Another One!

Evidence Grows to Prove Black Holes Really Exist

by Robert Sanders

Berkeley astronomers have identified a second black hole in our galaxy, a discovery made possible by the superior light-gathering ability of the 10-meter Keck Telescope, the largest optical and infrared telescope in the world.

Called GS 2000+25, it has been a suspected black hole since it flared to notice in 1988 as an X-ray nova. Confirmation has eluded astronomers, though, who through 1994 spent countless observing hours in a fruitless attempt to measure the velocity of its faint companion, which is needed to estimate the mass of the suspected black hole.

Because the suspected black hole is part of a binary star system, a lower limit for its mass can be calculated if the velocity and orbital period of its companion can be determined. If the lower limit is bigger than 3.2 times the mass of the sun there's a good chance the object is a black hole.

With five to 10 times more light gathering power than most competing telescopes, the Keck was able to measure both the velocity and orbital period with great precision. From observations in July, Berkeley astronomer Alex Filippenko and graduate students Thomas Matheson and Aaron Barth estimated a minimum mass of five times that of the sun--considerably above the limit of 3.2 solar masses and thus presumably a black hole.

"It's darn near certain," says Filippenko, a professor of astronomy and world authority on supernovas.

The Berkeley results will be published in early December in the Astrophysical Journal Letters.

Only one other black hole candidate with a minimum mass that is definitively greater than 3.2 solar masses has been found within our galaxy, Filippenko says. As more are discovered it becomes easier to rule out alternative though highly speculative explanations for the observations.

"If you find two or more within the same galaxy you are beginning to define a class of objects you can't explain in any other way," he says. "Observations like these are important because they rule out any alternative hypothesis."

The X-ray nova GS 2000+25 was observed in 1988 by the Japanese Ginga satellite in the constellation Vulpecula (the fox), and faded from view nearly a year later. Because of the ultrasoft X-rays it emitted, most astronomers guessed it was a black hole. The nova presumably was caused by matter from the faint companion star falling onto it, making it flare brightly and emit lots of X-rays before settling again into obscurity.

After the nova dimmed sufficiently to see the companion star, astronomers were able to measure the orbital period of the binary system, but the companion was too dim for most telescopes to obtain a spectrum and calculate the velocity. The pair is about 14,000 light years from Earth.

Using the low resolution imaging spectrometer mounted on the Keck the Berkeley team easily measured the spectrum of the companion star, from which they calculated its velocity. They also recalculated the orbital period, confirming previous measurements of 8.3 hours.

While the calculated minimum mass for the black hole is five times that of the sun, a more likely value is six or seven solar masses, Filippenko says, and it could be as massive as 14 of our suns.

Only lower limits on the black hole mass are obtainable because the orbital plane of the binary star system may be tilted with respect to Earth. Since spectra reveal only radial velocity--motion toward or away from us--and not total speed, the measured velocity is merely a minimum. If the orbital plane is tipped at a large angle relative to our line of sight, the total speed could be much greater, implying a much larger mass for the black hole.

The higher the mass the more likely it is a black hole and not a neutron star, Filippenko says. Neutron stars--the presumed source of pulsars--are dense compact objects formed from the cores of exploding stars. Their mass can only be so big, however, before they collapse of their own weight into an even denser object, a black hole. The theoretical upper limit for a neutron star's mass is between 3.0 and 3.2 times that of the sun, which is why astronomers consider any dark object having a greater mass to be a probable black hole.

Nevertheless Filippenko cautions that a black hole is not the only explanation for the observations. The primary (more massive) star in the binary system could be a cluster of two or three neutron stars, or an object composed of a previously unknown state of dense matter, though the chances of this are extremely slim.

The other black hole candidate with a minimum mass definitively above the theoretical maximum for a neutron star is V404 Cygni (also called GS 2023+33).


Copyright 1995, The Regents of the University of California.
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