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A guide to the black holes in our backyard

Black holes are the most astounding objects in the universe. And at least 19 of them lurk within the Milky Way.
The Milky Way's core contains a black hole weighing some 4 million solar masses. This supermassive object, dubbed Sagittarius A*, glows at radio wavelengths (seen here) and in X-rays but disappears in visible light because so much dust lies between Earth and the galactic center.
The Swan’s song
The most famous stellar-mass black hole is Cygnus X-1 (its designation signifies it as the first X-ray source discovered in Cygnus the Swan), which lies some 6,100 light-years from Earth. It is the only one in a high-mass X-ray binary system — its ­companion star is a blue supergiant that tips the scales at approximately 19 solar masses. In fact, this luminous companion shines brightly enough that it appears in our sky as a 9th-magnitude star visible through amateur telescopes.

The black hole in Cygnus X-1 weighs close to 15 solar masses, which makes it the heaviest one known in a binary system. The two objects orbit each other once every 5.6 days at an average distance about half that between the Sun and Mercury. As matter in the accretion disk falls toward the black hole, magnetic fields channel some of it into a pair of high-speed jets that emerge perpendicular to the disk. Recent observations show that the black hole rotates at more than 90 percent of the ­theoretical maximum.

The other 17 stellar-mass black holes reside in low-mass X-ray binaries. Most of their companion stars have masses similar to or somewhat smaller than the Sun. Still, a few of these objects stand out. Astronomers estimate the mass of GRS 1915+105 (a designation that comes from the Russian Granat satellite and the object’s sky coordinates) in Aquila at 14 Suns, but with an uncertainty of 4 solar masses, it could be the heavyweight champ. This object also spews jets that appear to travel faster than the speed of light — an optical illusion that arises because the jets move at about 90 percent light-speed toward Earth. It marked the first time scientists had seen such superluminal motion within our galaxy.

Meanwhile, GX 339-4 lies in the southern constellation Ara and experiences frequent X-ray outbursts followed by periods when its emission decreases, but never so far as to let its companion star shine through. It’s the only binary black hole whose companion still eludes detection.

Just because a binary system behaves oddly doesn’t mean it possesses a black hole, though. Few objects in the galaxy sport the peculiarities of the high-mass X-ray binary SS 433, which lies inside a 10,000-year-old supernova remnant called W50. The explosion that created this glowing remnant gave birth to a compact object that now steals material from a massive companion star. The gas forms an accretion disk that powers two jets beaming in opposite directions like a pair of rotating lighthouse beacons. Many astronomers think
SS 433’s compact object is a black hole, but they can’t rule out a neutron star.

The supermassive black hole at our galaxy’s heart is not a voracious eater. This X-ray image reveals lobes of hot gas extending a dozen light-years from the black hole (Sagittarius A*) but only a small glow for the black hole itself.
NASA/CXC/MIT/F. K. Baganoff, et al.

Black holes in globulars?
The biggest stellar-mass black holes in our galaxy appear to top out at about 15 to 20 times the Sun’s mass. Yet a number of scientists think much larger ones exist in some of the galaxy’s 150 or so globular star clusters. Star-sized black holes likely formed in these clusters early in their histories, more than 10 billion years ago, and fairly quickly sunk to the center. But what happened to these black holes? Some theorists think that they merged with other black holes or neutron stars and grew much bigger, while others suspect that they encountered other stars that then ejected them from the cluster.

Thomas Maccarone of the University of Southampton in England and his colleagues reported in 2007 on the best candidate for a black hole in any globular cluster. They found an X-ray source in a globular circling the giant elliptical galaxy M49, located some 50 million light-years away in the Virgo cluster. The object emits far too many X-rays to be a neutron star and so must be an accreting black hole. Its mass exceeds 20 Suns but could be much higher.

The case for black holes in our galaxy’s globulars is more tenuous. Unless a black hole actively feeds on material from another star, it won’t have an accretion disk that glows in X-rays. The best alternative method is to look at the brightnesses and motions of the stars near a cluster’s center.

Several research teams have examined the Milky Way’s largest globular, Omega Centauri, to do just that. In 2008, a group led by Eva Noyola of the Max Planck Institute for Extraterrestrial Physics in Germany reported a black hole weighing 40,000 solar masses. Just two years later, however, Jay Anderson and Roeland P. van der Marel of the Space Telescope Science Institute in Baltimore, Maryland, found no evidence for a black hole of that size. At this stage, neither side seems to be winning the debate.

A similar argument rages over the relatively nearby globular M15 in Pegasus. Some researchers claim the presence of an intermediate-mass black hole weighing about 4,000 Suns, while others find no such evidence. Earlier this year, Jay Strader of the Harvard-Smithsonian Center for Astrophysics and his colleagues announced that new observations show M15 can’t have a black hole larger than 980 solar masses.

The beast in the middle
While strong evidence for intermediate-mass black holes is lacking, the same can’t be said for their big brothers. Astronomers have found compelling signs for a supermassive black hole in the center of almost every large galaxy they have scrutinized, and the Milky Way is no exception. The core of our galaxy harbors an object called Sagittarius A* (pronounced A-star) — a black hole with about 4 million times the Sun’s mass. It’s the 19th confirmed black hole in the Milky Way, and it sits dead in the center.

The evidence takes several forms. First, intense radio waves and X-rays flow from an accretion disk that spans a region no bigger than our solar system. But the proof comes from careful tracking of the motions of stars as they orbit the central mass. It’s the same method astronomers use to hunt for globular cluster black holes, but the huge size of the object in the Milky Way’s heart makes these motions far easier to see. Analyzing the stellar orbits leads directly to the black hole’s mass.

The count of black holes in our galaxy likely will continue to grow in the years ahead, but it never will outpace the flood of planet discoveries. The ability to find planets has reached the stage where it’s surprising when a week goes by without a new detection. Black holes hide their identities much better, either behind the cloak of an event horizon or in isolation from other objects. Perhaps the biggest surprise in the study of our galaxy’s black holes is that we’ve already found 19.
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