A powerful stellar
explosion detected March 19 by NASA's Swift satellite has shattered the
record for the most distant object that could be seen with the naked eye.
The explosion was a gamma ray burst. Most gamma ray bursts occur when
massive stars run out of nuclear fuel. Their cores collapse to form black
holes or neutron stars, releasing an intense burst of high-energy gamma
rays and ejecting particle jets that rip through space at nearly the speed
of light like turbocharged cosmic blowtorches. When the jets plow into
surrounding interstellar clouds, they heat the gas, often generating bright
afterglows. Gamma ray bursts are the most luminous explosions in the
universe since the big bang.
"This burst was a whopper," said Swift principal investigator Neil
Gehrels of NASA's Goddard Space Flight Center in Greenbelt, Md. "It blows
away every gamma ray burst we've seen so far."
Swift's Burst Alert Telescope picked up the burst at 2:12 a.m. EDT,
March 19, and pinpointed the coordinates in the constellation Bootes.
Telescopes in space and on the ground quickly moved to observe the
afterglow. The burst is named GRB 080319B, because it was the second gamma
ray burst detected that day.
Swift's other two instruments, the X-ray Telescope and the
Ultraviolet/Optical Telescope, also observed brilliant afterglows. Several
ground-based telescopes saw the afterglow brighten to visual magnitudes
between 5 and 6 in the logarithmic magnitude scale used by astronomers. The
brighter an object is, the lower its magnitude number. From a dark location
in the countryside, people with normal vision can see stars slightly
fainter than magnitude 6. That means the afterglow would have been dim, but
visible to the naked eye.
Later that evening, the Very Large Telescope in Chile and the
Hobby-Eberly Telescope in Texas measured the burst's redshift at 0.94. A
redshift is a measure of the distance to an object. A redshift of 0.94
translates into a distance of 7.5 billion light years, meaning the
explosion took place 7.5 billion years ago, a time when the universe was
less than half its current age and Earth had yet to form. This is more than
halfway across the visible universe.
"No other known object or type of explosion could be seen by the naked
eye at such an immense distance," said Swift science team member Stephen
Holland of Goddard. "If someone just happened to be looking at the right
place at the right time, they saw the most distant object ever seen by
human eyes without optical aid."
GRB 080319B's optical afterglow was 2.5 million times more luminous
than the most luminous supernova ever recorded, making it the most
intrinsically bright object ever observed by humans in the universe. The
most distant previous object that could have been seen by the naked eye is
the nearby galaxy M33, a relatively short 2.9 million light-years from
Earth.
Analysis of GRB 080319B is just getting underway, so astronomers don't
know why this burst and its afterglow were so bright. One possibility is
the burst was more energetic than others, perhaps because of the mass,
spin, or magnetic field of the progenitor star or its jet. Or perhaps it
concentrated its energy in a narrow jet that was aimed directly at Earth.
GRB 080319B was one of four bursts that Swift detected, a Swift record
for one day. "Coincidentally, the passing of Arthur C. Clarke seems to have
set the universe ablaze with gamma ray bursts," said Swift science team
member Judith Racusin of Penn State University in University Park, Pa.
Swift is managed by Goddard. It was built and is being operated in
collaboration with Penn State, the Los Alamos National Laboratory, and
General Dynamics in the U.S.; the University of Leicester and Mullard Space
Sciences Laboratory in the United Kingdom; Brera Observatory and the
Italian Space Agency in Italy; plus partners in Germany and Japan.
Source: NASA
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