Astronomers identified a class of luminous cosmic emitters dubbed hypersoft X-ray sources that release radiation predominantly below 0.3 kiloelectronvolts, according to research published in Nature Astronomy on September 9, 2026.
Researchers Mustafa Muhibullah and Jimmy A. Irwin from the University of Alabama, alongside Rosanne Di Stefano from the Center for Astrophysics | Harvard & Smithsonian, detected these point-like, non-nuclear objects using archival data from the Chandra X-ray Center.
Astronomical surveys previously missed the objects because their emission spectra peak in the extreme ultraviolet, creating a long-standing observational blind spot. Typical X-ray binaries emit primarily above 0.3 keV. In contrast, hypersoft X-ray sources radiate primarily or entirely below that energy boundary.
The brightest detected examples emit near 1038 erg s−1 within this narrow X-ray band. Spectral models indicate that the total bolometric luminosities of the sources reach even higher levels, with the majority of their energy emerging in the extreme ultraviolet band.
The research team proposed that hypersoft sources represent X-ray binaries from several distinct physical categories. These include accreting white dwarfs, post-nova systems that could serve as progenitors of type Ia supernovae, and systems that host accreting black holes. Beyond their elusive nature, the emitters may play a role in ionizing gas within galaxies.
Muhibullah and Irwin developed the study's methodology, while Irwin and Di Stefano contributed equally to the work. The Chandra X-ray Center supported the project under grant AR4-25004X, with additional support from the National Science Foundation under grant AST-2009520. The authors made their analysis code and minimum replication dataset available on Zenodo.
