An Extreme Ultraviolet Explorer Atlas of Seyfert Galaxy Light Curves: Search for Periodicity

The Deep Survey instrument on the Extreme Ultraviolet Explorer (EUVE) satellite obtained long, nearly continuous soft X-ray light curves of 5-33 days duration for 14 Seyfert galaxies and quasars. We present a uniform reduction of these data, which account for a total of 231 days of observation. Several of these light curves are well suited to a search for periodicity or quasi-periodic oscillations in the range of hours to days that might be expected from dynamical processes in the inner accretion disk around ~108 M☉ black holes. Light curves and periodograms of the three longest observations show features that could be transient periods: 0.89 days in RX J0437.4-4711, 2.08 days in Ton S180, and 5.8 days in 1H 0419-577. The statistical significance of these signals is estimated using the method of Timmer & König, which carefully takes into account the red-noise properties of Seyfert light curves. The result is that the signals in RX J0437.4-4711 and Ton S180 exceed 95% confidence with respect to red noise, while 1H 0419-577 is only 64% significant. These period values appear unrelated to the length of the observation, which is similar in the three cases, but they do scale roughly as the luminosity of the object, which would be expected in a dynamical scenario if luminosity scales with black hole mass.