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Spying new quasar class, peering at M32.

Two Earth-orbiting spacecraft have found new evidence for black holes lurking near the edge of the observable universe as well asin our own cosmological backyard.

The more distant finding comes from an ongoing sky survey of gamma rays, radiation more energetic than X-rays. Last week in Washington, D.C., scientists reported that the Compton Gamma Ray Observatory (GRO) had detected strong gamma-ray emissions from three quasar-like objects that lie billions of light-years from Earth. The intensity of gamma radiation from each source -- quasar relatives called BL Lac objects -- represents 10 trillion times the output of the sun at all wavelengths.

GRO had previously identified eight quasars with similarly high-intensity gammas (SN: 1/25/92, p.60). Those findings, combined with the new observations, may represent a new class of quasars that radiate predominantly at gamma-ray wavelengths, says GRO scientist Neil Gehrels of NASA's Goddard Space Flight Center in Greenbelt, Md.

While the mechanism to produce such emissions remains a mystery, the underlying powerhouses must be supermassive black holes, Gehrels asserts. No other known phenomenon could account for such intense radiation spewed from such small volume sof space, he contends. The new findings point to a surprising similarity between certain types of a quasars and quasar-like objects. They also solve a key puzzle about the gamma-ray radiation from the quasars, Gehrels says.

When GRO first began detecting gamma rays from quasars, researchers had trouble believing the results. If some quasars beamed that much radiation toward Earth, they likely radiated a total of 10 to 100 times that amount in all directions -- and that was impossible, since the concentration of gammas needed to produce such intense radiation would cause the gammas to collide and annihilate each other, leaving pairs of oppositely charged subatomic particles in the stead.

Enter the BL Lacs. Researchers already had evidence suggesting that BL Lacs radiate light in a single beam. Since certain quasars, called blazard, share several other features with BL Lacs, scientists reasoned that blazars, too, might emit a single beam. Thus, the total gamma-ray energy emitted by these quasars wouldn't far exceed the intensity recorded and would match that permitted by theory, Gehrels says.

Rapid fluctuations in gamma rays emitted by the blazars and BL Lacs support this model. It's as if each object beams its energy like a flashlight that sometimes strikes the GRO detector dead on and sometimes all but misses it.

Another finding announced last week involves visible-light emissions from an object much closer to home. The Hubble Space Telescope's planetary camera has detected unusually intense starlight from the center of a tiny galaxy called M32, a satellite to the Andromeda galaxy, the Milky Way's nearest spiral neighbor. The intensity indicates that M32 packs 100 million times more stars near its core than the distribution of stars in the vicinity of the sun, reports Tod R. Lauer of the National Optical Astronomy Observatories in Tucson, Ariz. He and his colleagues suggest that the high density stems from a small black hole at M32's center, with a mass about 3 million times that of the sun.

Lauer odds that researchers had suspected that M32 harbors a black hole ever since ground-based observations in the 1980s showed that the orbital velocities of stars rose toward its center, as if they were circling a massive object.

But caveats abound, Lauer notes. Hubble's blurred vision (SN: 7/7/90, p.4) can resolve only the very beginning of a rise in star density; the density might level off farther into the core. And unlike other recently reported black hole candidates (SN: 1/25/92, p.52), M32 has no central jet, nor does its core radiate intensely at any wavelength. Lauer suggests that smaller black holes, like the one proposed for M32, may "eat" surrounding gas less frequently, spewing out less energy.

Jeremy Goodman of Princeton (N.J.) University notes that other nearby galaxies suspected of harboring black holes also radiate relatively little. The final verdict, he says, awaits high-resolution observations of star velocities at M32's core, a feat that researchers can't perform until Hubble gets corrective optics in 1994.
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Author:Cowen, Ron
Publication:Science News
Date:Apr 18, 1992
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