A longer XMM-Newton look at I Zwicky 1: variability of the X-ray continuum, absorption and iron K alpha line

We present the second XMM-Newton observation (85 ks) of the narrow-line Seyfert 1 galaxy (NLS1) I Zw 1 and describe its mean spectral and timing characteristics. On average, I Zw 1 is similar to 35 per cent dimmer in 2005 than in the shorter (20 ks) 2002 observation. Between the two epochs the intrinsic absorption column density diminished, but there were also subtle changes in the continuum shape. Considering the blurred ionized reflection model, the long-term changes can be associated with a varying contribution of the power-law component relative to the total spectrum. Examination of normalized light curves indicates that the high-energy variations are quite structured and that there are delays, but only in some parts of the light curve. Interestingly, a hard X-ray lag first appears during the most distinct structure in the mean light curve, a flux dip similar to 25 ks into the observation. The previously discovered broad, ionized Fe K alpha line shows significant variations over the course of the 2005 observation. The amplitude of the variations is 25-45 per cent and they are unlikely due to changes in the Fe K alpha-producing region, but perhaps arise from orbital motion around the black hole or obscuration in the broad iron-line-emitting region. The 2002 data are re-examined for variability of the Fe K alpha line at that epoch. There is evidence of energy and flux variations that are associated with a hard X-ray flare that occurred during that observation.