16 January 2012
An international team of researchers including astronomers from the University of Amsterdam (UvA), Radboud University Nijmegen and ASTRON has captured a black hole in the Milky Way in the act of firing super fast ‘bullets’ of gas into space. The observations were made using NASA’s Rossi X-ray Timing Explorer (RXTE) and the VLBA radio telescope and were presented at the 219th meeting of the American Astronomical Society in Austin, Texas.
Hurled out at a quarter of the speed of light, the balls of ionised gas originated in an area just outside the black hole’s event horizon, the point beyond which nothing can escape.
The astronomers were studying H1743-322, a binary star system located 28,000 light years away towards the constellation of Scorpio, where an outburst occurred in mid-2009. The normal star and the black hole making up the binary system revolve around each other in a matter of days, and are so close together that the black hole sucks a constant stream of matter from the star. Flowing towards the black hole, this gas forms a flattened accretion disk millions of kilometres across, making it many times wider than our sun. As the matter swirls inwards it is compressed and heated millions of degrees, until it starts emitting X-rays.
Some of that inward-spiralling matter is ejected back out of the accretion disk in a jet bursting in two opposite directions. Usually the jet contains a constant stream of particles, but occasionally it fires off huge bullets of gas at an enormous speed.
Early June of 2009 marked one such transition in H1743-322, when the changes in the binary system’s X-ray and radio emissions were recorded by the RXTE, VLBA and the Australia Telescope Compact Array (ATCA). Aside from an increase in cyclical X-ray variations, these changes were relatively stable between 28 May and 2 June; but on 4 June ACTA recorded a significant decrease in radio emissions. When RXTE looked again on 5 June, the variations had disappeared. On the same day, radio emissions grew, and the VLBA observed a transparent gas bubble travelling in the direction of the jet. A second gas bullet was observed travelling in the opposite direction the next day.
Astronomers had always assumed that these bullets were ejected at the time of the radio burst. However, VLBI data now reveals that they had already been fired off on 3 June - two days before the spike in radio emissions.
The research offers new leads for understanding how such jets occur and what exactly happens afterwards. Co-author Diego Altamirano (UvA) hopes to discover if the findings are universal and whether scheduled follow-up observations will reveal the same phenomenon at other black holes.
Some super-heavy black holes have much more powerful jets than others, which has sometimes been credited to a difference in black holes’ rotation. ‘However, this single outburst from an X-ray binary system has revealed two types of jets, while nothing in the rotation has changed’, says Sera Markoff (UvA). ‘So we need to look elsewhere for the physics behind this. Objects such as H1743-322 can expand our understanding of this phenomenon for black holes of all sizes.’