Abstract

We report on the results of detailed X-ray spectroscopy of the Fe-K region in the Seyfert 1 galaxy NGC 3783 using the Chandra High Energy Grating Transmission Spectrometer ( HETGS ) .
There were five observations over an interval of \sim 125 days in 2001 , each with an exposure time of \sim 170 ks .
The combined data constitute the highest signal-to-noise Fe-K spectrum having the best velocity resolution in the Fe-K band to date ( FWHM \sim 1860 km s ^ { -1 } ) .
The combined data show a resolved Fe K \alpha line core ( FWHM = 1700 ^ { +410 } _ { -390 } km s ^ { -1 } ) with a center energy of 6.397 \pm 0.003 keV , consistent with an origin in neutral or lowly ionized Fe , located between the BLR and NLR , as found by Kaspi et al . ( 2002 ) .
We also find that excess flux around the base of the Fe K \alpha line core can be modeled with either a Compton scattering “ shoulder ” or an emission line ( with about the same flux as the line core ) from a relativistic accretion disk , having an inclination angle of 11 ^ { \circ } or less .
This disk line model is as good as a Compton-shoulder model for the base of the Fe K \alpha line core .
In the latter model , we measured the column density to be 7.5 ^ { +2.7 } _ { -0.6 } \times 10 ^ { 23 } cm ^ { -2 } , which corresponds to a Thomson optical depth of \sim 0.60 , so the line-emitting matter is not quite Compton-thick .
An intrinsic width of 1500 ^ { +460 } _ { -340 } km s ^ { -1 } FWHM is still required in this model .
Moreover , more complicated scenarios involving both a Compton-shoulder and a disk line can not be ruled out .
We confirm an absorption feature due to He-like Fe ( FWHM = 6405 ^ { +5020 } _ { -2670 } km s ^ { -1 } ) , found in previous studies .

Keywords : accretion disks – galaxies : active – line : profile – X-rays : galaxies

To appear in the Astrophysical Journal , 10 July 2005