Multiwavelength Monitoring of the Narrow‐Line Seyfert 1 Galaxy Arakelian 564. I.ASCAObservations and the Variability of the X‐Ray Spectral Components

Abstract

We present a 35 day ASCA observation of the Narrow-Line Seyfert 1 galaxy Ark 564, yielding an on-source exposure of ~1 Ms. The ASCA observation 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 to a factor of ~16 and changes in the fractional amplitude of variability across the observation. Ark 564 shows small variations in photon index across the observation, with Γ 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 components over a timescale of weeks, with the hump varying by a factor of ~6 across the 35 day observation compared to a factor of 4 in the power law. This difference in the long-term amplitudes of variation causes changes in the softness ratio across the observation. Flux variations in the power-law component are measured down to a timescale of ~1000 s with 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 ~1 day. We detect Fe Kα and a blend of Fe Kβ plus Ni Kα, the line energies indicating an origin in highly ionized gas. Variability measurements constrain the bulk of the Fe Kα line to originate within approximately 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.