Issue |
A&A
Volume 396, Number 3, December IV 2002
|
|
---|---|---|
Page(s) | 761 - 772 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361:20020918 | |
Published online | 05 December 2002 |
The soft X-ray spectrum from NGC 1068 observed with LETGS on Chandra
1
SRON, National Institute for Space Research, Sorbonnelaan 2, 3548 CA Utrecht, The Nether lands
2
Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY 10027, USA
3
Theoretical Astrophysics and Space Radiation Laboratory, California Institute of Technology, MC 130-33, Pasadena, CA 91125, USA
Corresponding author: J. S. Kaastra, J.Kaastra@sron.nl
Received:
15
April
2002
Accepted:
17
June
2002
Using the combined spectral and spatial resolving power of the Low
Energy Transmission Grating (LETGS) on board Chandra, we obtain separate
spectra from the bright central source of NGC 1068 (Primary region), and from a
fainter bright spot 4´´ to the NE (Secondary region). Both
spectra are dominated by discrete line emission from H- and He-like ions of C
through S, and from Fe L-shell ions, but also include narrow radiative
recombination continua (RRC), indicating that most of the observed soft X-ray
emission arises in low-temperature ( few eV) photoionized
plasma. We confirm the conclusions of Kinkhabwala et al.
([CITE]), based on XMM-Newton Reflection Grating
Spectrometer (RGS) observations, that the entire nuclear spectrum can be
explained by recombination/radiative cascade following photoionization, and
radiative decay following photoexcitation, with no evidence for the presence of
hot, collisionally ionized plasma. In addition, we show that this same model
also provides an excellent fit to the spectrum of the Secondary region, albeit
with radial column densities roughly a factor of three lower, as would be
expected given its distance from the source of the ionizing continuum. The
remarkable overlap and kinematical agreement of the optical and X-ray line
emission, coupled with the need for a distribution of ionization parameter to
explain the X-ray spectra, collectively imply the presence of a distribution of
densities (over a few orders of magnitude) at each radius in the ionization
cone. Relative abundances of all elements are consistent with Solar abundance,
except for N, which is 2–3 times Solar. Finally, the long wavelength spectrum
beyond 30 Å is rich of L-shell transitions of Mg, Si, S, and Ar, and
M-shell transitions of Fe. The velocity dispersion decreases with increasing
ionization parameter, which has been deduced from the measured line intensities
of particularly these long wavelength lines in conjunction with the Fe-L
shell lines.
Key words: galaxies: individual: NGC 1068 / galaxies: Seyfert / quasars: emission lines / X-rays: galaxies
© ESO, 2002
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