Washington, April 2: Two NASA scientists have identified the smallest known black hole, which has a mass of only about 3.8 times greater than our Sun and a diameter of only about 15 miles. This particular size lies very close to the minimum size predicted for black holes that originate from dying stars. According to Nikolai Shaposhnikov of NASA's Goddard Space Flight Center in Greenbelt, Maryland, "This black hole is really pushing the limits. 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."
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. NASA's Rossi X-ray Timing Explorer (RXTE) satellite discovered the system in 2001. Astronomers realized soon after J1650's discovery that it harbors a normal star and a relatively lightweight black hole. But the black hole's mass had never been measured to high precision.
Shaposhnikov and his Goddard colleague Lev Titarchuk 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.
When Shaposhnikov and Titarchuk applied their method to XTE J1650-500, they calculated a mass of 3.8 Suns, with a margin of uncertainty of only half a Sun. This value is well below the previous black hole record holder with a reliable mass measurement, GRO 1655-40, which tips the scales at about 6.3 Suns.