Swift X-ray observations of galaxy NGC 5408 indicate its ultraluminous X-ray source undergoes periodic changes every 115.5 days. This cycle, astronomers suspect, is linked to the orbit of a donor star around a middleweight black hole, as shown in this artist’s view.
Ultra-luminous X-ray sources, are sources that are less luminous than Active Galactic Nuclei but more luminous than any known stellar process. Their origin remains unknown.
Since their discovery in the 1980s, by the Einstein Observatory, all the major X-ray telescopes, e.g. ROSAT, XMM-Newton, Chandra, have provided us with a wealth of data, increasing our knowledge for these enigmatic sources. Studies have shown that most of Ultra-luminous X-ray sources are associated with the star-forming regions of the galaxies and are surrounded with nebulae.
Recently, a team of astronomers from Russia and Japan has compared the optical spectra of Ultra-luminous X-ray sources to SS 433. SS 433 is a highly exotic eclipsing binary star system in the Milky Way, whose primary object is likely a black hole and its secondary companion is probably a late A-type star. SS 433 is the only known supercritical accretor in our galaxy. Supercritical accretion discs are those with accretion rates above the Eddington limit, i.e. the maximum possible luminosity a body can achieve when there is balance between the force of radiation acting outward and the gravitational force acting inward. The study revealed that all spectra of Ultra-luminous X-ray sources are surprisingly similar to one another and are also similar to that of SS 433. Scientists concluded that SS 433 is intrinsically the same as Ultra-luminous X-ray sources and that Ultra-luminous X-ray sources must constitute a homogeneous class of objects, which most likely have supercritical accretion discs. Further study of these sources will help us understand the origin of supermassive black holes that live in most of the galaxies.