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S.B. Kraemer (CUA/NASA's GSFC), D.M. Crenshaw (Georgia State Univ.), I.M. George (UMBC/NASA's GSFC), J.R. Gabel (Univ. of Colorado), NGC 4151 Team
We present a detailed analysis of the intrinsic X-ray absorption in the Seyfert 1 galaxy NGC 4151 using Chandra/High Energy Transmission Grating Spectrometer data obtained 2002 May as part of a program which included simultaneous ultraviolet (UV) spectra using the Hubble Space Telescope/Space Telescope Imaging Spectrograph and the Far Ultraviolet Spectrographic Explorer. Previous studies, most recently using ASCA spectra, revealed a large (> 1022 cm-2) column of intervening gas, which has varied both in ionization state and total column density. NGC 4151 was in a relatively low flux state during the observations reported here , although roughly 2.5 times as bright in the 2--10 keV band as during a Chandra observation in 2000. At both epochs, the soft X-ray band was dominated by emission lines, which show no discernible variation in flux between the two observations. The 2002 Chandra data show the presence of a very highly ionized absorber, in the form of H-like and He-like Mg, Si, and S lines, as well as lower ionization gas via the presence of inner-shell absorption lines from lower-ionization species of these elements. The latter accounts for both the bulk of the soft X-ray absorption and the high covering factor UV absorption lines of O~VI, C~IV, and N~V with outflow velocities ~ 500 km s-1. The presence of high ionization gas, which is not easily detected at low resolution (e.g., with ASCA), appears common among Seyfert galaxies. Since this gas is too highly ionized to be radiatively accelerated in sources such as NGC 4151, which is radiating at a small fraction of its Eddington Luminosity, it may be key to understanding the dynamics of mass outflow. We find that the deeper broad-band absorption detected in the 2000 Chandra data is the result of both 1) lower ionization of the intervening gas due to the lower ionizing flux and 2) a factor of ~ 3 higher column density of the lower ionization component. To account for this bulk motion, we estimate that this component must have a velocity > 1250 km s-1 transverse to our line-of-sight. This is consistent with the rotational velocity of gas arising from the putative accretion disk. While both thermal wind and magneto-hydrodynamic models predict large non-radial motions, we suggest that the latter mechanism is more consistent with the results of the photoionization models of the absorbers
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.