Tuesday, July 1, 2014

Streamer from a Supermassive Black Hole

A supermassive black hole with 65 million times the Sun’s mass sits in the heart of NGC 5548, a galaxy located approximately 245 million light years away. The centre of NGC 5548 is known to be particularly luminous due to the prodigious amounts of energy generated from the accretion of matter by the supermassive black hole. Much of that energy is in the form of X-rays as well as some ultraviolet radiation. An observing campaign running from May 2013 to February 2014 revealed something obscuring 90 percent of the X-rays emitted by the supermassive black hole when compared with past observations.

To piece together the puzzle of why this galaxy’s core went dark, the observing campaign utilized six space observatories - XMM-Newton, Hubble Space Telescope (HST), Swift, Nuclear Spectroscopic Telescope Array (NuSTAR), International Gamma-Ray Astrophysics Laboratory (INTEGRAL) and Chandra X-ray Observatory. In a paper published in the June 19 issue of the journal Science, the researchers who made the observations show that the X-ray obscuration is caused by the presence of a fast-moving stream of gas flowing outward from the galaxy’s supermassive black hole. The stream of gas is blocking 90 percent of the X-rays emitted by the accretion of matter around the supermassive black hole.

Figure 1: A digital rendition of the gas stream obscuring the supermassive black hole at the centre of NGC 5548. The green line denotes the Hubble Space Telescope’s line of sight. Image credit: Person Renaud & Pierre-Olivier Petrucci.

Figure 2: An artist’s impression of a gas stream flowing outward from the supermassive black hole at the heart of NGC 5548. The arrow indicates the Hubble Space Telescope’s line of sight. Image credit: NASA / ESA / A. Field, STScI.

Supermassive black holes in the hearts of massive galaxies eject huge amounts of matter in the form of powerful winds of ionized gas. NGC 5548 is known to have a persistent wind of ionized gas flowing away from the supermassive black hole at speeds of ~1000 km/s. Unlike the persistent wind, the newly discovered stream of gas travels up to five times faster, at speeds of ~5000 km/s. The gas stream is believed to have originated much closer to the supermassive black hole than the persistent wind, likely gas from the accretion disk that is swirling closely around the supermassive black hole.

This discovery is the first direct evidence for the long-predicted X-rays shielding process that can accelerate winds of ionized gas to very high speeds. Such high wind speeds can only exist if they originate from an area that is shielded from X-rays. The obscuration of X-rays by the gas stream has lasted for at least 2.5 years, possibly as long as 6 years. The paper concludes: “Although the outflow in NGC 5548 is not strong enough to influence its host galaxy, it gives us unique insight into how the same mechanism may be at work in much more powerful quasars.”

J. S. Kaastra et al., “A fast and long-lived outflow from the supermassive black hole in NGC 5548”, Science, published online June 19, 2014; doi: 10.1126/science.1253787