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H.L. Marshall (MIT CSR), J.P. Halpern (Columbia U.), K.M. Leighly (U. Oklahoma)
The Deep Survey instrument on the Extreme Ultraviolet Explorer satellite (EUVE\/) obtained a 33 day long, nearly continuous soft X-ray light curve of the Seyfert galaxy Ton S180 that is well suited to a search for transient periodicity or QPO in the range of hours to days that might be expected from dynamical processes in the inner accretion disk around a ~108\,M\odot black hole. Periodogram analysis of this highly variable light curve shows a feature at 2.08 days. We estimate the statistical significance of the signal using a method that carefully takes into account the red-noise properties of Seyfert light curves. The result is that the signal is significant at the 98% confidence level with respect to power-law noise.
We also performed a uniform reduction of all of the EUVE\/ light curves of 14 Seyfert galaxies and QSOs that exceed 5 days in duration, which account for a total of 231 days of observation. From the two observations (in addition to Ton S180) that were longer than 20 days in duration, we see a possible 0.89 day period in RX J0437.4--4711 with 96% significance, and a less significant signal at 5.8 days in 1H~0419--577. 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 dynamical scenarios such as orbital motion or trapped oscillations in the inner accretion disk, if luminosity scales with black-hole mass.
Although it is generally true that variability amplitude in AGNs increases with increasing photon energy, it is now clear that EUV variability is as dramatic as any detected at higher energies. In many objects this component contains the bulk of the bolometric luminosity. Theory is in fact quite challenged to produce the large-amplitude EUV variability that we observe. Because of the energetics, one cannot beg the question of the mechanism of variability, as is often done, by appealing to reprocessing of harder X-rays. The EUV is where we must search for the fundamental reason why AGNs vary.
Bulletin of the American Astronomical Society,
© 2003. The American Astronomical Soceity.