Bibcode
Mateos, S.; Barcons, X.; Carrera, F. J.; Ceballos, M. T.; Hasinger, G.; Lehmann, I.; Fabian, A. C.; Streblyanska, A.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 444, Issue 1, December II 2005, pp.79-99
Fecha de publicación:
12
2005
Revista
Número de citas
85
Número de citas referidas
81
Descripción
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 5×10-15 erg cm-2
s-1 (flux limit of 6×10-16 erg
cm-2 s-1 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
{NH ˜ 1.6 × 1021{-}1.2 ×
1022 cm-2}) and 77% (rest-frame {NH
˜ 1.5 ×1021{-}4× 1023
cm-2}) 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
({NH<1021 cm-2}). 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 (<
{kT}>=0.26±0.08) than in type-1 AGN (<
{kT}>=0.09±0.01). 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 (Γ=1.5{-}1.6)
than the background itself, implying that fainter sources need to be
more absorbed.