EVIDENCE FOR X-RAY FLUX AND SPECTRAL MODULATION BY ABSORPTION IN NGC-6814 .1. THE NATURE OF THE MOST RAPID VARIABILITY

The Seyfert 1 galaxy NGC 6814 was observed using the Japanese X-ray as tronomy satellite Ginga in 1990 April and October. The rapid variabili ty characteristically associated with this source was reconfirmed. Spe cifically, three dips were found in the April observation in which the flux dropped to nearly zero in similar to 300 s. The doubling timesca le was similar to 50 s. A similar but separated drop and rise in flux was observed in the October data, different in that the flux did not d ecrease completely to zero. A detailed analysis of the data around the structures of most rapid variability found spectral variability and l ags in flux between different energy bands. Lags were on the order of a few to tens of seconds for the April data, and on the order of tens to a couple of hundred seconds for the October data. The sense of the lags was such that during flux decreases the hard flux lagged, while d uring flux increases the soft flux lagged. Associated significant appa rent hardening of the spectrum at low flux was observed in the April d ata. Apparent hardening of the spectrum also occurred in the October f lux decrease, to a photon index of Gamma = 0.85; however, the spectrum softened at lowest flux to the index of the predecrease level, Gamma = 1.54. In the April dips, the line flux was found to decrease signifi cantly. A marginal decrease in line flux was also observed in the Octo ber data. The variability of the line flux reconfirmed the result of K unieda et al. (1990), who found that the line producton region must be within similar to 300 light-seconds from the source. To explain the o bservational results, a variable-absorption model was proposed, in whi ch the column density was assumed to vary as a function of time. The t ime dependence of the column was determined in two ways. First, the fu nctional dependence was assumed to be exponential, and model parameter s were derived by fitting explicitly to the lag data obtained from the October observation. Second, a partial-covering spectral model was as sumed, and the column density implied by the change in flux was found for both observations. The exponential folding time of the column dens ity implied during the October flux decrease could be reliably determi ned to be about 100 s. The spectral variability could be qualitatively explained by the variable-absorption model, since superposition of co ntinuously variable columns can result in an apparent hardening of the spectrum when measured by moderate-resolution instruments. The differ ences between the April and October observations could be explained if the two model parameters, the fraction of unabsorbed flux and the gra dient with respect to time of the column density, were adjusted. Other physical processes, including intrinsic spectral changes and warm abs orber models, which could account for the fastest variability, could b e ruled out by the results of the data analysis. A primary constraint found for geometrical models is that the material which is doing the a bsorption must have relatively low ionization (xi < 100). This result leads to severe constraints on a general orbiting cloud model, requiri ng high densities (n similar to 10(16) cm(-3)) and sheetlike geometry.