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New signs of activity from the old nova DK Lacertae

posted May 29, 2013, 6:55 AM by Jeremy Drake   [ updated Jun 2, 2013, 7:22 AM ]
A nova outburst is a cataclysmic nuclear explosion on a close binary star.  Such a binary typically comprises a white dwarf and a red dwarf that loses out to its companion's gravity, transfering hydrogen-rich that accumulates on the white dwarf surface. When the amount of accreted material reaches a critical mass, sudden hydrogen fusion is triggered by a thermonuclear run-away on the white dwarf surface. Most novae are probably repeat phenomena, recurring at intervals from a few years, which is vert rare - see the posting on the recurrent nova U Sco - to a hundred thousand years or so.  Since nearly all novae we see go off only once on a human timescale, we do not know where they are before they explode. This makes it difficult to study them in their quiescent state, during the build up of hydrogen-rich gas that will fuel the next explosion. 

We used the Swift X-ray satellite to survey some "old" novae - cataloged nova events from the last hundred years or so. We were looking for signs of high energy emission, either from the accretion of gas onto the white dwarf, or from the shock waves from their earlier explosion. One target, DK Lacertae, was easily detected.  It is unlikely that these X-rays are from shocked gas remaining from the outburst.  In this case, we can use the X-ray flux to estimate the rate of transfer of gas onto the white dwarf and find that it is probably a few hundred millionths of a solar mass per year. This work was lead by postdoc Dai Takei, and was published in the 2013 January edition of the Astronomical Journal.