"Old Faithful" Black Hole Provides Insight into Jet Formation
Scientists observing a disk of matter surrounding a black
hole in our Galaxy have discovered that the disk is periodically
disrupted and hurled outward in opposite directions from the black
hole, in jets moving at nearly the speed of light. The black hole
replenishes the disk by pulling hot gas from the surface of a
nearby "companion" star, and then undergoes another disruption,
repeating the sequence at half-hour intervals.
Researchers from teams at the California Institute
of Technology, Pasadena, CA, the Massachusetts Institute of
Technology (MIT), Boston, and NASA's Goddard Space Flight Center,
Greenbelt, MD, all worked to correlate the disappearance of
X-ray emitting hot gas in the disk with the appearance, shortly
thereafter, of rapidly expanding jets. Dr. Ronald Remillard of
MIT and Dr. Jean Swank of Goddard are discovered the X-ray behavior
using data from NASA's Rossi X-ray Timing Explorer (RXTE); Dr. Stephen
Eikenberry of Caltech used infrared observations made at Mt. Palomar, CA to
demonstrate that when the X-rays from the disk vanish, the jets suddenly
appear.
The X-rays from the disk disappear almost completely every half hour.
About five minutes later, they return again. Very shortly
after the X-rays vanish, jets appear at infrared and radio
wavelengths. Since X-ray emitting hot gas in the disk disappears
during these episodes, it is believed that the matter in the
disk must have escaped, probably flung out in the jets. As the
black hole pulls more gas from its companion star, the disk
appears again (along with the X-rays) and the cycle starts over.
"The system behaves like the celestial version of Old
Faithful," notes Dr. Craig Markwardt, a researcher working with
Swank at Goddard. "At fairly regular intervals, the accretion
disk is disrupted and a fast moving jet is produced." Swank added, "However,
this jet is staggeringly more powerful than a geyser. Every half-hour, the
black hole throws off the mass equal to that of a 100 trillion ton asteroid at
nearly the speed of light (approximately 650 million miles per hour). This
process clearly requires a lot of energy -- each cycle is equivalent to six
trillion times the annual energy consumption of the entire United States."
Black holes are very massive objects with gravitational
fields so intense that near them, nothing (not even light), can
escape their pull. While this prevents anyone from observing
black holes directly, their presence can be inferred from effects
on nearby matter. Many of the known or suspected black holes are
orbiting a close "companion" star. The black hole's gravity pulls
gas from the companion star into a swirling disk of material which
orbits around the black hole, much as soap suds swirl around a
bathtub drain. As it falls into the black hole, the gas in the
disk is compressed and heated to millions of degrees, emitting
X-rays.
A disk of hot gas, known as an accretion disk, is
commonly observed around a black hole with an orbiting stellar
companion. However, the near simultaneous disappearance of the disk
and formation of the jet has never been seen before. It promises
to shed light on the origin of the enigmatic jets, also commonly
observed near accreting black holes, but poorly understood.
"What is even more amazing is that we are seeing the first
clues to the source of matter ejected in the jets -- the
correlations we discovered indicate that the jet material must
come from the inner disk. For years theorists have hypothesized
that the jets come from somewhere close to the black hole, but no
one had ever actually seen that direct link until now" said
Eikenberry.
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