Backyard telescope users seeking to spot a stellar-mass black hole, or the next closest thing, have one good option: Cygnus X-1, which is locked in a close, 5.6-day orbit with a 9th-magnitude blue-giant star partway down the Northern Cross. A 3-inch scope and the right chart do the trick. The star’s powerful X-ray companion was the first and best black-hole candidate starting in 1972. It has been studied intensively ever since.
But for four decades astronomers were unable to determine the hole’s exact nature, because the system’s distance remained stubbornly inexact. As of mid-2011 the best measures still ranged from 5,800 to 7,800 light-years.
Now we know. Using very-long-baseline interferometry, a radio-astronomy team led by Mark Reid (Harvard-Smithsonian Center for Astrophysics) measured the system’s tiny trigonometric parallax due to Earth’s annual motion around the Sun. Their parallax value of 0.539 ± 0.033 milliarcsecond yields a distance of 6,070 ±300 light-years, more than a threefold improvement.
Plug that number into previous studies, and the black hole’s mass comes out to be 14.8 ± 1.0 Suns. That means its event horizon is about 90 km (56 miles) across. The mass of the blue companion star works out to 19 ± 2 Suns.
In addition, the hole’s spin comes out to more than 800 revolutions per second, or more than 95% of the maximum spin possible (defined as the event horizon rotating at essentially lightspeed). The researchers say the black hole could not have accreted enough material from the companion to spin up to that speed, so it must have been born spinning very fast.
The accurate distance also allowed the group to find the system’s velocity with respect to its galactic surroundings: only 21 km/second. This indicates little or no kick from a supernova explosion, supporting the theory that the parent star collapsed to form the hole directly with no supernova fireworks involved.