BLACK HOLE
ATLANTA - The dramatic tantrum last fall from an often-overlooked
star has betrayed the existence of the nearest black hole yet
discovered in the Milky Way -- one that should be put in a class
all its own, a team of astronomers announced Friday.
The black hole, which is associated with a visible star called V
4641, is being called a micro-quasar because it exhibited for a few
days in September the brilliant behavior associated with quasars.
It sent out tremendous bursts of X-ray radiation and shot out jets
of plasma at some 90 percent the speed of light, said Robert
Hjellming, of the National Radio Astronomy Observatory.
Hjellming and colleagues from the Massachusetts Institute of
Technology made the announcement here at the meeting of the
American Astronomical Society.
Quasars are thought to be black holes billions of times more
massive than the sun that lie at the heart of active galactic
nuclei. They are extremely energetic X-ray emitters that shoot out
tremendous fountains of plasma at velocities approaching the speed
of light. These jets stretch for thousands of light-years, puzzling
astrophysicists who must struggle to answer how black holes provide
the oomph and direction for such powerful columns. (A light-year is
5.88 trillion miles.)
Although V 4641 is billions of times smaller than any quasar --
weighing somewhere between three and 10 solar masses -- astronomers
who were looking at the object recognized the behavior. Only three
other black holes have earned the micro-quasar distinction, but
this is the closest one ever seen. It lies just 1,600 light-years
from Earth on the way to the center of the Milky Way in the
direction of the constellation Sagittarius.
The object drew tremendous attention from astronomers last
September after it erupted in a giant X-ray burst, dimmed, and then
fired to life again the following day. (Click here to read
space.com's coverage of the burst.)
Within several hours of its initial detection, X-ray-observing
satellites and radio telescopes around the world were targeting the
object. Most were just in time to catch a few hiccups of activity
and watch as the emissions faded out of sight.
Using the Very Large Array, a collection of 27 radio antennas in
the New Mexico desert, Hjellming obtained a set of radio images
that revealed the objects near-light-speed jets. (To read more
about that discovery and what it tells physicists about black
holes, see space.com's October 25 article.)
Now, Hjellming has released a set of images that show just how fast
the objects jets faded.
http://www.space.com/scienceastronomy/astronomy/v4641_microquasar_000114.html