A NASA astronomer has spotted one of the most powerful nuclear explosions
ever seen in the Universe. Tod Strohmayer from the Goddard Spaceflight Center in
Greenbelt, Maryland, was watching a distant neutron star with the Rossi X-ray
Timing Explorer when the star suddenly erupted. The explosion lasted for three
hours and released more energy than all the world鈥檚 population would consume in
a hundred billion years.
The neutron star, called 4U 1820-30, is 20,000 light years away in the
constellation Sagittarius. It鈥檚 part of a binary system and the two stars take
only 11 minutes to orbit each other鈥攖he shortest orbital period known.
Helium gas from the companion white dwarf star gradually piles up on the surface
of the extremely small and compact neutron star. Pressure builds within the
helium layer until there鈥檚 a nuclear explosion, releasing large amounts of
X-rays.
These X-ray bursts happen almost every day and last for about 10 seconds. The
massive explosion Strohmayer witnessed lasted a thousand times longer and
produced almost a thousand times as much energy. Strohmayer thinks a shallow
layer of carbon ash from the regular helium outbursts fuelled the big blast. 鈥淚t
might well be that every accreting neutron star shows these carbon-fuelled
explosions once every year or so,鈥� he says.
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Strohmayer is not the first to see a super-explosion from a neutron star.
Dutch researchers using the Italian-Dutch satellite BeppoSAX reported some
earlier this year, but Strohmayer says they did not get the whole picture. The
higher sensitivity of the Rossi satellite has shown that super-explosions are
triggered by a smaller, normal X-ray burst. He suggested that BeppoSAX鈥檚
instruments couldn鈥檛 properly distinguish a precursor from the main event.
