Theoretically, supermassive black holes that occupy the centre of galaxies (including our own) are surrounded by a vast cloud of gas. Depending on the angle you are viewing this molecular torus will obscure the supermassive black hole’s bright accretion disk. Until now, this vast doughnut of matter has never been observed, but with the help of the supermassive black hole accretion disk and a dying star, there’s a possibility that the molecular torus will not only be observed, but also mapped…
On writing “Light Echo from X-Ray Flare Reveals Existence of a Molecular Torus Surrounding a Supermassive Black Hole“, I couldn’t help but be amazed that researchers had found a way to look deep into the heart of a galactic nucleus by using the echoed light from a large X-ray flare as the material from a star is ripped away and fed to the supermassive black hole.
In 2007, a very strong X-ray flare was observed by the Sloan Digital Sky Survey in a galaxy named SDSSJ0952+2143. Researchers at the Max Planck Institute very quickly got to work, taking spectroscopic measurements of the burst. The team, led by Stefanie Komossa, made a series of discoveries, perhaps the most critical being the opportunity to map the structure of the galactic nucleus. Komossa likened the X-ray burst to a firework lighting up a dark city, before which little is known about the buildings and streets, but for an instant all the structures are highlighted. An X-ray flare from the galactic core is just like this firework, its light reflects off surrounding structures, bringing them for the first time, into view. But like a firework, the X-ray flare is very quick, often leaving astronomers looking at the afterglow. But this is the strength of the Sloan survey; it is able to observe a quarter of the night sky, instantly reporting any flares to astronomers so instruments can be directed to the flash straight away.
It would seem the Max Planck group were quick off the mark. First, by observing the spectroscopic fingerprint of the echoed light, so much information about the origins of the X-ray flare can be understood. An unusually strong hydrogen line was seen in the data, leading the researchers to conclude the X-ray emission was coming from the matter of a star falling into the black hole’s accretion disk, blasting out huge amounts of energy as it does so. It seems almost definite that young star got too close to the tidal influence of the supermassive black hole, dragging its hydrogen fuel into the accretion disk.
Even more significant was the observation of a very strong iron line in the spectroscopic data. This line is due to the X-ray emission bouncing off the molecular torus surrounding the galactic core. Suddenly the researchers had a way to find observational evidence of a molecular torus, and with the help of the Chandra X-ray observatory, it is hoped the torus structure will be mapped.
In addition to the X-ray echoes, physicists believe there will be an infrared emission from the molecular torus too. As the high energy X-rays hit the cloud, strong heating will have occurred, vaporizing quantities of gas. In a “last cry for help” the gas will have radiated in the infrared. If infrared observations back up the X-ray evidence, this first observation of a molecular torus will be strengthened.