An orbiting X-ray observatory has discovered an exploding
star in the Milky Way which somehow escaped notice by the usual crowd of star
gazers.
Calculations show that the star’s sudden brightness was
clearly visible to the naked eye, but no one reported anything until the
European Space Agency’s XMM-Newton telescope spotted an unexpected burst of
cosmic X-rays.
On Oct. 9, 2007, XMM-Newton
was turning from one target to another when it passed across a bright source of
X-rays that no one was expecting. The source was not listed in any previous X-ray
catalog, yet the mysterious object was lighting up XMM-Newton’s view of the
cosmos.
The XMM-Newton team looked up three possible celestial
candidates as at this location, including a normally faint star known only by
its catalog number USNO-A2.0 0450-03360039. Acting quickly, Andy Read of the
University of Leicester and Richard Saxton of ESA's European Space Astronomy
Centre (ESAC), Spain, e-mailed other astronomers about the newly-discovered X-ray
source.
More sleuthing
Astronomers turned to the 6.5-meter Magellan-Clay telescope at Las Campanas
Observatory in Chile, and found that USNO-A2.0 0450-03360039 had become 600
times brighter than normal. Analyzing the light from the source meant that they
could classify the object as a
nova.
Novae occur when a small, compact star, called a white dwarf, feeds off the gas
of a nearby companion star. Gas builds up on the white dwarf until a nuclear
reaction begins releasing large quantities of
energy, causing the white dwarf to explode in brightness.
That led to a puzzle. An explosion of this type does not immediately release X-rays,
because the expanding cloud of debris created in the detonation temporarily
masks them. That meant the explosion must have taken place many days before
XMM-Newton spotted the X-ray burst, although no one reported seeing it.
Amateur and professional astronomers usually find novae by regularly sweeping
the night sky for stars or other objects that suddenly
brighten — but humans are not alone in watching the sky. Saxton contacted
the robotic All
Sky Automated Survey project and asked astronomers to check their data.
They found the nova had taken place on June 5, 2007, and had been clearly
visible, and that it would have been bright enough to see with the unaided eye.
"Anyone who went outside that night and looked towards the constellation
of Puppis would have seen it," Saxton says.
Still tracking
The nova has now received the official name of V598 Puppis and has become one
of the brightest for almost a decade, despite not getting spotted during its
brilliant peak. As news of its existence spread, the global effort to track its
fading light became intense.
"Suddenly there was all this data being collected
about the star,” Read says. “For variable star work like this, the contribution
of the amateur community can be at least as important as that from the
professionals.”
This story has a happy ending thanks to XMM-Newton, which has covered 30
percent of the sky and documented 7,700 X-ray sources. However, the event does
make astronomers wonder whether there are other discoveries going unnoticed.
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