NASA NTRS2001
We find that variability of the iron K alpha line is common in Seyfert 1 galaxies. Using data from the ASCA archive for objects that have been observed more than once during the mission, we study the time-averaged spectra from individual observations, thereby probing variability on timescales that range from days to years. Since the statistics of the data do not warrant searches for line variability in terms of a complex physical model, we use a simple Gaussian to model the gross shape of the line and then use the centroid energy, intensity, and equivalent width as robust indicators of changes in the line profile. We find that approximately 70% of Seyfert 1 galaxies (10 out of 15) show variability in at least one of these parameters: the centroid energy, intensity, and equivalent width vary in six, four, and eight sources, respectively. Because of the low signal-to-noise ratio, limited sampling, and time averaging, we consider these results to represent lower limits to the rate of incidence of variability. In most cases changes in the line do not appear to track changes in the continuum. In particular, we find no evidence for variability of the line intensity in NGC 4151, suggesting an origin in a region larger than the putative accretion disk, where most of the iron line has been thought to originate. Mrk 279 is investigated on short timescales. The time-averaged effective line energy (as measured by the Gaussian center energy, which is weighted by emission in the entire line profile) is 6.5 keV in the galaxy rest frame. As the continuum flux increases by 20% in a few hours, the Fe K line responds within approximately 10,000 seconds with the effective line energy increasing by 0.22 keV (approximately 10,500 kilometers per second). We also examine the ROSAT PSPC spectrum of Mrk 279 but find inconsistencies with ASCA. Problems with the ASCA and ROSAT calibration that affect simultaneous spectral fits at low energies are discussed in an appendix.