With a new technique, two NASA scientists have determined the lightest known black hole. With a mass only about 3.8 times larger than our sun and a diameter of only 15 miles, the black hole is very close to the minimum size predicted that black holes come from dying stars."This black hole is truly at the borders. For many years, the astronomers wanted to know the smallest possible size of a black hole, and this little guy is a big step toward answering this question," says Nikolai Shaposhnikov lead author of NASA's Goddard Space Flight Center in Greenbelt, Md.
Goddard Shaposhnikov and Lev Titarchuk his colleagues presented their results at the American Astronomical Society's High Energy Astrophysics Division meeting in Los Angeles, Calif. Titarchuk also works at George Mason University in Fairfax, Va., and the U.S. Naval Research Laboratory in Washington, DC .
The tiny black hole is in a binary system, Milky Way known as XTE J1650-500, named after its celestial coordinates in the southern constellation Ara. NASA's Rossi X-ray Timing Explorer (RXTE) satellite detected the scheme in 2001. Astronomers realized J1650 soon after the discovery that it holds a normal star and a relatively small black hole. But the black hole of mass had never been measured at high precision.
The method used by Shaposhnikov and Titarchuk has been described in several papers in the Astrophysical Journal. It uses a relationship between black holes and the inner part of their surrounding disks, where gas spirals inward before making the fatal plunge. When the feeding frenzy reaches a moderate rate, hot gas piles up near the black hole and radiates a torrent of X-rays. The X-ray intensity varies in a pattern that repeats itself over a nearly regular interval. This signal is called a quasi-periodic oscillation, or QPO.Astronomers have long suspected that a QPO's frequency depends on the black hole's mass. In 1998, Titarchuk realized that the congestion zone lies close in for small black holes, so the QPO clock ticks quickly. As black holes increase in mass, the congestion zone is pushed farther out, so the QPO clock ticks slower and slower. To measure the black hole masses, Shaposhnikov and Titarchuk use archival data from RXTE, which has made exquisitely precise measurements of QPO frequencies in at least 15 black holes.
Last year, Shaposhnikov and Titarchuk applied their QPO method to three black holes whose masses had been measured by other techniques. In their new paper, they extend their result to seven other black holes, three of which have well-determined masses. "In every case, our measurement agrees with the other methods," says Titarchuk. "We know our technique works because it has passed every test with flying colors."
If Shaposhnikov and Titarchuk application of their method on XTE J1650-500, they calculated a mass of 3.8 Suns, with a margin of uncertainty of only half a sun. This figure is well below the previous record holder black hole with a reliable mass measurement, Gro 1655-40, tips the scales at about 6.3 Suns.Here are some unknown critical threshold, a dying star, a neutron star instead of a black hole. Astronomers think the boundary between black holes and neutron stars is somewhere from 1.7 to 2.7 solar masses. Knowledge of this dividing line is important for basic research in physics, because they say scientists about the behavior of matter if it is scrunched conditions for the extremely high density.
Despite the small size of the new record holder, future space transportation systems, travelers had better watch out. Smaller black holes as in J1650 exercise tidal forces stronger than the much larger black holes in the centers of galaxies, the young guys more dangerous to approach. "If you ventured too close to J1650-black hole, whose gravity would tidally stretch your body into a strand of spaghetti," says Shaposhnikov.
Shaposhnikov adds that RXTE is the only instrument that high-precision timing observations made for this line of research. "RXTE is absolutely crucial for this black hole mass measurements," he says.
