Discovery of a 23.8h QPO in the SWIFT light curve of XMMU J134736.6+173403

Abstract: XMMU J134736.6+173403 is an X-ray source discovered serendipitously by XMM-Newton which was found to be spatially coincident with a pair of galaxies, including a Seyfert 2 galaxy, but presented in 2003 a very sharp persistent flux drop of a factor 6.5 within 1h. From the analysis of a set of 29 Swift observations conducted from the 6 February to the 23 May 2008, we discovered twin-peak quasi-periodic oscillations (QPOs) with periods of 23.82+-0.07 h and 71.44+-0.57 h. Using a Chandra observation of 2008, we evaluate more accurately the position of the X-ray source and show that the new source coordinates coincide with the position of the Seyfert 2 galaxy. We provide a detailed spectral energy distribution of the AGN counterpart using multi-wavelength observations. The AGN is radio-loud and the broadband SED modelling indicates a black hole with a mass of 9.8x10^6 Msun, that accretes at an Eddington ratio of 0.047. QPOs for active galaxies have been reported so far in only few cases, the most reliable one being from RE J1034+396 for which a 1 h periodicity has been discovered analysing a ~91 ks XMM-Newton observation. Twin peak QPOs with an observed frequency ratio of 3:1 have not been reported so far for any AGN. From resonance models of the epicyclic frequencies we evaluate the different possible mass-spin relations. It's still not clear what could have been the origin of the high flux and sharp drop only observed in 2003.