Astrophysics [Image] A Neutron Star Ripping a Blue Super-Giant to Pieces

This gamma-ray image from the Integral satellite shows Cygnus X-1 (the large white blob at center) and Cygnus X-3, an accretion disk encircling a neutron star and the third brightest high-energy emitter in the constellation of Cygnus, the Swan.

The Imager on Board the Integral Satellite (IBIS) captured this image during that test phase and shows not only Cygnus X-1 (center) but also Cygnus X-3 (upper left). Instead of a black hole, Cygnus X-3 is thought to be a neutron star (a tiny dead stellar core) or a quark star pulling its companion star to pieces. Taken on 16 November 2002, the new IBIS observations support this theory.

"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." 

"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." 

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. 

"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." 

Cygnus X-1 is about 10,000 light years from Earth and one of the brightest high-energy emitters in the sky. It was discovered in 1966 and is thought to be a black hole, ripping its companion star, a blue supergiant with a surface temperature of around 31,000 K, to pieces. The blue giant orbits the black hole once every 5.6 days.

Credits: ESA. Original image by the Integral IBIS team. Image processing by ESA/ECF.


Source: The Daily Galaxy - science.nasa.gov