XMM-Newton Discovers
The Star That Everyone Missed
(18 July 2008) XMM-Newton has
discovered an exploding star in the Milky Way.
Usually that
would be important in itself, but this time there is a special twist.
Calculations show that the explosion must have been clearly visible to the
unaided eye but was missed by the legions of star watchers around the
planet.
The nova V598 Puppis, accidentally discovered in the XMM-Newton slew survey. The X-ray contours, which indicate the position of the nova, are overlaid on an optical image taken from the Digitised Sky Survey. (courtesy: Contours: ESA/ XMM-Newton/ EPIC (adapted from A. Read et al.), Background: Digitised Sky Survey)
On 9 October 2007, ESA's orbiting X-ray observatory XMM-Newton was turning from one target to another. As it did so, it passed across a bright source of X-rays that no one was expecting. The source was not listed in any previous X-ray catalogue, yet XMM-Newton was receiving some 50 X-rays every second from this mysterious object.
The nova V598 Puppis, accidentally discovered in the XMM-Newton slew survey. An optical image is shown on the left, and the X-ray discovery is shown on the right. (courtesy: ESA/ XMM-Newton/ EPIC (adapted from A. Read et al.))
The only celestial object the XMM-Newton
team could find at this location was a faint star, known only by its catalogue
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, arranged for an astronomical telegram to be circulated across the
Internet, informing other astronomers of the newly-discovered X-ray
source.
Astronomers using the 6.5-m Magellan-Clay telescope at Las
Campanas Observatory in Chile, found that USNO-A2.0 0450-03360039 had
dramatically brightened by more than 600 times. Analysing the light from the
source meant that they could classify the object as a nova.
Novae occur
when a compact star, called a white dwarf, feeds off the gas of a nearby
companion star. When sufficient gas builds up on the white dwarf, a nuclear
reaction begins releasing large quantities of energy, prompting the white dwarf
to shoot up in brightness.
This map of our galaxy shows all the objects that were detected in the XMM-Newton slew survey, one of which was the nova V598 Puppis. The plot is colour-coded such that sources of a lower energy are red and those with a higher energy are blue. Also, the brighter the source, the larger it appears on the map. The plot is in galactic coordinates (the centre of the plot corresponds to the centre of the Milky Way). (courtesy: ESA/ XMM-Newton/ EPIC (adapted from R. Saxton et al.))
But there was a puzzle. The incandescent
explosion does not immediately release X-rays; the expanding cloud of debris
created in the detonation temporarily masks them. As this clears, the X-rays
shine through. So, for XMM-Newton to see this nova, the explosion must have
taken place many days before. Yet, no one had reported seeing
it.
Usually, dedicated amateur and professional astronomers find novae
by regularly sweeping the night sky for stars that suddenly brighten. This one,
it seemed, had slipped the net. Saxton contacted the robotic survey project
ASAS and asked them to check their data. They found the nova. It had taken
place on 5 June 2007 and had been clearly visible, even to the unaided
eye.
"Anyone who went outside that night and looked towards the
constellation of Puppis would have seen it," says Saxton.
An artist's impression of XMM-Newton. (courtesy: ESA (Image by C. Carreau))
The nova is now officially designated V598
Puppis and is one of the brightest for almost a decade, doubling the irony that
it was not spotted during its brilliant peak. As news of it spread, the global
effort to track its fading light became intense. "Suddenly there was all this
data being collected about the star. For variable star work like this, the
contribution of the amateur community can be at least as important as that from
the professionals," says Read.
Thanks to XMM-Newton, this story has a
happy ending, but it does make astronomers wonder whether there are other
discoveries going unnoticed too.
Notes:
This nova was discovered
in the XMM-Newton slew survey; a systematic processing of data taken while the
satellite moves between objects. To date, the survey has covered 30% of the sky
and produced a catalogue of 7700 X-ray sources that has been released to the
public.
XMM-Newton slew survey discovery of the nova XMMSL1
J070542.7-381442 by Read et al. is published in Astronomy and
Astrophysics.
The first XMM-Newton slew survey catalogue: XMMSL1 by
Saxton et al. is published in Astronomy and Astrophysics.
The XMM-Newton
science teams are based in several European and US institutes, grouped into
three instrument teams and the XMM-Newton Survey Science Centre (SSC). Science
operations are managed at ESA's European Space Astronomy Centre (ESAC), at
Villanueva de la Cañada near Madrid, Spain. Spacecraft operations are
managed at ESA's European Space Operations Centre (ESOC) in Darmstadt,
Germany.
(source: ESA)
