The X-ray Spectral Variability of the Seyfert Galaxy NGC 3227

Authors: George, I.M. 1,2 , Mushotzky, R.F. 1 , Turner, T.J., 1,2 Yaqoob, T. 1,2 , Ptak, A. 1,3 , Nandra, K. 1,4 , Netzer, H. 5 ,
Affiltns: 1 LHEA, NASA/GSFC, 2 USRA, 3 Carnegie Mellon Univ, 4 NAS/NRC, 5Tel Aviv University,
Journal: ApJ, 509, 146
Publication Date: 1998 Dec 10

Abstract

We present the results from ASCA observations of NGC 3227 performed during 1993 and 1995, along with those from a ROSAT observation performed pseudo-simultaneously with the former. We find the 0.6-10 keV continuum to be consistent with a powerlaw with a photon index Gamma approx 1.6, flatter than that typically observed in Seyfert galaxies confirming previous results. Significant Fe K-shell emission is observed during both epochs, with an equivalent width and profile typical of Seyfert 1 galaxies. The ASCA observations in 1993 reveal absorption by a screen N_{H,z}^{ion} approxeq 3x10^{21} cm^{-2} of ionized material with an X-ray ionization parameter Ux approxeq 0.01. Both the column and ionization-state of this material are at the low end of the distribution of parameters observed for Seyfert 1 galaxies. Joint analysis of the ASCA and ROSAT data at this epoch show an additional screen of neutral material instrinsic to NGC 3227 with N_{H,z}^{neu} approx few x10^{20} cm^{-2}.

We find NGC 3227 to exhibit significant spectral variability both within and between the observations. The most likely explanation involves short-term variability in the continuum emission and longer-term variability in the column density of the ionized material. Time-resolved spectroscopy and color-color analysis indicate that the slope of the continuum steepened by DeltaGamma approxeq 0.1 during a flare of duration approx 10^4s, within the 1993 observation. However we were unable to distinguish between a steepening of the 'primary' continuum and a change in the relative strengths of the power-law and a putative Compton-reflection component. The absorbing column increased by a factor of approx 10 by the 1995 epoch, while the continuum is consistent with that observed in 1993. The 1995 data also show evidence that the warm absorber allows approx 10% of the nuclear emission to escape without attenuation.

We review our findings in the context of the previous results from this and similar objects and discuss the prospects of future observations.


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