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Summary
Many bright astronomical systems produce their light through accretion. In this process, matter falling toward a gravitating object (a star or a black hole) gives up its gravitational binding energy to electromagnetic radiation. In practice, the matter always has angular momentum, so the infall occurs through a sequence of near-circular orbits of decreasing radius and specific angular momentum, forming an accretion disc. The total radiation output of the disc depends on the rate at which matter spirals inwards and the depth of the effective gravitational potential well around the accreting object. Black holes have the deepest potential wells, and so are the brightest astronomical systems at all mass scales. Supermassive black holes power quasars, and stellar-mass black holes power the most luminous x-ray binary systems. On page 1330 of this issue, Soria et al. (1) discuss observations of a bright stellar-mass binary system, called MQ1, in the galaxy M83. Focusing on the fact that there is an expected limit to the total luminosity of any system held together by gravity, they find that MQ1 may exceed this limit, thereby suggesting a rethink on present accretion models.
