XMM-Newton slew survey discovery of the nova XMMSL1 J070542.7-381442 (V598 Puppis)

Aims. 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 are being processed and cross-correlated with other X-ray observations as soon as the slew data appear in the XMM- Newton archive. Methods. 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 XMMslew data and optical data acquired with the Magellan Clay 6.5 m telescope were used to classify the new object. Results. No XMM slew X-ray counts are detected above 1 keV and the source is seen to be similar to 750 times brighter than the ROSAT All-Sky Survey upper limit at that position. The normally m(V) similar to 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 that 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) approximate to 35 eV). We estimate the distance to the nova to be similar to 3 kpc. Analysis of archival robotic optical survey data shows a rapid-decline light curve consistent with what is expected for a very fast nova. Conclusions. 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.