LOS ANGELES, March 22 (Xinhuanet) -- US scientists said on Tuesdaythat the Chandra X-ray Observatory of National Aeronautics and Space Administration (NASA) found evidence of a new class black hole with a mass of about 10,000 Suns.

    In a paper published on the latest issue of Astrophysical Journal Letters, the researchers said the timing and regularity ofthese outbursts make the object one of the best candidates yet fora so-called intermediate-mass black hole.

    Earlier studies have found evidence for the stellar black holesthat are about 10 times as massive as the Sun, while supermassive black holes with masses as large as billions of Suns were also detected. But the latest evidence has suggested that intermediate-mass black holes may exist between these extremes.

    This new finding is important because intermediate-mass black holes would bridge the gap between stellar-mass black holes and supermassive black holes in the centers of galaxies, according to Jifeng Liu, lead author of the paper at the University of Michiganin Ann Arbor.

    The new observations are strong evidence in favor of the existence of intermediate-mass black holes, he noted.

    The researchers used Chandra to observe a black hole in the galaxy Messier 74 (M74), which is about 32 million light years from the Earth, and they found that this source exhibits strong, nearly periodic variations in its X-ray brightness every two hours,providing an important clue to the black holes' mass.

    The black hole also fell into a class of sources called ultraluminous X-ray sources (ULXs), because they radiate 10 to 1,000 times more X-ray power than neutron stars and stellar mass black holes.

    Some astronomers believe these mysterious ULXs are more powerful because they are intermediate mass black holes. Others think the ULXs are regular stellar-mass black holes that appear tobe much more powerful in X-rays because their radiation is beamed in a jet toward the Earth.

    Chandra's discovery of the persistence and long time period of the X-ray variations (quasi-periodic oscillations) of the M74 is an argument against a beamed jet. These variations are likely produced by changes in a disk of hot gas around the black hole. More massive black holes have larger disks, which in turn are expected to vary over longer periods.

    Independent observations of a wide range of black hole X-ray sources with masses ranging from 10 to tens of millions solar masses have revealed a relationship between the time scale of quasi-periodic oscillations and the mass of the underlying black hole. Using this technique, the observed two-hour variation implies that this ULX has a mass of about 10,000 Suns.

    Such a large mass would place this black hole well above the stellar-mass black hole limit of a few dozen solar masses.

    How then did it form? The leading theories under considerationare that intermediate-mass black holes form as dozens or even hundreds of black holes merge in the center of a dense star cluster, or that they are the remnant nuclei of small galaxies that are in the process of being absorbed by a larger galaxy, according to the researchers. Enditem