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S. Kaspi, M. J. Collinge, W. N. Brandt (PSU)
We present the first grating-resolution X-ray spectra of the Seyfert~1 galaxies NGC~3783 and NGC~4051, obtained with the High Energy Transmission Grating Spectrometer on the Chandra X-ray Observatory.
The spectrum of NGC~3783 reveals many narrow absorption lines from the H-like and He-like ions of O, Ne, Mg, Si, S and Ar, as well as Fe\,{\sc xvii}--Fe\,{\sc xxi} L-shell lines. We also have identified several weak emission lines, mainly from O and Ne. The absorption lines are blueshifted by a mean velocity of \approx 440±200~km\,s-1 and are not resolved, indicating a velocity dispersion within the absorbing gas of few hundred km\,s-1 or less. The emission lines have a marginally significant redshift of about 200 km\,s-1. We compare the Chandra spectrum to simultaneous ASCA and RXTE spectra and discuss the X-ray time variability of the object. We use photoionization models to explain the observed spectrum and to constrain the physical conditions within the nucleus of NGC~3783.
The spectrum of NGC~4051 shows fewer emission and absorption lines from H-like and He-like ions of O, Ne, Mg, and Si. The strongest emission lines we identify are from the O\,{\sc vii} triplet at 21.6, 21.8, and 22.1 Å. The spectrum is dominated by a continuum power law with no obviously detected oxygen absorption edges. As in NGC\,3783 the absorption lines are blueshifted by a few hundred km\,s-1 and the emission lines have a redshift of about one hundred km\,s-1. A simultaneous high-resolution UV spectrum taken by HST/STIS reveals several deep absorption systems in C\,{\sc iv} and N\,{\sc v}. The blueshift of the UV absorption systems relative to the systemic velocity ranges from 0 to 700 km\,s-1.
In both objects we investigate the relation between the X-ray absorber and the UV absorber, and we try to resolve whether both types of absorptions arise in the same region or in distinct ones. We also study the possibility that the x-ray emission from both objects is extended.