By Ron Cowen
A brilliant flash of high-energy radiation recorded on Feb. 22 lasted for less
than a minute. But this gamma-ray burst, one of the brightest ever detected, is
providing the strongest evidence so far that these cosmic flashbulbs originate in
star-forming regions of distant galaxies and are generated by the explosive death
of massive stars.
The findings support the notion that these brilliant bursts and their afterglows
can enable astronomers to study galaxies that lie too far away and are too dusty
for the scientists to easily observe.
The Feb. 22 burst and its X-ray afterglow, first detected by the Italian satellite
BeppoSAX, was also examined by NASA’s Chandra X-ray Observatory. The burst
originated in a galaxy some 8 billion light-years from Earth.
A gamma-ray burst produces a blast of material that expands into surrounding space
like a rapidly inflating balloon. This expanding blast wave produces a steady
stream of X rays. However, BeppoSAX found that the intensity of the emissions took
a sudden downturn, reports Luigi Piro of the Consigilio Nazionale delle Ricerche
in Rome.
He suggests the drop in intensity occurred because the blast wave encountered a
dense wall of gas that dramatically slowed its expansion. Gas at this density “can
only be found in very crowded regions where stars are formed,” Piro says.
Radio observations also suggest that the burst exploded in a galaxy undergoing
intense star formation. Five hours after BeppoSAX detected the burst, the James
Clerk Maxwell telescope atop Hawaii’s Mauna Kea recorded a submillimeter radio
source at the location of the burst. Surprisingly, the source remained constant
rather than fading over time, says Fiona Harrison of the California Institute of
Technology in Pasadena. These and other observations indicate the radio emission
is not part of the burst’s waning afterglow but is radiation from the burst’s home
galaxy, she and her colleagues assert.
The submillimeter radiation observed on Earth would have begun as infrared light
from the distant galaxy. Expansion of the universe then shifted this light to
longer wavelengths. The intensity of the infrared light indicates that at the time
the galaxy emitted the radiation, about 8 billion years ago, it was a veritable
stellar nursery, churning out the equivalent of 500 suns per year, Harrison says.
In a separate study, Piro has obtained evidence linking the origin of several
gamma-ray bursts to the explosion of massive stars. Analyzing the X-ray afterglow
of four bursts that predated the Feb. 22 event, he and his colleagues found an
abundance of iron. The Feb. 22 burst also bears signs of iron.
Only supernova explosions, which mark the demise of heavyweight stars, can produce
iron. Such stars typically weigh a few times as much as the sun. However, the
amount of iron observed in these five events indicates that the stars that
generated the bursts were more than 10 times as heavy as the sun. The explosive
death of such an extremely massive star, which is a sort of souped-up supernova,
has become known as a hypernova.
Piro and Harrison presented their teams’ findings April 4 at a meeting on gamma-
ray astronomy in Baltimore.
