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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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Normally, J1550-564 is almost invisible at x-ray wavelengths, but its intensity varies in a seemingly random pattern of powerful flares. In 1998, for example, it was 1.5 times brighter than the Crab Nebula for several days. [Click to view the 1998-1999 light curve.]

"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."

View a time series and power spectrum of J1550's
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.
Dieters first coined the term "humming black holes" when he was explaining another black hole binary system to his mother, who is not an astrophysicist. The "sound" from one of these black hole systems wouldn't be a pure tone, recounted Dieters, because the spectrum of oscillations contains a whole range of frequencies. Scientists call these Quasi Periodic Oscillations, or QPOs.

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.

Web Links

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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