The Humming Black Hole
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The Humming Black Hole
Scientists are monitoring a curious black hole binary
star system that has become one of the brightest sources in the
sky at x-ray wavelengths.
May 15, 2000
-- The comic-book character Superman is well known for his x-ray
vision. Last month, if he had looked up at the southern sky for
a bit of x-ray stargazing, the brightest thing to see wouldn't
have been the Sun or the Moon. It would have been an exotic black
hole binary star system known by astronomers as XTE J1550-564.
"In April 2000, J1550-564 was nearly as bright as the Crab
Nebula, which is the brightest hard x-ray source in the entire
sky," says Dr. Mike McCollough of the NASA/Marshall Space
Flight Center. "Since then it's faded to about one-tenth
the x-ray luminosity of the Crab, but that's still very bright."
Right: You can't see it, but it's there. Right now XTE
J1550-564 is one of the brightest objects detectable by x-ray
telescopes. If human eyes were sensitive to x-radiation,
we could see J1550-564 shining brightly in the southern constellation
Norma. This image shows the sky around Norma as it would appear
looking southeast from Sydney Australia near local midnight.
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"That was the brightest eruption we know of," says McCollough, "It flared again in early 1999, but since then J1550 has been quiescent -- until lately. BATSE [the Burst and Transient Source Experiment on the Compton Gamma-ray Observatory] detected an outburst in the hard x-ray band [20-300 kilo electron-volts (keV)] on April 6, 2000, then the Rossi X-ray Timing Explorer confirmed it at lower energies."
Above: In early April 2000 the x-ray emission from XTE J1550-564 skyrocketed until it was nearly as bright as the Crab Nebula in BATSE's 20 - 300 keV x-ray band.
McCollough and colleagues believe that XTE J1550 is a black
hole with an orbiting companion star. Gaseous material from the
star spilling toward the black hole forms a swirling disk of
material that heats up as it falls through the black hole's event
horizon. The disk, called an "accretion
disk", becomes so hot and glows so brightly at x-ray
wavelengths that it's visible to Earth-orbiting x-ray telescopes
from 10,000 or more light years away. 
Left:
An artist's concept of an x-ray binary system. Gas from an evolved
star feeds the accretion disk of an orbiting black hole or neutron
star. The accreting gas heats up and shines brightly as an X-ray
source.
What causes J1550's massive flares?
"It's probably blobs of material from the companion star
cascading down onto the accretion disk," explains McCollough.
When J1550 is "on," as it is now, its unpredictable
x-ray flux oscillates by about 50% every 3 seconds or so. To
astrophysicists, these oscillations are one of the most intriguing
aspects of J1550's enigmatic behavior.
"If you converted the x-ray oscillations from J1550 into
sound waves it would feel like a low, rumbling hum," says
Dr. Stefan Dieters, an astronomer at the NASA/Marshall Space
Flight Center. "It's the sort of sound you feel in your
chest from a very large bass speaker at a rock-and-roll concert.
The dominant frequency component is around 0.3 Hz -- that's too
low for the human ear to hear -- but its spectrum contains frequencies
all the way up to 20 or 30 Hz, which is near the lower limit
of human hearing."
quasi-periodic x-ray oscillations.
 Hey, what happened to the Sun!? If human eyes could see x-rays, the sky
would look very unfamiliar. The usual bright spots -- the Sun
and the Full Moon -- are practically invisible at hard x-ray
and soft gamma-ray wavelengths. (What's a "hard x-ray?"
Click here
to find out.) The Crab
Nebula, the remnant of a supernova explosion in 1054 AD pictured
below, is usually the most brilliant object in the hard x-ray
sky.  It shines so steadily that x-ray astronomers
use a unit called "Crabs" to define how bright other
sources are (the flux from the Crab Nebula is, by definition,
1 Crab). The Sun usually registers less than 0.01 Crabs, although
sometimes a powerful solar flare will make the Sun shine brightly
at hard x-ray wavelengths. Black hole binary systems like XTE
J1550 can also erupt and briefly upstage the Crab Nebula, the
"Old Reliable" of x-ray astronomy. Another black hole,
Cygnus
X-1, is a persistent hard x-ray source that is also sometimes
brighter than the Crab. |
What causes QPOs in these systems?
"It could be that the accretion disk [that gives rise to
the x-ray emission] is simply vibrating," says McCollough.
"Or the QPOs could be a beat frequency between the spin
period of the central object and the orbital period of the disk's
inner edge. We just don't know."
"It's all very speculative," agrees Dieters. "At
the beginning of a flare the dominant QPO frequency is often
low. During the 1998 outburst from J1550, for example, QPOs started
out vibrating at 0.06 Hz (16-17 sec period), then the frequency
increased by a factor of 20 over a 10 day period [click
for a plot].
"There are lots of theoretical models to explain this, but
the basic idea is that some kind of boundary in the accretion
disk is moving in toward the black hole. It might be the inner
boundary of the disk, or perhaps a transition region between
two different parts of the disk. Whatever it is, it starts outside,
where the disk's orbit is bigger and the orbital period is longer.
Then it moves into a tighter, faster orbit that gives rise to
higher-frequency oscillations.
"During the most recent eruption in April, the QPO frequency
started low and stayed low. Why did it work differently this
time?" asked Dieters. "It's a mystery...."
"The
list of black hole binaries with QPOs is getting longer all the
time," he continued. "Right now we know of at least
10 of them. As we look more closely at these objects it seems
like just about every one has oscillations at some level."
Not all of the QPO sources studied by McCollough and Dieters
vibrate at low frequencies. Black hole systems can oscillate
as fast as 250 Hz, while QPOs from neutron star binaries have
frequency components extending as high as 1.25 kilohertz.
Above: An artist's impression of space and time twisting
around a spinning
black hole. Credit: Joe Bergeron of Sky & Telescope magazine.
"When we examine these fast oscillations in black hole
systems, we're really sensing what's going on in the inner accretion
disk, near the point of no return where material flows across
the event horizon," says McCollough. "It strains the
imagination. We're getting close to a region where space and
time as we know it doesn't exist any more."
Stay tuned to Science@NASA for more news and updates about
exotic objects like XTE J1550-564.
Compton Gamma-ray Observatory -the second of NASA's four Great Observatories.
Burst and Transient Source Detector -on the Compton Gamma-ray Observatory
Rossi X-ray Timing Explorer
Learning Center
-The RXTE probes the physics of cosmic X-ray sources by making
sensitive measurements of their variability over time scales
ranging from milliseconds to years.
A
Cool Black Hole Animation
Black Holes -a tutorial about black holes and accretion disks
X-Rays - Another Form of Light - the basics of X-rays from
the Chandra home page at Harvard
Links to
science papers about kilohertz QPOs....
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an email message every time we post a new story!!!| For lesson plans and educational activities related to breaking science news, please visit Thursday's Classroom |
Author: Dr.
Tony Phillips Production Editor: Dr. Tony Phillips Curator: Bryan Walls Media Relations: Steve Roy Responsible NASA official: John M. Horack |