Multiwavelength monitoring of the Narrow-Line Seyfert 1 galaxy Arakelian 564. I. ASCA observations and the variability of the X-ray spectral components

We present a 35 day ASCA observation of the Narrow-Line Seyfert 1 galaxy Ar k 564, yielding an on-source exposure of similar to1 Ms. The ASCA observati on was part of a multiwavelength AGN Watch monitoring campaign. The soft X- ray light curve binned to 256 s reveals trough-to-peak flux variations up t o a factor of similar to 16 and changes in the fractional amplitude of vari ability across the observation. Ark 564 shows small variations in photon in dex across the observation, with Gamma in the range 2.45-2.72. The presence of the soft hump component below 1 keV, previously detected in ASCA data, is confirmed. Time-resolved spectroscopy with approximately daily sampling reveals a distinction in the variability of the soft hump and power-law com ponents over a timescale of weeks, with the hump varying by a factor of sim ilar to6 across the 35 day observation compared to a factor of 4 in the pow er law. This difference in the longterm amplitudes of variation causes chan ges in the softness ratio across the observation. Flux variations in the po wer-law component are measured down to a timescale of similar to 1000 s wit h accompanying spectral variability. Some correlated events are observed in the soft hump, UV flux, and hard X-ray flux when all are sampled daily. No significant UV to X-ray lags are found, with upper limits of similar to1 d ay. We detect Fe K alpha and a blend of Fe K beta plus Ni K alpha, the line energies indicating an origin in highly ionized gas. Variability measureme nts constrain the bulk of the Fe K alpha line to originate within approxima tely a light week of the nucleus. The large EW of the emission lines may be due to high metallicity in Narrow-line Seyfert 1 galaxies, supporting some evolutionary models for active galactic nuclei.