No question: supermassive black holes get a lot of the glory, thanks to their obvious presence at the centers of many galaxies. However, stars more than 20 times the mass of our Sun leave behind smaller, stellar-mass black holes after their violent supernova deaths. Despite this model’s wide acceptance, astronomers have only identified about 50 stellar-mass black holes in the Milky Way to date, but there must be many more lurking. A new study may have revealed why: the black holes are shrouded by a thick donut of gas that blocks much of their X-ray light from reaching us.

J. M. Corral-Santana and colleagues based this hypothesis on a detailed study of a relatively faint, fluctuating X-ray source in the Milky Way. Their observations in X-ray and visible light revealed the signs of a binary system: an ordinary star in orbit around a black hole, similar to other systems, but with some key differences. For one, the star and black hole were so close together that the orbital period of the system was only 2.8 hours. For another, the matter being drawn off the star was obscuring the black hole when viewed from Earth. The authors hypothesized that many other black holes may be similarly hidden, and future searches should take that possibility into account. [Read more…]