This Black Hole is too Bright
The extreme gravity of a black hole attracts nearby material, funneling it into what’s called an accretion disk. Friction among gas particles within the disk causes it to glow across a range of wavelengths. When this emitted radiation balances the inward gravitational pull, astronomers say the black hole has reached the Eddington limit. Because each black hole’s limit depends on its mass, they have a way to measure the black hole’s mass.
After studying the light output of an ultraluminous x-ray source in spiral galaxy M101, called M101 ULX-1, researchers thought the source was a black hole between 100 and 1,000 times the Sun’s mass. However, observations and the following analysis reported in November 28 issue of Nature (See “Puzzling accretion onto a black hole in the ultraluminous x-ray source M 101 ULX-1, Nature 503, November 28, 2013), pp. 500-503) suggest instead that the black hole is between 20 and 30 solar masses.
Ji-Feng Liu of the Chinese Academy of Sciences in Beijing and colleagues come to this conclusion after studying M101 ULX-1’s spectrum, which plots the detected brightness of each wavelength. From that , they learned that the black hole’s companion— the object that donates material to the accretion disk—is a Wolf-Rayet star with 19 solar masses. A wind of particles and radiation lows from the star and likely feeds the black hole’s accretion disk. The team also determined that the two complete a full rotation around each other in 8.2 days. From those two pieces of information, Liu’s team calculated the mass of the black hole: 20-30 times the Sun.
This stellar-mass black hole is much more energetic than it should be given its mass; the astronomers refer to the object as having super-Eddington luminosity. “These findings show that our understanding of black hole accretion is incomplete and needs revision,” says Liu.
 See “Astronews,” Astronomy (42, 3, March 2014, p. 14)