SOLVING THE MYSTERY OF THE X-RAY PERIODICITY IN THE SEYFERT-GALAXY NGC6814

ACTIVE galaxies, of which Seyfert galaxies are a subgroup, are thought to be powered by the accretion of gas onto a massive black hole at the galaxy centre; the X-rays emitted by active galactic nuclei arise from the heated, infalling gas. The observed rapid variability of this X-ray emission requires a compact source, providing additional support for the black-hole hypothesis. The Seyfert galaxy NGC6814 has been thought to be unique among active galaxies, in exhibiting not simply variability but a periodicity of approximately 12,100 s in its X-ray luminosity1-4. Many exotic theories have been proposed to explain this periodicity, including, gravitational lensing of hotspots on the accretion disk by the central black hole5-7 or the effects of a captured star orbiting the black hole4,8-10. Here we show, using data from the Rosat X-ray telescope, that although the 12,100 s period is indeed real and stable, it is not associated with NGC6814. Instead, the periodic emission arises from another source, 37 arcmin away from NGC6814, which is probably an object in our Galaxy-for example, a white dwarf accreting gas (from a giant companion) onto its poles.