The bright X-ray source known as LMC X-3 resides in the Large Magellanic Cloud, the dwarf galaxy that is the Milky Way’s nearest neighbor. Two decades ago astronomers discovered that the source is actually a binary system with a normal star rapidly orbiting a nearby black hole (whose mass is about 2.3 solar-masses) in only 1.7 days. In X-ray binary systems like this one, material from the normal star falls onto a disk around the black hole, causing it to glow and emit radiation – at X-ray wavelengths from the inner portion of the disk closest to the black hole, and at infrared wavelengths from the outer portions of the disk. The emission typically varies in time, presumably because the infalling matter arrives in clumps or in an uneven stream. The infrared and X-ray emissions also vary from one another, and astronomers have long thought that modeling their behaviors might lead to an enhanced understanding of black hole accretion processes.
CfA astronomers James Steiner and Jeff McClintock, along with a team of five colleagues, analyzed a ten-year collection of optical, infrared and X-ray data on LMC X-3. They discovered from the relative timing of the flares as seen in the two bands that the X-ray emission events lagged the infrared emission by about two weeks, and were able to develop a model that can successfully explain the processes at work. They considered the radiation as coming from three locations: the star itself (normal starlight dominates the emission), the disk (it is heated by accretion and emits in both X-rays and infrared), and other hot material in the disk and/or the star (it is heated by X-rays from the hot inner disk).
The scientists are able to conclude that the infrared probably arises from a narrow annular region of the disk, a somewhat surprising result because it had been thought that infrared would come from a much wider area. They also derive a more precise orbital period for the binary (1.704805 days) and key parameters of the disk. The authors note, however, that their model has about thirty parameters; their proposed scenario is the one that best fits the whole set of data. The new work is an impressive success at understanding a complex and dramatic extragalactic black hole system.
"Modeling the Optical–X-ray Accretion Lag in LMC X-3: Insights into Black-Hole Accretion Physics," James F. Steiner, Jeffrey E. McClintock, Jerome A. Orosz, Michelle M. Buxton, Charles D. Bailyn, Ronald A. Remillard, and Erin Kara, ApJ 783, 101, 2014.