Research Highlights

doi:10.1038/nindia.2014.138 Published online 20 October 2014

Ultraluminous X-ray source discovered in nearby galaxy

By analysing observational data, astrophysicists have discovered a system of ultraluminous X-ray binary stars in NGC 3384, a spiral galaxy about 37 million light years from Earth1. This binary system harbours a black hole and emits powerful X-rays. The discovery of this binary system may advance our understanding of similar binary systems containing black holes.

X-ray binary stars are systems consisting of a normal star and a compact object such as a black hole or a neutron star. In such systems, the compact object continuously sucks in matter from a companion star, a process known as accretion. The interaction between the accreted matter and the huge gravitational field of the compact object heats the accreting matter, causing the compact object to radiate X-rays. This enhances the luminosity of the binary star systems.

Ultraluminous binary systems are thought to contain intermediate mass black holes that are potential candidates for a gap in black hole masses lying between stellar-mass black holes and super-massive black holes in the centre of galaxies. In the hope of identifying such luminous binary systems with intermediate black holes, the researchers analysed X-ray images captured by NASA’s Chandra X-ray Observatory, a powerful X-ray telescope.

During an observation that lasted for 28.67 kiloseconds, Chandra detected eight point sources of X-ray binary candidates. From these binary point sources, the researchers singled out one that was ultraluminous — ULX (X-8). They found that the luminosity resulting from the powerful X-ray emission of this point source is greater than that of one million suns.

In addition, they claim that the ultraluminous binary system ULX harbours a 30-solar-mass black hole. “It is an intermediate mass black hole, which is accreting matter from a companion star,” says K. Yugindro Singh, a senior author of the study.

The authors of this work are from: Assam University, Silchar and Manipur University, Imphal, India.


References

1. Devi, A. S. et al. Discovery of an ultra-luminous X-ray source candidate, ULX (X-8) in NGC 3384 with Chandra. Astrophys. Space. Sci. (2014) doi: 10.1007/s10509-014-2091-8