In an attempt to catch new X-ray transients while they are still bright, the data taken by XMM-Newton as it slews between targets is being processed and cross-correlated with other X-ray observations as soon as the slew data appears in the XMM-Newton archive. A bright source, XMMSL1 J070542.7-381442, was detected on 9 Oct 2007 at a position where no previous X-ray source had been seen. The XMM slew data and optical data acquired with the Magellan Clay 6.5m telescope were used to classify the new object. No XMM slew X-ray counts are detected above 1keV and the source is seen to be ~750 times brighter than the ROSAT All-Sky Survey upper limit at that position. The normally m(V)~16 star, USNO-A2.0 0450-03360039, which lies 3.5 from the X-ray position, was seen in our Magellan data to be very much enhanced in brightness. Our optical spectrum showed emission lines which identified the source as a nova in the auroral phase. Hence this optical source is undoubtedly the progenitor of the X-ray source - a new nova (now also known as V598 Pup). The X-ray spectrum indicates that the nova was in a super-soft state (with kT(eff)~35eV). We estimate the distance to the nova to be ~3kpc. Analysis of archival robotic optical survey data shows a rapid decline light curve consistent with that expected for a very fast nova. The XMM-Newton slew data present a powerful opportunity to find new X-ray transient objects while they are still bright. Here we present the first such source discovered by the analysis of near real-time slew data.
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