Multi-wavelength spectroscopic analysis of the ULX Holmberg II X-1 and its nebula suggests the presence of a heavy black hole accreting from a B-type donor

Vol. 690
4. Extragalactic astronomy

Multi-wavelength spectroscopic analysis of the ULX Holmberg II X-1 and its nebula suggests the presence of a heavy black hole accreting from a B-type donor

UltraLuminous X-ray sources (ULXs) are extragalactic X-ray sources brighter than ~5x1039 erg s-1, so that the radiation pressure, in normal conditions, can stop accretion unless the central object has a mass larger than ~40 M⊙. ULXs were discovered in the first X-ray images obtained by the Einstein satellite, back in the 1980s. These sources were indeed interpreted as the result of accretion onto intermediate-mass black holes in binary systems. This framework was revolutionised in 2014 by the discovery of coherent pulsations from one of the ULXs in the star-forming galaxy M82. Since then, the presence of intermediate-mass black holes in ULXs has always been regarded with caution. Beamed emission or a high magnetic field neutron star were often invoked to explain the overwhelming luminosity. In this paper, Reyero Serantes and collaborators carried out a multiwavelength spectral analysis of one of the brightest ULXs, Holmberg II X-1, and its circumstellar nebula. Studying the nebula, which acts as a calorimeter, and the ULX source itself, the authors were not able to find evidence of beaming. All the observational properties were accounted for by a close binary consisting of ≳ 66 M⊙ black hole accreting matter from a ~22 M⊙ B-supergiant companion. Holmberg II X-1 provides one of the first pieces of evidence for the progenitors of gravitational wave black hole binary mergers.