741 research outputs found
The soft X-ray excess AGN RE J2248-511
We model the spectral energy distribution of the ultrasoft broad-line AGN RE
J2248-511 with Comptonised accretion disc models. These are able to reproduce
the steep optical and ultrasoft X-ray slopes, and the derived black hole mass
is consistent with independent mass estimates. This AGN displays properties of
both broad and narrow line Seyfert 1 galaxies, but we conclude that it is
intrinsically a `normal' Seyfert 1 viewed at high inclination angle.Comment: 4 pages, 1 figure. MG10 Proceeding
Full-Bridge Modular Multilevel Converter for the Four-Quadrant Supply of High Power Magnets in Particle Accelerators
Ponencia presentada en 2022 24th European Conference on Power Electronics and Applications (EPE'22 ECCE Europe), 5-9 September 2022, Hanover, Germany.Many particle accelerators require to supply chains of magnets with high quality, high magnitude, cycling currents. To do this, the power converters need to provide high output voltages, reaching in some cases tens of kilovolts. Additionally, converters are required to store the magnet energy during de-magnitasion cycles. For such application, Full-bridge Modular Multilevel Converters (FB-MMC) could be used given their capacity to store energy, their inherent reliability and their good harmonic performance. This paper studies how this converter topology could be used for this application, proposing a method to recover and store the energy of the magnet using the converter submodules
X-ray Spectra of the RIXOS source sample
We present results of an extensive study of the X-ray spectral properties of
sources detected in the RIXOS survey, that is nearly complete down to a flux
limit of 3e-14 cgs (0.5-2 keV). We show that for X-ray surveys containing
sources with low count rate spectral slopes estimated using simple hardness
ratios in the ROSAT band can be biased. Instead we analyse three-colour X-ray
data using statistical techniques appropriate to the Poisson regime which
removes the effects of this bias. We have then applied this technique to the
RIXOS survey to study the spectral properties of the sample. For the AGN we
find an average energy index of 1.05+-0.05 with no evidence for spectral
evolution with redshift. Individual AGN are shown to have a range of properties
including soft X-ray excesses and intrinsic absorption. Narrow Emission Line
Galaxies also seem to fit to a power-law spectrum, which may indicate a
non-thermal origin for their X-ray emission. We infer that most of the clusters
in the sample have a bremsstrahlung temperature >3 keV, although some show
evidence for a cooling flow. The stars deviate strongly from a power-law model
but fit to a thermal model. Finally, we have analysed the whole RIXOS sample
containing 1762 sources. We find that the mean spectral slope of the sources
hardens at lower fluxes in agreement with results from other samples. However,
a study of the individual sources demonstrates that the hardening of the mean
is caused by the appearance of a population of very hard sources at the lowest
fluxes. This has implications for the nature of the soft X-ray background.Comment: 31,LaTeX file, 2 PS files with Table 2 and 22 PS figures. MNRAS in
pres
Nanodielectric mapping of a model polystyrene-poly(vinyl acetate) blend by electrostatic force microscopy
4 páginas, 4 figuras.-- PACS number(s): 61.41.+e, 62.23.-c.-- et al.We present a simple method to quantitatively image the dielectric permittivity of soft materials at nanoscale using electrostatic force microscopy (EFM) by means of the double pass method. The EFM experiments are based on the measurement of the frequency shifts of the oscillating tip biased at two different voltages. A numerical treatment based on the equivalent charge method allows extracting the values of the dielectric permittivity at each image point. This method can be applied with no restrictions of film thickness and tip radius. This method has been applied to image the morphology and the nanodielectric properties of a model polymer blend of polystyrene and poly(vinyl acetate).The financial support of Donostia International Physics
Center (DIPC) is acknowledged. The authors would like to
acknowledge the financial support provided by the Basque
Country Government (Reference No. IT-436-07, Depto. Educación,
Universidades e Investigación), the Spanish Ministry
of Science and Innovation (Grant No. MAT 2007-63681), the
European Community (SOFTCOMP program), and the PPF
Rhéologie et plasticité des matériaux mous hétérogènes
2007–2010 Contract No. 20071656.Peer reviewe
Fault-tolerant strategies in MMC-based high power magnet supply for particle accelerator
[EN] Many particle accelerators require to supply chains of magnets with high quality, high magnitude, cycling currents. To
do this, the power converters need to provide high output voltages, reaching in some cases tens of kilovolts. Additionally,
converters are required to store the magnet energy during de-magnetization cycles. For such application, Full-bridge Modular
Multilevel Converters (FB-MMC) could be used given their capacity to store energy and their inherent reliability. In this sense,
one of the most interesting features of the proposed topology is the possibility of bypassing one or several submodules in
the event of a fault or malfunction. By doing this, it is possible to ride-through the failure of a component and avoid the
interruption of the accelerator operation.
However, when the number of submodules is small, this operation could lead to an excessive charge of the healthy cells,
increasing the risk of secondary failures. Besides, undesired harmonic content could appear on the output current, degrading the
operation of the accelerator. It is then necessary to implement strategies that allow to remove a faulty cell without significantly
impacting the operation of the remaining ones and of the converter itself.
Accordingly, the purpose of this article is to investigate several of these strategies and assess them. By means of detailed
computer simulations, the behaviour of the converter during normal and submodule fault conditions is analysed. Then, several
fault-tolerant strategies are described, verified and compared with the aid of simulation tools. The results show the effectiveness
of the analysed strategies in avoiding the overvoltage on the healthy submodules after a cell bypass and the little impact of
this operation on the quality of the converter output current.Colmenero Moratalla, M.; Vidal-Albalate, R.; Blanquez Delgado, FR.; Blasco-Gimenez, R. (2023). Fault-tolerant strategies in MMC-based high power magnet supply for particle accelerator. Mathematics and Computers in Simulation. 1-17. https://doi.org/10.1016/j.matcom.2023.07.01411
Supersoft X-ray sources identified with Be binaries in the Magellanic Clouds
We investigated four luminous supersoft X-ray sources (SSS) in the Magellanic
Clouds suspected to have optical counterparts of Be spectral type. If the
origin of the X-rays is in a very hot atmosphere heated by hydrogen burning in
accreted envelopes of white dwarfs (WDs), like in the majority of SSS, these
objects are close binaries, with very massive WD primaries. Using the South
African Large Telescope (SALT), we obtained the first optical spectra of the
proposed optical counterparts of two candidate Be stars associated with SUZAKU
J0105-72 and XMMU J010147.5-715550, respectively a transient and a recurrent
SSS, and confirmed the proposed Be classification and Small Magellanic Clouds
membership. We also obtained new optical spectra of two other Be stars proposed
as optical counterparts of the transient SSS XMMU J052016.0-692505 and
MAXI-J0158-744. The optical spectra with double peaked emission line profiles,
are typical of Be stars and present characteristics similar to many high mass
X-ray binaries with excretion disks, truncated by the tidal interaction with a
compact object. The presence of a massive WD that sporadically ignites nuclear
burning, accreting only at certain orbital or evolutionary phases, explains the
supersoft X-ray flares. We measured equivalent widths and distances between
lines' peaks, and investigated the variability of the prominent emission lines'
profiles. The excretion disks seem to be small in size, and are likely to be
differentially rotating. We discuss possible future observations and the
relevance of these objects as a new class of type Ia supernovae progenitors.Comment: Accepted for publication in teh Astrophysical Journa
A Medium Survey of the Hard X-Ray Sky with ASCA. II.: The Source's Broad Band X-Ray Spectral Properties
A complete sample of 60 serendipitous hard X-ray sources with flux in the
range \ecs to \ecs (2 - 10
keV), detected in 87 ASCA GIS2 images, was recently presented in literature.
Using this sample it was possible to extend the description of the 2-10 keV
LogN(>S)-LogS down to a flux limit of \ecs (the
faintest detectable flux), resolving about a quarter of the Cosmic X-ray
Background. In this paper we have combined the ASCA GIS2 and GIS3 data of these
sources to investigate their X-ray spectral properties using the "hardness"
ratios and the "stacked" spectra method. Because of the sample statistical
representativeness, the results presented here, that refer to the faintest hard
X-ray sources that can be studied with the current instrumentation, are
relevant to the understanding of the CXB and of the AGN unification scheme.Comment: 28 pages plus 6 figures, LaTex manuscript, Accepted for publication
in the Astrophysical Journal, Figure 5 can retrieved via anonymous ftp at
ftp://ftp.brera.mi.astro.it/pub/ASCA/paper2/fig5.ps.g
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