Astro News Briefs: April 14–20

Two More Moons for Jupiter Make 60

April 14, 2003 | With Jupiter high in the sky, astronomers are taking advantage. In past few weeks nearly a dozen new planetary moons have been discovered — two of which were certified by the International Astronomical Union on April 12th and 14th. The latest pair, dubbed S/2003 J19 and S/2003 J20, were located using the Canada-France-Hawaii Telescope atop Mauna Kea, Hawaii. J19 was spotted by a four-member group led by Brett Gladman (University of British Columbia); J20 was identified by Scott S. Sheppard and David C. Jewitt (University of Hawaii), along with Jan Klenya (Cambridge University).

Sheppard provides more information on Jupiter's satellite family at http://www.ifa.hawaii.edu/~sheppard/satellites/.

For a tabulation of all known moons see SkyandTelescope.com's Guide to Planetary Satellites.


Monster Gamma Burst Proves Supernova Link

April 14, 2003 | The record-breaking gamma-ray burst seen on March 29th is yielding a huge payoff: conclusive proof that at least some classical, long-duration gamma-ray bursts are associated with supernovae. Krzysztof Z. Stanek (Harvard-Smithsonian Center for Astrophysics) and colleagues find telltale broad emission lines characteristic of a supernova beginning to emerge in the burst's visible-light afterglow. "I used to be very skeptical about the supernova connection," says Stanek, "but our spectroscopy data are really clinching the case."

Details are in a Harvard press release.


No Dark Matter For Elliptical Galaxies?

April 14, 2003 | A mysterious, invisible substance called dark matter exerts gravitational forces that keep galaxies and galaxy clusters from flying apart. So say scores of textbooks and review papers. But while astronomers have made a solid case for dark-matter halos surrounding numerous Frisbee-shaped spiral galaxies (our own Milky Way included), a recent probe of several spherical or football-shaped elliptical galaxies has failed to turn up similarly conclusive evidence. University of Nottingham astronomer Aaron J. Romanowsky put forth this finding at an astronomy meeting in Dublin last week. He and fellow researchers used a new spectrograph on the 4.2-meter William Herschel Telescope to measure the velocities of dozens of planetary nebulae in the outskirts of each elliptical galaxy. The finding builds on a seminal 1993 study of 9th-magnitude M105, in Leo, and it suggests that ellipticals may lose their dark-matter halos in gravitational tussles with neighboring galaxies.


Listening for Gravitational Waves

April 14, 2003| The Laser Interferometer Gravitational-wave Observatory, or LIGO, was back "on the air" for two months with its 4-km-long "antennas" in Livingston, Louisiana, and Hanford, Washington — and it's closer than ever to achieving its intended sensitivity to gravitational waves, ultrafeeble ripples in space-time. Einstein's general theory of relativity predicts that gravitational waves will emanate from certain astronomical events, such as out-of-round supernova explosions and the mergings of neutron stars paired in very close orbits. At its best frequencies (those near 150 cycles per second), LIGO's sensitivity to these waves is about 10 times poorer than intended — down from the factor of 100 that prevailed during the instrument's first science run last year.

Unfortunately, the system's duty cycle remains well below 50 percent, largely because of ground vibrations at LIGO's Louisiana site. Attempts to better isolate the instrument from seismic "noise" are ongoing. In the meantime, LIGO scientists are gearing up to promptly analyze their latest data, which in principle should allow them to "hear" merging neutron-star binaries as distant as the Andromeda Galaxy. According to their presentations at last week's American Physical Society meeting, the noisier data acquired in 2002 set only a loose upper limit on such events: no more than 164 yearly in our Milky Way.


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