The lowest-mass known black hole belongs to a binary system named XTE J1650-500. The black hole has about 3.8 times the mass of our sun, and is orbited by a companion star, as depicted in this illustration. 
In this top-down illustration of a black hole and its surrounding disk, gas spiraling toward the black hole piles up just outside it, creating a traffic jam. The traffic jam is closer in for smaller black holes, so X-rays are emitted on a shorter timescale. Credit: NASA
NASA scientists have identified the smallest, lightest black hole yet found.
The new lightweight record-holder weighs in at about 3.8 times the mass of our sun and is only 15 miles (24 kilometers) in diameter.
"This black hole is really pushing the limits," said study team leader Nikolai Shaposhnikov of NASA's Goddard Space Flight Center in Greenbelt, Md. "For many years astronomers have wanted to know the smallest possible size of a black hole, and this little guy is a big step toward answering that question."
The low-mass black hole sits in a binary system in our galaxy known as XTE J1650-500 in the southern hemisphere constellation Ara. NASA's Rossi X-ray Timing Explorer (RXTE) satellite discovered the system in 2001, and astronomers soon realized that the system harbored a relatively lightweight black hole. But the black hole's mass had never been precisely measured.
Black holes can't be seen, but they're identified by the activity around them, which also helps astronomers estimate a size of the region inside the activity, and how much mass must be in that confined region to generate all the surrounding activity. More specifically, astronomers can weigh black holes by using a relationship between the apparent size of the black hole and the X-rays emitted by the torrent of gas that swirls into the black hole's disk from its companion star.
As the gas piles up near the black hole, it "becomes very dense and congested," like a traffic jam, Shaposhnikov said at a press conference announcing the find. "So matter has to literally squeeze into the black hole."
As it is squeezed, the gas heats up and radiates X-rays. The intensity of the X-rays varies in a pattern repeated over a nearly regular interval. Astronomers have long suspected that the frequency of this signal, called the quasi-periodic oscillation, or QPO, depends on the mass of the black hole.
As the black hole gets bigger, the zone of swirling gas is pushed farther out, so the QPO ticks away slowly. But for smaller black holes, the gas sits closer in and the QPO ticks rapidly.
Shaposhnikov and his colleague Lev Titarchuk of George Mason University used this method to "weigh" XTE J1650-500 and found a mass of 3.8 suns. This value is well below the previous record holder GRO 1655-40, which tips the scales at about 6.3 suns.
This new mass measurement could help shed light on what the smallest star that will produce a black hole is. Astronomers know that some unknown critical threshold, possibly between 1.7 and 2.7 solar masses, marks the boundary between a star that generates a black hole upon its death and one that produces a neutron star.
"This new result brings us much closer to the theoretically predicted limit," Shaposhnikov said.
Knowing this boundary would help scientists understand the behavior of matter when it is scrunched to extraordinarily high densities.
"The question of black hole masses has concerned us for more than a decade now," said astrophysicist Vicky Kalogera of Northwestern University, who was not involved with the study, during the press conference. Scientists had predicted that their should be more black holes at the lower end of the mass range than astronomers had identified, so this study helps clear up some confusion as to where these lightweight black holes were, she added.
Kalogera did caution that the method used by Shaposhnikov and Titarchuk is not the main way that black hole masses are measured, but noted that their measurements of the masses of other black holes agreed well with the results from the standard method.
Shaposhnikov and Titarchuk presented their findings on March 31 at the American Astronomical Society's High-Energy Astrophysics Division meeting in Los Angeles.
