Cygnus X-1, Exactly

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.

Cygnus X-1
An artist's highly symbolic representation of the Cygnus X-1 black hole and its X-ray-hot inner accretion disk.
NASA / CXC / M.Weiss
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.

Here are the research team’s three papers regarding the hole’s distance, mass, and spin.

5 thoughts on “Cygnus X-1, Exactly

  1. Rod

    The current mass measurements for the binary system indicates the primary (19 M_sun) is a spectral class O star on the H-R diagram. This raises the issue of past, stellar evolution of the system. Wikipedia reports that the progenitor star for the 14.8 M_sun BH was perhaps originally 40 M_sun. Thus Cygnus X-1 binary system 5 million years ago had much more mass than presently observed. We have plenty of unobservable evolution used to explain the current Cygnus X-1 system.

  2. Anthony BarreiroAnthony Barreiro

    Bald Emotions — You can’t see a black hole directly, of course. But you could see the ninth-magnitude companion star in a 4-inch telescope. The trick will be deciding which ninth-magnitude star is the right one! Cygnus lies right along the brightest part of the Milky Way, and it’s just lousy with stars. Find fourth-magnitude Eta Cygni, the brightest star in the middle of the swan’s neck, or halfway down the base of the cross. Eta is halfway from second-magnitude Sadr (the intersection of the cross or the base of the swan’s neck) to Albireo (the base of the cross or the swan’s beak). From Eta, scan one-half degree (the width of the full moon) toward the northeast. One of those dim stars is HDE 226868, the visible companion of the black hole. If you have a go-to scope you can point it to RA 19h 58m 21.6756s, Dec +35° 12′ 05.775″ — but that would be cheating. 😉

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