Nailing Another Short GRB

August 5, 2005 | NASA's Swift satellite continues to catch and pinpoint gamma-ray bursts (GRBs) like no instrument ever before. On July 24th it observed another mysterious short GRB well enough for ground-based astronomers to follow up rapidly. The first time this happened, in May, astronomers gained good evidence that short GRBs come from the mergers of binary neutron stars — very unlike the more common long GRBs, which come from the supernova-like collapse of a high-mass star's core.

The July 24th event, dubbed GRB 050724, strengthens this conclusion. Judging from its radio and visible-light afterglows, Edo Berger and 23 coauthors will report in Nature that GRB 050724 went off in a region of purely old, low-mass stars in an elliptical galaxy at a redshift of 0.257, that it had only a tenth to a thousandth of a typical long burst's total energy, and that the energy was apparently emitted in narrow beams about 9° wide. A preprint of their paper is here.

Says independent GRB expert William H. Lee (Instituto de Astronomia, Mexico), "This is encouraging for the [neutron-star] merger scenario, and we all hope that Swift will continue to work as well as it has and keep us busy! Things are moving fast indeed."

Cosmic-Ray Milestone

August 4, 2005 | Astrophysicists have laid eyes on the first data from the Pierre Auger Observatory, a sprawling array of cosmic-ray detectors dotting 1,500 square kilometers of the Argentine pampas. Released in July, the Pierre Auger Collaboration's inaugural scientific publication plots the energy spectrum of 3,525 cosmic-ray events that the particle-sensing system observed last year. Unfortunately, Auger cannot yet resolve a longstanding discrepancy over the so-called GZK cutoff: an expected absence of cosmic-ray particles with energies above 1020 electron volts, the result of cosmic-ray particles colliding with the photons that make up the cosmic microwave background. However, "in the next few years we'll probably find out whether the GZK cutoff is real," says University of Chicago physicist James Cronin.

Messenger Buzzes Earth

August 3, 2005 | After a year orbiting the Sun, the Mercury Surface, Space Environment, Geochemistry, and Ranging mission, known as Messenger, swooped within 2,350 kilometers (1,460 miles) of Earth's surface yesterday. At 3:13 p.m. Eastern Daylight Time the NASA spacecraft flew above central Asia and received a trajectory shift from our planet’s gravity, setting up Messenger for the first of two Venus flybys in October 2006. The spacecraft was launched from Cape Canaveral on August 3, 2004, and will make its first flyby of Mercury in January 2008. After three more loops around the Sun, Messenger will settle into Mercury orbit in 2011.

See the Messenger Web site for more information.

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