XMM - Newton VIEW OF THE PROTOTYPE NARROW - LINE SEYFERT 1 GALAXY I ZW 1

We present results based on an XMM-Newton observation of the prototype narrow-line Seyfert 1 galaxy I Zw 1 performed in June 2002. The 0.6−12 keV spectrum is well described by a steep power-law (Γ ∼ 2.3) and a weak soft blackbody component (kT ∼ 200 eV) below 2 keV. The soft X-ray excess emission is featureless and contributes only ∼ 10% to the total X-ray emission in the 0.6− 2 keV band. There are tentative evidences for an iron iron Kα line and an iron K edge, both from high ionized He-like iron. The 0.6 − 12 keV continuum of I Zw 1 can be explained in terms of a blackbody (kT ∼ 40 eV), thermal comptonization and its reflection off the surface of an ionized accretion disk. We note that characterizing the soft excess copmonent ‘above a power law continuum’ may not be appropriate due to the low energy curvature of a comptonized spectrum. X-ray emission from I Zw 1 was highly variabile during the observation. I Zw 1 showed a large, symmetric X-ray flare during which the X-ray luminosity increased by ∆L ∼ 6 × 10 erg s in an interval of ∼ 2800 s only. The X-ray spectral variability was unusual during the flare. In contrast to the general observation of spectral steepening with flux from Seyfert galaxies, there is a clear evidence for hardening of the 0.6 − 10 keV spectrum of I Zw 1 with increasing flux. This hardening is due to a stronger and slightly flatter power-law component at higher flux without any change in the soft X-ray excess component. The large flare, the accompanying spectral flattening , and the steady nature of the soft excess component may be due to an additional flatter power-law component during the flare possibly arising as a result of ejection of electron plasma and the SSC process. Subject headings: galaxies: active – galaxies: individual: I Zw 1 – galaxies: Seyfert – X-rays: galaxies