Astronomers have, for the first time ever, spotted a black hole spitting out super-fast ‘bullets’ from the disk of material torn from a neighbouring star. The results were presented in the 219th meeting of the American Astronomical Society in Austin, Texas.
A lot of black hole regions are known to be surrounded by hot disks of material, called accretion disks, captured from their environments, usually a companion star. Like an extreme plughole, the material circles the black hole, heating up and emitting radiation as it does so, until eventually disappearing into the event horizon. But not all of it. Some is ejected perpendicular to the disk in powerful cosmic jets, or as transient outbursts.
In a cosmic version of the “showdown at the O.K. Corral,” a large, international team of astronomers cast their steely gaze on H1743-322, lying 28,000 light years from Earth. This object, discovered in 1977, has undergone several known outbursts. The “bullets” in question, moving at significant portions of the speed of light, are lobes of hot material ejected from the black hole’s polar regions. A two-hour long such-outburst can put out as much energy as our Sun does in an entire decade.
As Professor Gregory Sivakoff of Alberta University, who presented the findings, says: “Until now, we’ve never been able to determine the exact time when the super-fast ejection occurred. With this new result, we can clearly see how the change in jet behaviour is accompanied by dramatic changes in the X-ray emission coming from around the black hole.”
The astronomers in fact had a “posse” to help them stare down their quarry: a quartet of instruments, quick on the draw. The X-ray emission data came from NASA’s Rossi X-ray Timing Explorer (RXTE) satellite. Radio observations came from NRAO’s Very Long Baseline Array (VLBA), the Very Long Array (VLA), and CSIRO’s Australia Telescope Compact Array. By measuring the motion of the material the astronomers, like town sheriffs, were able to do a bit of detective work and find the exact moment of the outburst. “Pinpointing the ejection time was only possible by simultaneously using RXTE and the VLBA,” says study team leader Dr James Miller-Jones, of ICRAR -Curtin University in Australia.
The exact mechanism for the formation of cosmic jets is still a mystery, which is why the team’s discovery is so important. It could offer clues as to how and why such jets form. But Dr Diego Altamirano of the University of Amsterdam strikes a word of caution: “To really understand how these things work, we need to perform the same type of simultaneous radio and X-ray observations on many more stellar-mass black holes with transient jets.” Even though the team were able to observe an entire cycle of changes in the correlated X-ray and radio emissions for H1743-322, the characteristics for other such-objects could look very different.
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