Washington: Researchers have now captured the exact moment when a black hole in our galaxy triggered gigantic and powerful gas “bullets” during its mid-2009 outburst.
The data comes from observations from NASA’s Rossi X-ray Timing Explorer (RXTE) satellite and the National Science Foundation''s (NSF) Very Long Baseline Array (VLBA) radio telescope.
Racing outward at about one-quarter the speed of light, these “bullets” of ionized gas are thought to arise from a region located just outside the black hole’s event horizon, the point beyond which nothing can escape.
“Like a referee at a sports game, we essentially rewound the footage on the bullets’ progress, pinpointing when they were launched,” said Gregory Sivakoff of the University of Alberta in Canada.
“With the unique capabilities of RXTE and the VLBA, we can associate their ejection with changes that likely signaled the start of the process.”
The research centered on the mid-2009 outburst of a binary system known as H1743–322, located about 28,000 light-years away toward the constellation Scorpius.
Discovered by NASA’s HEAO-1 satellite in 1977, the system is composed of a normal star and a black hole of modest but unknown masses. Their orbit around each other is measured in days, which puts them so close together that the black hole pulls a continuous stream of matter from its stellar companion.
The flowing gas forms a flattened accretion disk millions of miles across, several times wider than our sun, centered on the black hole. As matter swirls inward, it is compressed and heated to tens of millions of degrees, so hot that it emits X-rays.
Some of the infalling matter becomes re-directed out of the accretion disk as dual, oppositely directed jets. Most of the time, the jets consist of a steady flow of particles. Occasionally, though, they morph into more powerful outflows that hurl massive gas blobs at significant fractions of the speed of light.
In early June 2009, H1743–322 underwent this transition as astronomers watched with RXTE, the VLBA, the Very Large Array near Socorro, N.M., and the Australia Telescope Compact Array (ATCA) near Narrabri in New South Wales. The observatories captured changes in the system''s X-ray and radio emissions as the transformation occurred.
From May 28 to June 2, the system’s X-ray and radio emissions were fairly steady, although RXTE data show that cyclic X-ray variations, known as quasi-periodic oscillations or QPOs, gradually increased in frequency over the same period. On June 4, ATCA measurements showed that the radio emission had faded significantly.
The same day, the radio emission increased. An extremely detailed VLBA image revealed a bright, radio-emitting bullet of gas moving outward from the system in the direction of one of the jets. On June 6, a second blob, moving away in the opposite direction, was seen.
Until now, astronomers had associated the onset of the radio outburst with the bullet ejection event. However, based on the VLBA data, the team calculated that the bullets were launched on June 3, about two days before the main radio flare.
The study will be published in the Monthly Notices of the Royal Astronomical Society.
First Published: Wednesday, January 11, 2012, 13:07