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Modeling the Radiation of Black Holes

Modeling the Radiation of Black Holes

An X-ray image of two ultraluminous X-ray sources (ULXs) in the Andromeda galaxy. New calculations confirm that ULXs are usually stellar-mass black holes rapidly accreting material and emitting radiation in a narrow beam.

NASA/Swift/Stefan Immler

Ultraluminous X-ray sources (ULXs) are extremely luminous, compact X-ray sources found in some nearby spiral galaxies. The nature of these mysterious sources is not well understood, but they are thought to be black holes of about ten solar-masses accreting material, and are distinct both in size and character from the supermassive black holes in the nuclei of galaxies that also emit bright X-rays. The class of ULXs appears to contain several physical variants: one subsample shows coherent pulsations and is thought to be composed of neutron stars rather than black holes, while another set might be more massive than a star. Even a single type might change its emission character with time between the several morphological classes identified.

Astronomers trying to model ULXs face several challenges. The strong gravitational field means that the calculations must be done in a full general relativity context, and moreover the observations indicate that the emission is not spherical but instead is typically highly beamed. Not least, powerful magnetic fields are expected to be present and must in included in the simulations. CfA astronomer Ramesh Narayan and two colleagues used their new computer codes to calculate the emission properties and spatial appearances of ULXs, taking into account the complexities of relativity, magnetic fields, beaming, and varying accretion rates. Their results are in good agreement with previous, less sophisticated calculations. They conclude that observed large luminosities are in part caused by focusing of the emission by the geometry of the system. This raises the question, to be pursued in further research, of whether the observed population of ULXs is only the tip of the iceberg with many more ULXs oriented away from our line-of-sight, and if so, where this large population comes from.

"Spectra of Black Hole Accretion Models of Ultra-Luminous X-ray Sources," Ramesh Narayan, Aleksander Sadowski, Roberto Soria, MNRAS 2017 (in press).