513 research outputs found
On Estimating the High-Energy Cutoff in the X-ray Spectra of Black Holes via Reflection Spectroscopy
The fundamental parameters describing the coronal spectrum of an accreting
black hole are the slope of the power-law continuum and the energy
at which it rolls over. Remarkably, this parameter can be accurately
measured for values as high as 1 MeV by modeling the spectrum of X-rays
reflected from a black hole accretion disk at energies below 100 keV. This is
possible because the details in the reflection spectrum, rich in fluorescent
lines and other atomic features, are very sensitive to the spectral shape of
the hardest coronal radiation illuminating the disk. We show that fitting
simultaneous NuSTAR (3-79 keV) and low-energy (e.g., Suzaku) data with the most
recent version of our reflection model RELXILL, one can obtain reasonable
constraints on at energies from tens of keV up to 1 MeV, for a source
as faint as 1 mCrab in a 100 ks observation.Comment: Accepted for publication in ApJL, 6 pages, 5 figure
Modeling of quantum dot lasers with microscopic treatment of Coulomb effects
We present a spatially resolved semiclassical model for the simulation of semiconductor quantum-dot lasers including a multi-species description for the carriers along the optical active region. The model links microscopic determined quantities like scattering rates and dephasing times, that essentially depend via Coulomb interaction on the carrier densities, with macroscopic transport equations and equations for the optical field
Irradiation of an Accretion Disc by a Jet: General Properties and Implications for Spin Measurements of Black Holes
X-ray irradiation of the accretion disc leads to strong reflection features,
which are then broadened and distorted by relativistic effects. We present a
detailed, general relativistic approach to model this irradiation for different
geometries of the primary X-ray source. These geometries include the standard
point source on the rotational axis as well as more jet-like sources, which are
radially elongated and accelerating. Incorporating this code in the relline
model for relativistic line emission, the line shape for any configuration can
be predicted. We study how different irradiation geometries affect the
determination of the spin of the black hole. Broad emission lines are produced
only for compact irradiating sources situated close to the black hole. This is
the only case where the black hole spin can be unambiguously determined. In all
other cases the line shape is narrower, which could either be explained by a
low spin or an elongated source. We conclude that for all those cases and
independent of the quality of the data, no unique solution for the spin exists
and therefore only a lower limit of the spin value can be given.Comment: accepted by MNRAS for publication; now proof corrected Versio
Influence of the carrier reservoir dimensionality on electron-electron scattering in quantum dot materials
We calculated Coulomb scattering rates from quantum dots (QDs) coupled to a 2D carrier reservoir and QDs coupled to a 3D reservoir. For this purpose, we used a microscopic theory in the limit of Born-Markov approximation, in which the numerical evaluation of high dimensional integrals is done via a quasi-Monte Carlo method. Via a comparison of the so determined scattering rates, we investigated the question whether scattering from 2D is generally more efficient than scattering from 3D. In agreement with experimental findings, we did not observe a significant reduction of the scattering efficiency of a QD directly coupled to a 3D reservoir. In turn, we found that 3D scattering benefits from it?s additional degree of freedom in the momentum space
First Results on the DOAS-Retrieval of OClO from SCIAMACHY Nadir Measurements
The Scanning Imaging Absorption Spectrometer for
Atmospheric Chartography was launched successfully
onboard ENVISAT on March 1, 2002. It observes solar
radiation transmitted, backscattered from the atmosphere
and reflected from the ground in nadir, limb and occultation
viewing modes. Chlorinedioxide (OClO), an important
indicator for stratospheric chlorine activation, can be
measured in the UV spectral range by Differential Optical
Absorption Spectroscopy (DOAS).
First results of the DOAS retrieval of OClO slant column
densities (SCDs) from the SCIAMACHY measurements
are presented. The influence of several parameters like the
wavelength range chosen as fitting window or the reference
spectra included in the fit on the quality of the retrieval is examined. It is found that a proper correction of
polarisation features in the spectra is essential for a good
DOAS analysis of OClO. The OClO SCDs derived from SCIAMACHY are compared to measurements of the Global Ozone Monitoring Experiment (GOME) which has successfully measured OClO since 1995. SCIAMACHY flies in the same orbit, but measures approx. 30 minutes earlier than GOME. As OClO shows a strong diurnal variation, this leads to differences in the observed column densities, which may be useful to investigate the photochemistry of OClO and related compounds. Also, the spatial resolution of SCIAMACHY is higher (30*60 km^2 compared to 40*320 km^2 for GOME), which will allow a more detailed study of small scale effects like e.g. chlorine activation in mountain waves
Chandra X-ray spectroscopy of the focused wind in the Cygnus X-1 system III. Dipping in the low/hard state
We present an analysis of three Chandra High Energy Transmission Gratings
observations of the black hole binary Cyg X-1/HDE 226868 at different orbital
phases. The stellar wind that is powering the accretion in this system is
characterized by temperature and density inhomogeneities including structures,
or "clumps", of colder, more dense material embedded in the photoionized gas.
As these clumps pass our line of sight, absorption dips appear in the light
curve. We characterize the properties of the clumps through spectral changes
during various dip stages. Comparing the silicon and sulfur absorption line
regions (1.6-2.7 keV 7.7-4.6 {\AA}) in four levels of varying column
depth reveals the presence of lower ionization stages, i.e., colder or denser
material, in the deeper dip phases. The Doppler velocities of the lines are
roughly consistent within each observation, varying with the respective orbital
phase. This is consistent with the picture of a structure that consists of
differently ionized material, in which shells of material facing the black hole
shield the inner and back shells from the ionizing radiation. The variation of
the Doppler velocities compared to a toy model of the stellar wind, however,
does not allow us to pin down an exact location of the clump region in the
system. This result, as well as the asymmetric shape of the observed lines,
point at a picture of a complex wind structure.Comment: 19 pages, 15 figures, accepted for publication in A&
2XMMi J225036.9+573154 - a new eclipsing AM Her binary discovered using XMM-Newton
We report the discovery of an eclipsing polar, 2XMMi J225036.9+573154, using
XMM-Newton. It was discovered by searching the light curves in the 2XMMi
catalogue for objects showing X-ray variability. Its X-ray light curve shows a
total eclipse of the white dwarf by the secondary star every 174 mins. An
extended pre-eclipse absorption dip is observed in soft X-rays at phi=0.8-0.9,
with evidence for a further dip in the soft X-ray light curve at phi~0.4.
Further, X-rays are seen from all orbital phases (apart from the eclipse) which
makes it unusual amongst eclipsing polars. We have identified the optical
counterpart, which is faint (r=21), and shows a deep eclipse (>3.5 mag in white
light). Its X-ray spectrum does not show a distinct soft X-ray component which
is seen in many, but not all, polars. Its optical spectrum shows Halpha in
emission for a fraction of the orbital period.Comment: Accepted for publication in MNRA
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