XMM-Newton discovers the star that everyone missed (7/23/2008)

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 image composite (infrared, red and blue channels) from the SuperCOSMOS Sky Surveys (SSS), Royal Observatory, Edinburgh. Credits: Contours: ESA/ XMM-Newton/ EPIC (adapted from A. Read et al.), Background: SSS

Nova missed by amateur and professional astronomers spotted by team led by Leicester scientist.

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.

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 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, formerly of Leicester and now at 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.

Andy is a senior calibration scientist in the EPIC team at the University of Leicester. EPIC, the 'European Photon Imaging Camera', is the major instrument on ESA's cornerstone mission, XMM-Newton. Two of the three cameras on EPIC were built at Leicester.

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.

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. Read and 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.

Andy Read said: "The most amazing thing about this nova is that it was discovered in X-rays, and not in optical light - All previous Galactic novae have been discovered in the optical, but the optical explosion from this nova was not seen or reported, neither by professionals nor amateurs.

It was only when we discovered it in X-rays later with XMM-Newton that we went back through archival robotic data and there it was. Ironically, it was one of the brightest optical novae for many years, at its peak visible to the naked eye - That it was missed by everyone is amazing." "Anyone who went outside that night and looked towards the constellation of Puppis would have seen it," added Saxton.

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, and Read and colleagues were able to follow the decay of the nova both with XMM-Newton and with the Leicester-built X-ray Telescope (XRT) on board the NASA satellite Swift. "Suddenly there was all this data being collected about the star" 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.

Note: This story has been adapted from a news release issued by the University of Leicester

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