Variable iron K alpha lines in Seyfert 1 galaxies

We find that variability of the iron K-alpha line is common in Seyfert 1 ga laxies. Using data from the ASCA archive for objects that have been observe d more than once during the mission, we study the time-averaged spectra fro m individual observations, thereby probing variability on timescales that r ange from days to years. Since the statistics of the data do not warrant se arches 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 centr oid energy, intensity, and equivalent width as robust indicators of changes in the line profile. We find that similar to 70% of Seyfert 1 galaxies (10 out of 15) show variability in at least one of these parameters : the cent roid energy, intensity, and equivalent width vary in six, four, and eight s ources, respectively. Because of the low signal-to-noise ratio, limited sam pling, and time averaging, we consider these results to represent lower lim its to the rate of incidence of variability. In most cases changes in the l ine 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 o f the iron line has been thought to originate. Mrk 279 is investigated on s hort timescales. The time-averaged effective line energy (as measured by th e 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 increas es by 20% in a few hours, the Fe K line responds within similar to 10,000 s , with the effective line energy increasing by 0.22 keV (similar to 10,500 km s(-1)). We also examine the ROSAT PSPC spectrum of Mrk 279 but find inco nsistencies with ASCA. Problems with the ASCA and ROSAT calibration that af fect simultaneous spectral fits at low energies are discussed in an appendi x.