A very capable Swift detects 500th gamma-ray burst

University Park, PA--NASA's Swift satellite, which includes gamma-ray, x-ray, and UV/visible telescopes and which was launched into space five years ago to detect gamma-ray bursts (GRBs) from distant astronomical objects, has spotted its 500th burst.

A GRB consists of an initial blast of gamma rays, then a longer x-ray afterglow, and often a visible-light afterglow as well. Gamma-ray bursts come in two versions: "long" and "short." Long GRBs are now understood to occur at the moment a massive star collapses to form a black hole; short GRBs are less well understood. However, the emerging picture is that short GRBs arise when two compact objects -- either a pair of neutron stars or a neutron star and a black hole -- collide and merge.

GRBs were discovered in 1967 by unclassified military satellites designed to look for clandestine nuclear tests; the first observations required extensive analysis to be sure that the bursts were truly originating beyond the solar system, and they weren't published until 1973.

Some Swift optical specs
Swift detects about 100 GRBs a year, pinpointing their positions to between 0.5 and 5 arcsec. It performs spectroscopy in the 180 to 600 nm wavelength range (optical) and the 0.3 to 150 keV energy range (x-ray and gamma ray). The results are transmitted and distributed to the public within seconds.

--The Burst Alert Telescope detects gamma rays at 15 to 150 keV, uses a coded-aperture mask, and has a wide 2-steradian field of view.

--The X-ray Telescope sees at 0.3 to 10 keV and is a focusing telescope with an 18-arcsec resolution.

--The UV/Optical Telescope allows 0.5-arcsec position localizations and uses a grism (a grating combined with a prism) filter to obtain spectra of GRB afterglows to determine the red shift, and thus the distance, of the GRB.

In addition to finding GRBs, Swift also does ultraviolet studies of exploding stars, monitors black holes and neutron stars for surges of high-energy radiation, and is carrying out a long-term X-ray survey of the entire sky.

Burst 500
Burst 500, officially known as GRB 100413B, occurred in the constellation Cassiopeia as a "long" burst. It wasn't detected in the onboard analysis of data from the spacecraft's Burst Alert Telescope (BAT), which was interrupted 18 seconds after the burst as Swift slewed to a pre-planned target; instead, GRB 100413B came to light when David Palmer, an astrophysicist at Los Alamos National Laboratory in New Mexico, later analyzed the data. "The BAT team regularly digs through the data once it comes to the ground and finds weak bursts like this one that take a bit of special care," said Judith Racusin at Goddard Space Flight Center (Greenbelt, MD), who coordinated burst observations that day.

Measuring GRB distance
Most of the time, the hard task of measuring burst distances falls to ground-based observatories, which can target a burst's location with telescopes far larger than the Ultraviolet/Optical Telescope aboard Swift.

"Getting on the afterglows quickly with large ground-based telescopes remains a key element in understanding GRBs," said Derek Fox, a Swift team member at Penn State (University Park, PA), whose research focuses on follow-up observations. "It's this synergy between Swift and ground observatories that has really moved the ball forward, especially for short bursts."

And the farther the burst, the more important rapid ground follow-up becomes. At distances greater than about 12 billion light years, gas clouds block UV wavelengths before they can reach Earth, and all optical light becomes shifted into IR wavelengths only detectable by specially-equipped ground-based telescopes. Astronomers scramble to detect afterglow from new bursts as soon as they can.

"Thanks to such efforts, we know Swift has seen GRBs as close as about 100 million light years and as far away as 13 billion light years," adds Neil Gehrels, Swift's lead researcher at Goddard Space Flight Center. Put another way, Swift sees gamma-ray bursts over a span of time equivalent to about 95% of the universe's age.

For more on Swift, visit: http://www.nasa.gov/mission_pages/swift/main/index.html