Issue |
A&A
Volume 444, Number 1, December II 2005
|
|
---|---|---|
Page(s) | 79 - 99 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20052881 | |
Published online | 21 November 2005 |
XMM-Newton observations of the Lockman Hole IV: spectra of the brightest AGN
1
Instituto de Física de Cantabria (CSIC-UC), 39005 Santander, Spain e-mail: sm279@star.le.ac.uk
2
X-ray Astronomy Group, Department of Physics and Astronomy, Leicester University, Leicester LE1 7RH, UK
3
Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, Garching 85748, Germany
4
Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 OHA, UK
Received:
15
February
2005
Accepted:
27
June
2005
This paper presents the results of a detailed X-ray spectral analysis
of a sample of 123 X-ray sources detected with XMM-Newton
in the Lockman Hole field.
This is the deepest observation carried out with XMM-Newton with
more that 600 ks of good EPIC-pn data.
We have spectra with good signal to noise (>500 source counts) for all objects down to 0.2-12 keV fluxes of
~ (flux limit of ~
in the 0.5-2 and 2-10 keV bands). At the time of the analysis, we
had optical spectroscopic identifications for 60% of the
sources, 46 being optical type-1 AGN and 28 optical type-2 AGN.
Using a single power law model our sources' average spectral slope
hardens at faint 0.5-2 keV fluxes but not at faint 2-10 keV fluxes. We have been able to explain
this effect in terms of an increase in X-ray absorption at faint fluxes.
We did not find in our data
any evidence for the existence of a population of faint intrinsically harder sources. The average
spectral slope of our sources is ~1.9,
with an intrinsic dispersion of ~0.28.
We detected X-ray absorption (F-test significance ≥95%) in 37% of the sources, ~10% in type-1 AGN (rest-frame
)
and ~
(rest-frame
) in type-2 AGN.
Using X-ray fluxes corrected for absorption,
the fraction of absorbed objects and the absorbing column density distribution did not
vary with X-ray flux.
Our type-1 and type-2 AGN do not appear to have
different continuum shapes, but the distribution of intrinsic
(rest-frame) absorbing column densities is different among both classes.
A significant fraction of our type-2 AGN (5 out of 28) were found to display
no substantial absorption (
).
We discuss possible interpretations to this in terms of
Compton-thick AGN and intrinsic Broad Line Region properties.
An emission line compatible with Fe Kα was detected in 8 sources (1 type-1 AGN, 5 type-2 AGN
and 2 unidentified) with rest frame equivalent widths 120-1000 eV. However weak
broad components can be easily missed in other sources
by the relatively noisy data.
The AGN continuum or intrinsic
absorption did not depend on X-ray luminosity and/or redshift.
Soft excess emission was detected in 18 objects, but only in 9 (including 4 type-1 AGN and 4 type-2 AGN)
could we fit this spectral component with a black body model.
The measured 0.5-2 keV luminosities of the fitted
black body were not significantly different in type-1 and type-2 AGN,
although the temperatures of the black body
were slightly higher in type-2 AGN (
) than
in type-1 AGN (
).
For 9 sources (including 1 type-1 AGN and 3 type-2 AGN) a
scattering model provided a better fit of the soft excess emission.
We found that the integrated contribution from our sources to the X-ray
background in the 2-7 keV band is softer (
) than the background itself, implying that fainter sources need to be more absorbed.
Key words: X-rays: general / X-rays: diffuse background / surveys / galaxies: active
© ESO, 2005
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