1,822 research outputs found
XMM-Newton observation of the ULIRG NGC 6240: The physical nature of the complex Fe K line emission
We report on an XMM-Newton observation of the ultraluminous infrared galaxy
NGC 6240. The 0.3-10 keV spectrum can be successfully modelled with: (i) three
collisionally ionized plasma components with temperatures of about 0.7, 1.4,
and 5.5 keV; (ii) a highly absorbed direct power-law component; and (iii) a
neutral Fe K_alpha and K_beta line. We detect a significant neutral column
density gradient which is correlated with the temperature of the three plasma
components. Combining the XMM-Newton spectral model with the high spatial
resolution Chandra image we find that the temperatures and the column densities
increase towards the center.
With high significance, the Fe K line complex is resolved into three distinct
narrow lines: (i) the neutral Fe K_alpha line at 6.4 keV; (ii) an ionized line
at about 6.7 keV; and (iii) a higher ionized line at 7.0 keV (a blend of the Fe
XXVI and the Fe K_beta line). While the neutral Fe K line is most probably due
to reflection from optically thick material, the Fe XXV and Fe XXVI emission
arises from the highest temperature ionized plasma component.
We have compared the plasma parameters of the ultraluminous infrared galaxy
NGC 6240 with those found in the local starburst galaxy NGC 253. We find a
striking similarity in the plasma temperatures and column density gradients,
suggesting a similar underlying physical process at work in both galaxies.Comment: 8 pages including 9 figures. Accepted for publication in A&
1ES 1927+654: Persistent and rapid X-ray variability in an AGN with low intrinsic neutral X-ray absorption and narrow optical emission lines
We present X-ray and optical observations of the X-ray bright AGN 1ES
1927+654. The X-ray observations obtained with ROSAT and Chandra reveal
persistent, rapid and large scale variations, as well as steep 0.1-2.4 keV
(Gamma = 2.6 +/- 0.3) and 0.3-7.0 keV (Gamma = 2.7 +/- 0.2) spectra. The
measured intrinsic neutral X-ray column density is approximately 7e20cm^-2. The
X-ray timing properties indicate that the strong variations originate from a
region, a few hundred light seconds from the central black hole, typical for
type 1 AGN. High quality optical spectroscopy reveals a typical Seyfert 2
spectrum with some host galaxy contamination and no evidence of Fe II
multiplets or broad hydrogen Balmer wings. The intrinsic optical extinction
derived from the BLR and NLR are A_V >= 3.7 and A_V=1.7, respectively. The
X-ray observations give an A_V value of less than 0.58, in contrast to the
optical extinction values. We discuss several ideas to explain this apparent
difference in classification including partial covering, an underluminous BLR
or a high dust to gas ratio.Comment: 8 pages including 10 figures. Accepted for publication in Astronomy
and Astrophysic
Intercomparison of stratospheric chemistry models under polar vortex conditions
Several stratospheric chemistry modules from box, 2-D or 3-D models, have been intercompared. The intercomparison was focused on the ozone loss and associated reactive species under the conditions found in the cold, wintertime Arctic and Antarctic vortices. Comparisons of both gas phase and heterogeneous chemistry modules show excellent agreement between the models under constrained conditions for photolysis and the microphysics of polar stratospheric clouds. While the mean integral ozone loss ranges from 4-80% for different 30-50 days long air parcel trajectories, the mean scatter of model results around these values is only about +/-1.5%. In a case study, where the models employed their standard photolysis and microphysical schemes, the variation around the mean percentage ozone loss increases to about +/-7%. This increased scatter of model results is mainly due to the different treatment of the PSC microphysics and heterogeneous chemistry in the models, whereby the most unrealistic assumptions about PSC processes consequently lead to the least representative ozone chemistry. Furthermore, for this case study the model results for the ozone mixing ratios at different altitudes were compared with a measured ozone profile to investigate the extent to which models reproduce the stratospheric ozone losses. It was found that mainly in the height range of strong ozone depletion all models underestimate the ozone loss by about a factor of two. This finding corroborates earlier studies and implies a general deficiency in our understanding of the stratospheric ozone loss chemistry rather than a specific problem related to a particular model simulation
Development of superconducting and cryogenic technology in the Institute for Technical Physics (ITP) of the Research Center Karlsruhe
Optical spectroscopy of EX Lupi during quiescence and outburst: Infall, wind, and dynamics in the accretion flow
We explore the accretion mechanisms in EX Lupi, prototype of EXor variables,
during its quiescence and outburst phases. We analyse high-resolution optical
spectra taken before, during, and after its 2008 outburst. In quiescence and
outburst, the star presents many permitted emission lines, including typical
CTTS lines and numerous neutral and ionized metallic lines. During the
outburst, the number of emission lines increases to over a thousand, with
narrow plus broad component structure (NC+BC). The BC profile is highly
variable on short timescales (24-72h). An active chromosphere can explain the
metallic lines in quiescence and the outburst NC. The dynamics of the BC line
profiles suggest an origin in a hot, dense, non-axisymmetric, and non-uniform
accretion column that suffers velocity variations along the line-of-sight on
timescales of days. Assuming Keplerian rotation, the emitting region would be
located at ~0.1-0.2 AU, consistent with the inner disk rim, but the velocity
profiles of the lines reveal a combination of rotation and infall. Line ratios
of ions and neutrals can be reproduced with a temperature of T~6500 K for
electron densities of a few times 10cm in the line-emitting
region. The data confirm that the 2008 outburst was an episode of increased
accretion, albeit much stronger than previous EX Lupi and typical EXors
outbursts. The line profiles are consistent with the infall/rotation of a
non-axisymmetric structure that could be produced by clumpy accretion during
the outburst phase. A strong inner disk wind appears in the epochs of higher
accretion. The rapid recovery of the system after the outburst and the
similarity between the pre-outburst and post-outburst states suggest that the
accretion channels are similar during the whole period, and only the accretion
rate varies, providing a superb environment for studying the accretion
processes.Comment: 15 pages plus 26 pages online material, accepted by A&
Transport properties of CuGaSe(2)-based thin-film solar cells as a function of absorber composition
The transport properties of thin-film solar cells based on wide-gap CuGaSe(2) absorbers have been investigated as a function of the bulk [Ga]/[Cu] ratio ranging from 1.01 to 1.33. We find that (i) the recombination processes in devices prepared from absorbers with a composition close to stoichiometry ([Ga]/[Cu] = 1.01) are strongly tunnelling assisted resulting in low recombination activation energies (E(a)) of approx. 0.95 eV in the dark and 1.36 eV under illumination. (ii) With an increasing [Ga]/[Cu] ratio, the transport mechanism changes to be dominated by thermally activated Shockley-Read-Hall recombination with similar E(a) values of approx. 1.52-1.57 eV for bulk [Ga]/[Cu] ratios of 1.12-1.33. The dominant recombination processes take place at the interface between CdS buffer and CuGaSe(2) absorber independently from the absorber composition. The increase of E(a) with the [Ga]/[Cu] ratio correlates with the open circuit voltage and explains the better performance of corresponding solar cells
The XMM-SSC survey of hard-spectrum XMM-Newton sources 1: optically bright sources
We present optical and X-ray data for a sample of serendipitous XMM-Newton
sources that are selected to have 0.5-2 keV vs 2-4.5 keV X-ray hardness ratios
which are harder than the X-ray background. The sources have 2-4.5 keV X-ray
flux >= 10^-14 cgs, and in this paper we examine a subsample of 42 optically
bright (r < 21) sources; this subsample is 100 per cent spectroscopically
identified. All but one of the optical counterparts are extragalactic, and we
argue that the single exception, a Galactic M star, is probably a coincidental
association. The X-ray spectra are consistent with heavily absorbed power laws
(21.8 < log NH < 23.4), and all of them appear to be absorbed AGN. The majority
of the sources show only narrow emission lines in their optical spectra,
implying that they are type-2 AGN. Only a small fraction of the sources (7/42)
show broad optical emission lines, and all of these have NH < 10^23 cm^-2. This
implies that ratios of X-ray absorption to optical/UV extinction equivalent to
> 100 times the Galactic gas-to-dust ratio are rare in AGN absorbers (at most a
few percent of the population), and may be restricted to broad absorption-line
QSOs. Seven objects appear to have an additional soft X-ray component in
addition to the heavily absorbed power law. We consider the implications of our
results in the light of the AGN unified scheme. We find that the soft
components in narrow-line objects are consistent with the unified scheme
provided that > 4 per cent of broad-line AGN have ionised absorbers that
attenuate their soft X-ray flux by >50 per cent. In at least one of the X-ray
absorbed, broad-line AGN in our sample the X-ray spectrum requires an ionised
absorber, consistent with this picture.Comment: accepted for publication in MNRA
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