The Milky Way’s Supermassive Black Hole
The Nuclear Spectroscopic Telescope Array or NuSTAR satellite was launched on 13 June 2012 on a journey to discover the mysteries of the high-energy universe at the centre of our Milky Way. The crown jewel of our galaxy is a black hole packing a mass of more than 4 million Suns, called Sagittarius A, which is not directly visible. This region of some 230 light-years on either side, contains thousands of objects of dense cores of stars, filaments of hot magnetic gas, clouds of cold gas and dust, and the scattered remains of dead massive stars.
This black hole lets out frequent blasts of energy, the first flares were detected in 1999, averaging twice a day in infrared and once per day in low-energy ‘soft’ X-rays. In this region lie many binary systems, each containing a neutron star and a lower-mass companion Sun. As the companion dumps material onto the neutron star, that material heats up and emits X-rays, complicating the measurements of flares monitoring. Some of these neutron stars are magnetars. They are the most magnetic objects in the universe, hundreds to thousands of times stronger than normal neutron stars, which are already a trillion times that of Earth. Astronomer have found 28 of these magnetars in the vicinity of Sagittarius A. The detection of the magnetar SGR JI745-2900 opened up the possibility of future testing of the general theory of relativity and measuring the black hole’s mass precisely.
However, it was the unexpected discovery of a haze of soft X-rays in the Milky Way’s central region that excited astronomers. The cause of this haze remains a mystery. The theoretical possibility of so many massive stars, about 30 times our Sun’s mass, all congregating near the galactic centre and how did they all get there is intriguing. Presently, NuSTAR is gathering information on all the individual stars that lie just about one-quarter degree, about 115 light-years, north of the galactic centre, and comparing their spectral properties.