Hypersoft X-ray Sources: A New Class of Luminous Cosmic Emitters

Abstract

X-ray binaries, powered by black holes, neutron stars, or white dwarfs accreting matter from a companion star, are among the brightest beacons in galaxies, outshining the Sun by a factor of millions. Most emit primarily above 0.3 keV in X-rays, but cooler thermal sources peaking in the extreme ultraviolet (EUV) would be much more difficult to detect due to astronomy's critical blind spot in EUV. Here, we report the discovery of a remarkable new class of luminous, point-like, non-nuclear X-ray objects in galaxies-hypersoft X-ray sources -- that have been missed by all previous surveys to date. Detected primarily or exclusively below 0.3 keV, with 0.15-0.3 keV to 0.3-1.0 keV photon ratios >8, the most luminous examples radiate >1E38 erg/s in the narrow X-ray band, with spectral models indicating even greater bolometric luminosities, largely emitted in the EUV. They rank among the most energetic sources in galaxies, yet their EUV-peaking spectra evaded earlier detections. We propose that hypersoft sources are X-ray binaries spanning multiple physical classes, including accreting white dwarfs or post-nova systems-potential Type Ia supernova progenitors-and systems hosting accreting black holes. Beyond their elusive nature, they may play crucial role in ionizing gas within galaxies.