Astronomers using NASA’s Imaging X-ray Polarimetry Explorer (IXPE) have made a discovery that could change how we think about black holes. The team studied a system called IGR J17091-3624, about 28,000 light-years away, and found that the X-rays coming from it were much more polarized than expected.
Polarization is a property of light that shows how its electric field is aligned, and measuring it helps scientists figure out what is happening around a black hole. In March 2025, IXPE detected X-ray polarization from this system for the first time. The measurement was taken in the 2–8 keV energy range and showed a polarization degree of 9.1 percent with strong statistical confidence, along with a polarization angle of about 83 degrees. There was also a small hint that polarization increases with energy, though this was not statistically significant.
The black hole was observed in what scientists call a corona-dominated hard state, meaning the X-rays were mainly coming from the corona, a region of extremely hot plasma around the black hole. The spectrum showed a hard power-law shape with weak reflection features, and a Type-C quasi-periodic oscillation at about 0.2 Hz was detected.
“The black hole IGR J17091-3624 is an extraordinary source which dims and brightens with the likeness of a heartbeat, and NASA’s IXPE allowed us to measure this unique source in a brand-new way,” said Melissa Ewing of Newcastle University. Black holes in systems like this pull matter from a nearby star, which forms a rotating disc. The corona sits in the inner part of this disc and can reach temperatures of up to 1.8 billion degrees Fahrenheit, producing very bright X-rays. Even though it is so luminous, the corona is too small and far away to be imaged directly.
“Typically, a high polarization degree corresponds with a very edge-on view of the corona. The corona would have to be perfectly shaped and viewed at just the right angle to achieve such a measurement,” said Giorgio Matt of the University of Roma Tre. He added, “The dimming pattern has yet to be explained by scientists and could hold the keys to understanding this category of black holes.” The orientation of the system’s radio jet is not known, so scientists cannot compare it directly with the X-ray polarization angle. If the geometry is similar to other black hole systems previously studied, the two directions would likely be parallel.
To explain the high polarization, researchers tested different ideas. One possibility is that a wind of matter is being lifted from the disc and blown away. If X-rays from the corona scatter off this wind, it could produce the observed polarization. Another idea is that plasma in the corona is streaming outward at speeds up to 20 percent of the speed of light, which would create relativistic effects that boost polarization. Both scenarios fit with the Comptonization process, where geometry, viewing angle, and scattering in thick winds all play a role.
“These winds are one of the most critical missing pieces to understand the growth of all types of black holes,” said Maxime Parra of Ehime University. Future IXPE observations may reveal even more surprising results. For now, the unusually high polarization degree in IGR J17091-3624 gives astronomers new clues about how black holes behave and how their coronas generate X-rays.
Source: NASA, Oxford University Press | Image via Depositphotos
This article was generated with some help from AI and reviewed by an editor. Under Section 107 of the Copyright Act 1976, this material is used for the purpose of news reporting. Fair use is a use permitted by copyright statute that might otherwise be infringing