DETECTION OF ACCRETION X-RAYS FROM QS Vir: CATACLYSMIC OR A LOT OF HOT AIR?

作者：Matranga Marco^*; Drake Jeremy J; Kashyap Vinay; Steeghs Danny

来源：Astrophysical Journal, 2012, 747(2): 132.

DOI：10.1088/0004-637X/747/2/132

• accretion, accretion disks
• binaries: close
• binaries: eclipsing
• novae, cataclysmic variables
• stars: winds, outflows
• X-rays: binaries

An XMM-Newton observation of the nearby %26quot;pre-cataclysmic%26quot; short-period (P-orb = 3.62 hr) binary QS Vir (EC 13471-1258) revealed regular narrow X-ray eclipses when the white dwarf passed behind its M2-4 dwarf companion. The X-ray emission provides a clear signature of mass transfer and accretion onto the white dwarf. The low-resolution XMM-Newton EPIC spectra are consistent with a cooling flow model and indicate an accretion rate of (M) over dot = 1.7 x 10(-13) M-circle dot yr(-1). At 48 pc distant, QS Vir is then the second nearest accreting cataclysmic variable known, with one of the lowest accretion rates found to date for a non-magnetic system. To feed this accretion through a wind would require a wind mass-loss rate of (M) over dot similar to 2 x 10(-12) M-circle dot yr(-1) if the accretion efficiency is of the order of 10%. Consideration of likely mass-loss rates for M dwarfs suggests this is improbably high and pure wind accretion unlikely. A lack of accretion disk signatures also presents some difficulties for direct Roche lobe overflow. We speculate that QS Vir is on the verge of Roche lobe overflow, and that the observed mass transfer could be supplemented by upward chromospheric flows on the M dwarf, analogous to spicules and mottles on the Sun, that escape the Roche surface to be subsequently swept up into the white dwarf Roche lobe. If so, QS Vir would be in a rare evolutionary phase lasting only a million years. The X-ray luminosity of the M dwarf estimated during primary eclipse is L-X = 3 x 10(28) erg s(-1), which is consistent with that of rapidly rotating %26quot;saturated%26quot; K and M dwarfs.

• 出版日期2012-3-10