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Surprising X-rays from a pre-cataclysmic binary

posted May 25, 2013, 3:53 PM by Jeremy Drake   [ updated May 25, 2013, 3:53 PM ]
We looked at the pre-cataclysmic eclipsing binary QS Virginis with the XMM-Newton satellite in order to study the outer atmosphere, or corona, of an ultra-fast rotating M dwarf star.  The pre-cataclysmic binaries (pre-CVs) comprise a K or M dwarf in a very close orbit with a white dwarf star.  The orbital period of QS Vir, and the spin period of the M dwarf, is only 3.6 hours. In time - millions to billions of years, depending on the system - the orbit of a pre-CV will shrink such that the K or M dwarf will begin to lose material to the gravity of the white dwarf: it will become a cataclysmic variable star, as this accreting material forms a hot disk of gas that gradually spirals in toward the white dwarf surface.  

Instead of seeing X-rays from the corona of the M dwarf in QS Vir, we saw the signal shown in the plot here.  Their is a small flare from the M dwarf, but the X-rays are otherwise regularly eclipsed - the emission was mostly coming from the white dwarf, not the M dwarf.  These X-rays betray accretion of gas at a very low rate onto the white dwarf surface.  But how? The binary is still a tiny bit too wide to allow steady mass transfer.  We think that upward chromospheric flows or "spicules" on the M dwarf might reach high enough to fall under the influence of the white dwarf's gravity.  Other possibilities include the magnetic syphon flow described in an earlier study, or accretion of a coronal mass ejection. This work was lead by postdoc Marco Matranga and was published in the 2012 March 10 edition of the Astrophysical Journal.