Supermasssive black holes - giants with masses of millions or billions of suns - are relatively famous because they reside at the nuclei of galaxies like quasars where they are responsible for some of the most dramatic phenomena in the cosmos. Their environments can, for example, produce narrow jets of particles that travel across thousands of light-years of space at nearly the speed of light (relativistic speeds). Black holes come in all sizes, however, and small ones (about ten times
as massive as the sun) can also reproduce some of these fantastic phenomena. Stellar-mass black holes are found in all around our galaxy, and are thus much more convenient objects of study than the supermassive black holes in distant galactic nuclei. By studying these objects and their extreme environments, astronomers can learn about the physical processes around black holes like the mechanisms that trigger or suppress the formation of jets, processes that so far have remained mysterious.
CfA astronomers Joseph Neilsen and Julia Lee, writing in last month's Nature, report on their Chandra X-ray Observatory study of just such a local object: a fourteen solar-mass black hole in our galaxy called GRS 1915+105. This source is one of the brightest X-ray sources in the sky and was the first object in the galaxy found to have relativistic jets. This black hole has a small star orbiting it every 33.5 days, and material from the star's atmosphere falls onto an accretion disk around the black hole, providing the basis for the X-ray emission and outflowing jets.
The astronomers note that when the jets are prominent, the object is faint and emits predominantly high-energy X-rays; when the jets are faint, the object is bright and emits mostly lower-energy X-rays. By closely analyzing archival datasets spanning seven years, they hypothesize that during the high-energy phases, the powerful jets illuminate the accretion disk, causing it to glow. Furthermore, they argue that after a while a hot wind develops from the disk, and this wind can carry away enough matter to suppress the jets and the high-energy X-rays as the black hole enters its other, brighter state. The results provide significant new insights into the mechanisms at work around small black holes, and offer possible new explanations for the behaviors associated with the massive black holes that power quasars.