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Session 50 - Active Galaxies.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center
We present the first measurements of the Fe K_\alpha line profile in NGC 4151 from ASCA observations performed in May, November and December 1993. The apparent line profile is asymmetric, consisting of a peak at \sim 6.3-6.4 keV, a sharp drop on the blue side below \sim 7 keV and a broad red wing extending to \sim 4-5 keV. This first clear indication of complex structure confirms earlier tentative measurements of the Fe-K region with BBXRT. The line intensity, equivalent width and detailed shape are model-dependent. Interpretation of the profile in terms of emission from a disk rotating around a central black hole strongly constrains the putative disk to be face-on, the inferred inclination angle being 0^+19_-0 degrees. However this contradicts the current body of knowledge of the geometry of this source. If the apparent red wing is not in fact part of a peculiar continuum then a face-on disk may be allowed only if the collimated structure observed at the parsec scale is misaligned with respect to the disk normal by at least 20 degrees, iron is overabundant by a factor of \sim 2, with some additional caveats. Alternatively, a different geometry of line-emitting material may be required. Moreover, the narrow core of the iron line may have a separate origin in material further out from the center (e.g. a molecular torus). Even then, a putative disk producing the residual broad line is still required to be face-on. Finally, in contrast to clear variability in the soft X-ray line emission below \sim 1 keV between the observations, these data show no significant variability of the Fe K_\alpha overall line shape, intensity or equivalent width.
Program listing for Tuesday