Astronomers had been anticipating the 2010 explosion of U Scorpii and were ready to observe it with a battery of the world's best instrumentation. Novae are essentially thermonuclear explosions - colossal hydrogen bombs - detonated on the surface of a white dwarf star. The white dwarf in a nova is in a "cataclysmic binary" system, in close proximity to a companion star that loses gas to the white dwarf's strong gravity. This material accretes onto the white dwarf surface and over time builds up a layer sufficiently hot and dense that nuclear fusion is ignited, resulting in thermonuclear runaway and an explosion. This process can repeat many times during the evolution of a nova-like cataclysmic variable, but generally the time between explosions is too great -thousands to hundreds of thousands of years - to observe more than one explosion on a single star.  U Sco is different. It is a member of an exclusive class of only 10 known "recurrent novae", so-called because they have been observed to explode more than once. Its white dwarf is comparatively massive - close to the Chandrasekhar limit of 1.4 times the mass of the Sun. The higher mass means it needs less "fuel" to initiate an explosion and it gets enough of it from its companion star to erupt every 10 years or so. The timing is not precise though, with previous blasts occurring at intervals of 8-12 years. When an outburst is due, it then becomes a favourite target for amateur astronomers who vie to be the first to spot the new nova event. Amateur astronomer B. G. Harris discovered U Sco to have entered its latest outburst on 2010 January 28. A world-wide multi-wavelength campaign was triggered and the event was followed from X-rays to radio. The explosion flings off debris from the white dwarf surface at several thousand kilometers a second, and this debris and any surrounding circumstellar gas becomes shock-heated to millions of degrees. At such temperatures the gas emits most of its light in X-rays, and we were eagerly awaiting a detection of this X-ray signal. If we can observe this extremely hot gas, we can learn how energetic the explosion was and how much material has been ejected - parameters vital for understanding if the white dwarf is gaining more mass than it is losing and might become a Type 1a supernova once the white dwarf exceeds the Chandrasekhar limiting mass. But NASA's Swift and RXTE satellites failed to detect it. We studied it anyway and ran detailed hydrodynamic supercomputer models of the explosion to see what constraints the X-ray limits might place. We found that the blast was re-directed and "collimated" by the accretion disk of gas that would have surrounded the white dwarf prior to the blast. The X-ray limits are consistent with the ejected mass being less than 1 ten millionth of the mass of the Sun. The results were published in the 2010 September 10 edition of the Astrophysical Journal Letters. |
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