Session 24 - Seyfert & Starburst Galaxies; Winds, Outflows & Energetics.
Oral session, Monday, January 15
Corte Real, Hilton

[24.06] The Longest EUVE Observation of a Seyfert Galaxy: RX J0437.4--4711

H. L. Marshall (MIT), J. P. Halpern (Columbia U.)

We monitored the soft X-ray-selected Seyfert galaxy RX J0437.4--4711 with the EUVE\/ satellite continuously for 20 days in 1994 October-November, in parallel with the millisecond pulsar J0437--4715 that lies only 4^\prime\!.2 from it. An ordinary Seyfert 1 galaxy, RX J0437.4--4711 (z = 0.052) is one of the best detected by EUVE\/ in large part because of its fortuitously small Galactic N_H of (1.01 \pm 0.16) \times 10^20 cm^-2 (as determined by a fit to the ROSAT\/ data). Its EUVE\/ spectrum in the 70--110 Å\ range is consistent with the ROSAT\/ power-law energy index \alpha = 1.56 \pm 0.09 and shows no evidence of emission lines. Its light curve shows large-amplitude variability by a factor of 4 over the 20-day period, and a minimum doubling time of 5 hours.

These data constitute a uniformly sampled soft X-ray light curve of a highly variable Seyfert galaxy whose power spectrum can be examined on time scales from 3 hours to 10 days. The power spectrum is fitted by a logarithmic slope of -1.25 \pm 0.25. A possible period of 0.906 \pm 0.018 days is also apparent in the light curve and power spectrum, which might correspond to orbital motion in the inner accretion disk around a black hole of mass \sim 10^8\,M_ødot. The range of time scales sampled allows one to discriminate between shot-noise and rotating-spot models, the latter predicting periods or quasi-periods similar to the possible 0.9 d signal that we see. The shot-noise model, on the other hand, predicts two breaks in the power spectrum, corresponding to the shots with the shortest and longest decay times. Such breaks are not clearly seen. The power continues to diverge down to a time scale of 10 days, indicating that longer EUV observations are necessary to reveal the characteristic time scales and amplitudes of variability at the peak of the Seyfert spectral energy distribution.