most precise measurements ever recorded of high-energy light coming from the famous black hole Cygnus X-1, and the results are helping scientists better understand how matter behaves as it spirals into one of the most extreme objects in the universe. The work centers on a balloon-borne telescope called XL-Calibur, which recently completed a six-day flight from Sweden to Canada in July 2024. During this flight, the instrument collected detailed data about the X-ray light emitted near Cygnus X-1, located roughly 7,000 light-years from Earth.

Researchers at Washington University in St. Louis, along with collaborators from institutions across the world, are now analyzing these observations to improve computer models that simulate physical processes around black holes. Their latest analysis, published in The Astrophysical Journal, reports the most accurate measurement to date of hard X-ray polarization from Cygnus X-1. This kind of precision is crucial because polarization reveals how light waves are oriented, and that orientation provides clues about the shape, behavior, and structure of the extremely hot gas—called the accretion flow—surrounding the black hole.

The XL-Calibur instrument is specifically designed to measure this X-ray polarization. Instead of taking traditional images, which are nearly impossible at such distances and energy levels, the telescope studies the directional vibrations of incoming light. These patterns help scientists deduce the geometry of the matter swirling violently around the black hole. Since Cygnus X-1 appears only as a tiny point of X-ray light when observed from Earth, polarization is one of the few effective tools scientists have for understanding what is happening near its event horizon.

The newly published data comes from XL-Calibur’s July 2024 flight, when the telescope launched from the Esrange Space Center operated by the Swedish Space Corporation. Over six days, it drifted across the Arctic region before landing in Canada. This journey placed the telescope high above most of Earth’s atmosphere, allowing it to detect hard X-rays that typically get absorbed before reaching ground-based instruments. During the flight, the team pointed the telescope at Cygnus X-1 for extended periods, gathering a rich set of measurements that researchers are now using to refine theoretical models.

These findings are especially important because they allow scientists to compare XL-Calibur’s hard X-ray results with softer X-ray observations from NASA satellites like IXPE (Imaging X-ray Polarimetry Explorer). When data from balloon-borne and space-based systems align consistently, researchers gain stronger confidence in their interpretations of how black hole environments operate. In this case, the combined datasets may help resolve long-standing questions about the origin of X-ray emissions from black holes, including whether they come primarily from a compact corona, an extended corona, or another energetic structure near the black hole.

The project is part of a broad international collaboration involving Washington University, the University of New Hampshire, Osaka University, Hiroshima University, ISAS/JAXA, the KTH Royal Institute of Technology in Stockholm, NASA’s Goddard Space Flight Center and Wallops Flight Facility, and more than a dozen other research institutions. Graduate students, postdoctoral researchers, and senior scientists worked together to design, operate, and analyze the mission, making this a significant global effort in high-energy astrophysics.

The team is already planning its next major mission. In 2027, XL-Calibur is scheduled for a new and much longer flight—from Antarctica. This mission aims to observe not only more black holes but also neutron stars, incredibly dense remnants of massive stars. Because Antarctic flights can remain airborne for weeks thanks to stable polar winds, scientists expect to gather even more detailed polarization data. The hope is that by combining the upcoming flight’s results with the July 2024 dataset and NASA’s satellite observations, researchers may finally be able to answer key questions about how black holes release enormous amounts of energy and light, how material falls inward, and how jets form and evolve.

What Cygnus X-1 Is and Why It Matters

Cygnus X-1 is one of the most studied black holes in the universe. Discovered in the 1960s as a bright X-ray source, it later became the first object widely accepted as a true black hole. It is part of a binary system, meaning it orbits together with a massive companion star. The black hole’s intense gravitational pull strips material from this companion, creating a rapidly spinning disk of gas around it. As this gas spirals inward, it heats up to millions of degrees and emits X-rays.

Because Cygnus X-1 is relatively close to Earth compared to many other black holes, it serves as a natural laboratory for studying extreme physics. Understanding how light from the system becomes polarized helps reveal the structure and behavior of its accretion disk, corona, and relativistic jets—all of which are influenced by the black hole’s intense gravity and magnetic fields.

Polarization measurements at different X-ray energies can help distinguish between competing theoretical models. For example, if the corona is compact and located very close to the black hole, the polarization angle and degree would differ from what would be expected if the corona were extended or layered. The new XL-Calibur measurements contribute essential data to this debate.

How Balloon-Borne Telescopes Help Study Black Holes

Balloon-borne observatories like XL-Calibur are an ingenious solution to a unique challenge in astrophysics. Hard X-rays cannot be easily focused by traditional telescope mirrors, and Earth’s atmosphere blocks most of them. A huge stratospheric balloon can lift delicate scientific instruments high enough—often above 99.5% of the atmosphere—to make sensitive measurements at a fraction of the cost of launching a satellite.

XL-Calibur combines a focusing X-ray mirror with a specialized detector designed for polarization measurements. As X-rays strike the detector, they interact in a way that reveals the angle of their electric field vibrations. By tracking these patterns over long exposure times, scientists can determine how ordered or chaotic the emissions near a black hole are.

These flights also provide opportunities for rapid iteration and improvement. Unlike satellites, which can take years or decades of preparation, balloon missions can be upgraded and relaunched with relative flexibility. This makes them ideal platforms for cutting-edge experimental astrophysics.

Why These New Measurements Are Significant

The latest measurements offer several important insights:

They provide the most precise hard X-ray polarization data ever obtained from a black hole.
They help narrow down theoretical models describing the geometry of the corona and accretion flow.
They complement data from NASA’s IXPE mission, strengthening overall scientific conclusions.
They demonstrate that balloon-borne polarimeters can achieve results comparable to space telescopes.
They set the stage for far more extensive observations during the planned 2027 Antarctic mission.

As scientists analyze this growing dataset, we may be entering a period where long-standing mysteries about how black holes produce jets, how they convert gravitational energy into light, and how matter behaves under extreme conditions finally become clearer.