Waiting for Cygnus X-3

One of the brightest x-ray sources in the Milky Way seems about to erupt in a dazzling flare. By studying the explosion scientists hope to unravel an extragalactic mystery.

February 25, 2000 -- Astronomers are increasingly convinced that supermassive black holes lie at the centers of most large galaxies. It's a classic case of truth being stranger than fiction. Gigantic disks of gas -- called accretion disks -- swirl around central black holes that weigh in at millions or even billions of solar masses. As gas in the accretion disk falls into the hole it heats up and glows so brightly in x-rays that we can detect them a billion light years away. The cores of these systems, called active galactic nuclei (AGNs), outshine all of the stars in the host galaxy by factors of 10 to 1000.

About 10% of all AGNs are stranger still. They produce narrow beams of energetic particles and magnetic fields, and eject them outward in opposite directions away from the disk at nearly the speed of light. When one of these beams is pointed toward Earth, it looks especially bright and astronomers call it a blazar. Among all AGNs, blazars can be detected over the widest range of frequencies, from radio waves to gamma rays.

Many aspects of blazars remain a mystery. What accelerates the material in the jets to relativistic speeds? How are the jets collimated? What are they made of?

The answers to some of these questions about distant galaxies may lie right here in our own Milky Way, in the binary star system Cygnus X-3.

"Cygnus X-3 is a black hole or a neutron star that's accreting matter from an companion star," explains Mike McCollough of the NASA/Marshall Space Flight Center. "Because of the deep gravity well, a huge amount of energy can be released in x-rays and gamma-rays. It's also a very bright radio source that undergoes massive flares from time to time."

During an intense flare in 1997, McCollough and colleagues made a high-resolution radio map of Cygnus X-3 using the Very Long Baseline Array (VLBA), a continent-sized radio interferometer.

"When we looked at the images, lo and behold, there was definitely a one-sided radio jet, about 50 milliarcseconds long," recalled McCollough. "Two days later it extended to 120 milliarcseconds and then it disappeared. This likely makes Cyg X-3 a galactic blazar -- a jet source where we were looking straight down the jet."

Left: An artist's concept of a high-mass x-ray binary system like Cygnus X-3. Gas from a massive 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.

"Cygnus X-3 may be the first example of a blazar here in our own galaxy," he continued. "It's the only case known of a Wolf-Rayet star with a compact companion. Wolf-Rayet stars are massive stars -- 7 to 50 solar masses -- that have blown away their outer envelope of hydrogen. What's left is mostly helium. These types of stars have a very vigorous stellar wind, and that's probably what's driving things in this source."

"We can't see Cygnus X-3 optically because it's in the galactic plane where optical extinction by interstellar dust obscures the source. Fortunately, we can see it at infrared (IR) wavelengths and that's how we know it's a Wolf-Rayet star, from the IR spectral lines. Modulation of the IR and the X-ray emission gives us the orbital period of the binary, only 4.8 hours."

The next opportunity to study Cygnus X-3 during a bright flare may be just around the corner. McCollough and colleagues believe that another eruption is imminent.

"Just before a major flare, the radio and hard X-ray emission from Cygnus X-3 drops very low and stays there for days or weeks." explained McCollough. "It's as if something is building up before the explosion. This lets us predict major flares. On February 18 the radio emission from Cygnus X-3 dropped to very low levels and it's stayed there since. The hard X-ray (20-100 keV) emission which BATSE [on the Compton Gamma Ray Observatory, pictured right] usually detects from this source also vanished in late January. We believe this is the precursor of some major activity."

Right: The Compton Gamma Ray Observatory (CGRO) was the most massive instrument ever launched by a NASA Space Shuttle in 1991. Astronomers using CGRO data continue to make important discoveries, including mysterious gamma-ray bursts that uniquely illuminate the early universe and a whole new class of QSOs. The CGRO will be one of the primary satellites observing Cyg X-3 when that binary system erupts. McCollough also uses the Burst and Transient Source Experiment (BATSE) on CGRO to monitor precursor activity.

When Cygnus X-3 does erupt, McCollough is ready. He has been granted "Target of Opportunity" time to observe Cyg X-3 with the Chandra X-ray Observatory, the Compton Gamma Ray Observatory, and the Rossi X-ray Timing Explorer. When Cygnus X-3 erupts -- any day now, says McCollough -- all of these spaceborne observatories will turn toward the X-ray source and begin collecting critical data at X-ray and gamma-ray wavelengths.

Radio astronomers are also on standby. McCollough and colleagues are currently monitoring Cyg X-3 using the Green Bank interferometer in West Virginia, the Ryle telescope in Britain, the RATAN 600 radio telescope in Russia, and the Very Large Array in New Mexico. All of these instruments will spring into action when the flare begins. McCollough and his collaborators have been granted observing time as well on the Very Long Baseline Array, which will monitor Cyg X-3 for three days after the flare to make detailed radio images of the jet.

Left: One of the VLBA antennas at Caltech's Owens Valley Radio Observatory. Others are located at sites ranging from Hancock, New Hampshire to Mauna Kea, Hawaii. Together these antennas combine to form a powerful radio interferometer than can make detailed maps of celestial objects like Cygnus X-3.

"We expect to learn a lot," says McCollough. "If there really is a relativistic jet in Cyg X-3 we might get a glimpse of how it works. Some models predict matter-antimatter production in the jet. The Compton Gamma-Ray Observatory will be able to detect the spectral line at 511 keV that results from electrons and positrons annihilating one another. Jets like these might also entrain matter from the accretion disk or the stellar wind. If that happens we might be able to see that material by means of spectral line emission at x-ray energies. What we proposed to do with Chandra -- and this has just been approved -- is to use one of the high resolution spectrometers to look for spectral lines from entrained gas. If we see anything, the data will provide redshifts and composition. We'll actually measure the speed of the jet and what it's made of!"

We will also look for GeV emission (high energy gamma-rays) with the Compton Gamma Ray Observatory," concluded McCollough. "Since extragalactic blazars are known produce high-energy gamma rays, so might a galactic one."

Stay tuned to Science@NASA as the explosive story of Cygnus X-3 unfolds, with reports about the impending flare and updates about what scientists learn from their observations.

Related Links

Chandra home page -from Harvard

Chandra News -from NASA

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 -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.

The Very Long Baseline Array -A continent-sized radio telescope, operated by the National Radio Astronomy Observatory.

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

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