In the constellation Norma
10 to 15 times the mass of the Sun
Diameter roughly 35-55 miles (60-90 km)
In the quarter-century that orbiting X-ray telescopes had been watching the sky, a distant star system in the southern constellation Norma had never made a peep. But in 1998, that changed. On September 7, the Rossi X-Ray Timing Explore (RXTE) recorded an X-ray outburst that was 50 percent brighter than the Crab Nebula, which is normally the brightest X-ray object in the sky. Two weeks later, it flared even brighter. The system was designated XTE 1550-564 ("XTE" for the RXTE satellite, and the numbers for the system's coordinates in the sky).
From the satellite X-ray observations, combined with optical observations by ground-based telescopes, astronomers concluded that XTE 1550-564 consisted of a black hole and a normal companion star. It produced the intense X-ray flares when blobs of gas from the companion star fell onto the accretion disk of hot gas around the black hole. As the blobs hit the accretion disk the shock wave heated them to hundreds of millions of degrees, so they produced bright outbursts of X-rays.
XTE 1550-564 has produced several similar outbursts since then, although none of them was as powerful as the 1998 events. During an outburst, its X-ray brightness can vary by 50 percent in just a few seconds. The high-speed change indicates that the object is relatively small, which provides further evidence that it consists of a black hole with a surrounding accretion disk.
Two years after its discovery, the system was classified as a "microquasar" when another X-ray telescope, Chandra X-Ray Observatory, discovered that jets of charged particles were racing away from the black hole at about half of the speed of light. The jets form when powerful magnetic fields in the accretion disk shoot out particles before they can be lost in the black hole. Full-sized quasars are powered by supermassive black holes in the hearts of galaxies. Microquasars like XTE 1550-564 are powered by much smaller black holes.
As Chandra watched the system over the next few years, the jets slowed down as they rammed into clouds of gas around the star system.
One study has concluded that a relatively empty cavity surrounds XTE 1550-564. It extends about one light-year on the east side of the system, and about 1.5 light-years on the west. The gas outside this cavity is denser on the west side, so the jet slowed down more quickly as it rammed into the gas. The western jet also glowed brighter in X-rays because the interaction with the denser gas produced a more powerful shockwave.
The black hole in the XTE 1550-564 system is 10-15 times as massive as the Sun, with 10.5 solar masses seen as the most likely weight. The companion star is either a star similar to the Sun or one that has started to puff up as it nears the end of its life. The two stars orbit each other once every 1.54 days, indicating that they are much closer to each other than Mercury, the closest planet, is to the Sun.
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This document was last modified: February 12, 2012.