![]() As black holes increase in mass, the congestion zone is pushed farther out, so the QPO clock ticks slower and slower. In 1998, Titarchuk realized that the congestion zone lies close in for small black holes, so the QPO clock ticks quickly. This signal is called a quasi-periodic oscillation, or QPO.Īstronomers have long suspected that a QPO’s frequency depends on the black hole’s mass. The X-ray intensity varies in a pattern that repeats itself over a nearly regular interval. When the feeding frenzy reaches a moderate rate, hot gas piles up near the black hole and radiates a torrent of X-rays. It uses a relationship between black holes and the inner part of their surrounding disks, where gas spirals inward before making the fatal plunge. The method used by Shaposhnikov and Titarchuk has been described in several papers in the Astrophysical Journal. The traffic jam is closer in for smaller black holes, so X-rays are emitted on a shorter timescale. In this top-down illustration of a black hole and its surrounding disk, gas spiraling toward the black hole piles up just outside it, creating a traffic jam. But the black hole’s mass had never been measured to high precision. Astronomers realized soon after J1650’s discovery that it harbors a normal star and a relatively lightweight black hole. NASA’s Rossi X-ray Timing Explorer (RXTE) satellite discovered the system in 2001. The tiny black hole resides in a Milky Way Galaxy binary system known as XTE J1650-500, named for its sky coordinates in the southern constellation Ara. They will describe their results in more detail in a media telecon on April 1 at 1:30 p.m. Titarchuk also works at George Mason University in Fairfax, Va., and the US Naval Research Laboratory in Washington, DC. Shaposhnikov and his Goddard colleague Lev Titarchuk are presenting their results on Monday, March 31, at the American Astronomical Society High-Energy Astrophysics Division meeting in Los Angeles, Calif. For many years astronomers have wanted to know the smallest possible size of a black hole, and this little guy is a big step toward answering that question," says lead author Nikolai Shaposhnikov of NASA’s Goddard Space Flight Center in Greenbelt, Md. "This black hole is really pushing the limits. With a mass only about 3.8 times greater than our Sun and a diameter of only 15 miles, the black hole lies very close to the minimum size predicted for black holes that originate from dying stars. Using a new technique, two NASA scientists have identified the lightest known black hole. The black hole has about 3.8 times the mass of our sun, and is orbited by a companion star, as depicted in this illustration. The lowest-mass known black hole belongs to a binary system named XTE J1650-500.
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