199 research outputs found
An XMM-Newton view of the serendipitous sources in the PKS0312-770 field
We describe an XMM-Newton observation of the PKS0312-770 field, which facilitates the spectral analysis of serendipitous sources previously detected by CHANDRA. The combination of larger effective area and longer exposure duration allows a significant increase in detected photons, and a lower limit in source detection sensitivity. In particular the hard X-ray normal galaxy unveiled by Fiore et al (2000) is most likely explained as a moderately absorbed (N_H ~ 1e22 cm^-2) AGN. We detect 52 sources (45 previously unreported) at a limiting flux of ~2e-15 cgs in the 0.5-2keV band. The LogN-LogS curve is consistent with that derived from by XMM-Newton observations of the Lockman Hole field. The flux determinations allow to check for any inconsistency between the calibrations of the two observatories, which is discussed
In-orbit Vignetting Calibrations of XMM-Newton Telescopes
We describe measurements of the mirror vignetting in the XMM-Newton
Observatory made in-orbit, using observations of SNR G21.5-09 and SNR
3C58 with the EPIC imaging cameras. The instrument features that complicate
these measurements are briefly described. We show the spatial and energy
dependences of measured vignetting, outlining assumptions made in deriving the
eventual agreement between simulation and measurement. Alternate methods to
confirm these are described, including an assessment of source elongation with
off-axis angle, the surface brightness distribution of the diffuse X-ray
background, and the consistency of Coma cluster emission at different position
angles. A synthesis of these measurements leads to a change in the XMM
calibration data base, for the optical axis of two of the three telescopes, by
in excess of 1 arcminute. This has a small but measureable effect on the
assumed spectral responses of the cameras for on-axis targets.Comment: Accepted by Experimental Astronomy. 26 pages, 18 figure
Performance study of ground-based infrared Bracewell interferometers - Application to the detection of exozodiacal dust disks with GENIE
Nulling interferometry, a powerful technique for high-resolution imaging of
the close neighbourhood of bright astrophysical objets, is currently considered
for future space missions such as Darwin or the Terrestrial Planet Finder
Interferometer (TPF-I), both aiming at Earth-like planet detection and
characterization. Ground-based nulling interferometers are being studied for
both technology demonstration and scientific preparation of the Darwin/TPF-I
missions through a systematic survey of circumstellar dust disks around nearby
stars. In this paper, we investigate the influence of atmospheric turbulence on
the performance of ground-based nulling instruments, and deduce the major
design guidelines for such instruments. End-to-end numerical simulations allow
us to estimate the performance of the main subsystems and thereby the actual
sensitivity of the nuller to faint exozodiacal disks. Particular attention is
also given to the important question of stellar leakage calibration. This study
is illustrated in the context of GENIE, the Ground-based European Nulling
Interferometer Experiment, to be installed at the VLTI and working in the L'
band. We estimate that this instrument will detect exozodiacal clouds as faint
as about 50 times the Solar zodiacal cloud, thereby placing strong constraints
on the acceptable targets for Darwin/TPF-I.Comment: A&A, accepte
New high-sensitivity, milliarcsecond resolution results from routine observations of lunar occultations at the ESO VLT
(Abridged) Lunar occultations (LO) are a very efficient and powerful
technique, that achieves the best combination of high angular resolution and
sensitivity possible today at near-infrared wavelengths. Given that the events
are fixed in time, that the sources are occulted randomly, and that the
telescope use is minimal, the technique is very well suited for service mode
observations. We have established a program of routine LO observations at the
VLT observatory, especially designed to take advantage of short breaks
available in-between other programs. We have used the ISAAC instrument in burst
mode, capable of producing continuous read-outs at millisecond rates on a
suitable subwindow. Given the random nature of the source selection, our aim
has been primarily the investigation of a large number of stellar sources at
the highest angular resolution in order to detect new binaries. Serendipitous
results such as resolved sources and detection of circumstellar components were
also anticipated. We have recorded the signal from background stars for a few
seconds, around the predicted time of occultation by the Moon's dark limb. At
millisecond time resolution, a characteristic diffraction pattern can be
observed. Patterns for two or more sources superimpose linearly, and this
property is used for the detection of binary stars. The detailed analysis of
the diffraction fringes can be used to measure specific properties such as the
stellar angular size and the presence of extended light sources such as a
circumstellar shell. We present a list of 191 stars for which LO data could be
recorded and analyzed. Results include the detection of 16 binary and 2 triple
stars, all but one of which were previously unknown. The projected angular
separations are as small as 4 milliarcseconds and magnitude differences as high
as ?K=5.8 mag...Comment: 10 pages, 3 figures, to be published in A&
An XMM and Chandra view of massive clusters of galaxies to z=1
The X-ray properties of a sample of high redshift (z>0.6), massive clusters
observed with XMM-Newton and Chandra are described, including two exceptional
systems. One, at z=0.89, has an X-ray temperature of T=11.5 (+1.1, -0.9) keV
(the highest temperature of any cluster known at z>0.6), an estimated mass of
(1.4+/-0.2)x10^15 solar masses and appears relaxed. The other, at z=0.83, has
at least three sub-clumps, probably in the process of merging, and may also
show signs of faint filamentary structure at large radii,observed in X-rays. In
general there is a mix of X-ray morphologies, from those clusters which appear
relaxed and containing little substructure to some highly non-virialized and
probably merging systems. The X-ray gas metallicities and gas mass fractions of
the relaxed systems are similar to those of low redshift clusters of the same
temperature, suggesting that the gas was in place, and containing its metals,
by z=0.8. The evolution of the mass-temperature relation may be consistent with
no evolution or with the ``late formation'' assumption. The effect of point
source contamination in the ROSAT survey from which these clusters were
selected is estimated, and the implications for the ROSAT X-ray luminosity
function discussed.Comment: 9 pages, in Carnegie Observatories Astrophysics Series, Vol. 3:
Clusters of Galaxies: Probes of Cosmological Structure and Galaxy Evolution,
ed. J. S. Mulchaey, A. Dressler, and A. Oemler. See
http://www.ociw.edu/ociw/symposia/series/symposium3/proceedings.html for a
full-resolution versio
Redshift determination in the X-ray band of gamma-ray bursts
If gamma-ray bursts originate in dense stellar forming regions, the
interstellar material can imprint detectable absorption features on the
observed X-ray spectrum. Such features can be detected by existing and planned
X-ray satellites, as long as the X-ray afterglow is observed after a few
minutes from the burst. If the column density of the interstellar material
exceeds ~10^{23} cm^{-2} there exists the possibility to detect the K_alpha
fluorescent iron line, which should be visible for more than one year, long
after the X-ray afterglow continuum has faded away. Detection of these X-ray
features will make possible the determination of the redshift of gamma-ray
bursts even when their optical afterglow is severely dimmed by extinction.Comment: 15 pages with 5 figures. Submitted to Ap
An absorption event in the X-ray lightcurve of NGC 3227
We have monitored the Seyfert galaxy NGC 3227 with the Rossi X-ray Timing
Explorer (RXTE) since January 1999. During late 2000 and early 2001 we observed
an unusual hardening of the 2-10 keV X-ray spectrum which lasted several
months. The spectral hardening was not accompanied by any correlated variation
in flux above 8 keV. We therefore interpret the spectral change as transient
absorption by a gas cloud of column density 2.6 10^23 cm^-2 crossing the line
of sight to the X-ray source. A spectrum obtained by XMM-Newton during an early
phase of the hard-spectrum event confirms the obscuration model and shows that
the absorbing cloud is only weakly ionised. The XMM-Newton spectrum also shows
that ~10% of the X-ray flux is not obscured, but this unabsorbed component is
not significantly variable and may be scattered radiation from a large-scale
scattering medium. Applying the spectral constraints on cloud ionisation
parameter and assuming that the cloud follows a Keplerian orbit, we constrain
the location of the cloud to be R~10-100 light-days from the central X-ray
source, and its density to be n_H~10^8cm^-3, implying that we have witnessed
the eclipse of the X-ray source by a broad line region cloud.Comment: 5 pages, 6 figures, accepted for publication in MNRAS letter
Detailed diagnostics of an X-ray flare in the single giant HR 9024
We analyze a 96 ks Chandra/HETGS observation of the single G-type giant HR
9024. The high flux allows us to examine spectral line and continuum
diagnostics at high temporal resolution, to derive plasma parameters. A
time-dependent 1D hydrodynamic model of a loop with half-length cm (), cross-section radius
cm, with a heat pulse of 15 ks and ~erg cm s
deposited at the loop footpoints, satisfactorily reproduces the observed
evolution of temperature and emission measure, derived from the analysis of the
strong continuum emission. For the first time we can compare predictions from
the hydrodynamic model with single spectral features, other than with global
spectral properties. We find that the model closely matches the observed line
emission, especially for the hot ( K) plasma emission of the FeXXV
complex at \AA. The model loop has and aspect
ratio as typically derived for flares observed in active stellar
coronae, suggesting that the underlying physics is the same for these very
dynamic and extreme phenomena in stellar coronae independently on stellar
parameters and evolutionary stage.Comment: 26 pages. Accepted for publication on the Astrophysical Journa
Imaging Performance of the XMM-Newton X-ray telescopes
The in-orbit imaging performance of the three X-ray telescopes on board of
the X-ray astronomy observatory XMM-Newton is presented and compared with the
performance measured on ground at the MPE PANTER test facility. The comparison
shows an excellent agreement between the on ground and in-orbit performance.Comment: 9 pages, 10 Postscript figures, for SPIE 4012, paper 8
The small dispersion of the mid IR -- hard X-ray correlation in AGN
Context: We investigate mid-infrared and X-ray properties of the dusty torus
in unification scenarios for active galactic nuclei.
Aims: We use the relation between mid IR and hard X-ray luminosities to
constrain AGN unification scenarios.
Methods: With VISIR at the VLT, we have obtained the currently highest
angular resolution (0".35 FWHM) narrow-band mid infrared images of the nuclei
of 8 nearby Seyfert galaxies. Combining these observations with X-ray data from
the literature we study the correlation between their mid IR and hard X-ray
luminosities.
Results: We find that the rest frame 12.3 mircon (L_MIR) and 2-10 keV (L_X)
luminosities are correlated at a highly significant level. The best fit
power-law to our data is log L_MIR \propto (1.60 \pm 0.22) log L_X, showing a
much smaller dispersion than earlier studies.
Conclusions: The similarity in the og L_MIR / log L_X ratio between Sy1s and
Sy2s even using high angular resolution MIR data implies that the similarity is
intrinsic to AGN and not caused by contamination from extra-nuclear emission.
This supports clumpy torus models. The exponent of the correlation constrains
the inner geometry of the torus.Comment: 5 pages, 3 figures, accepted for publication in Astronomy &
Astrophysic
- …