13,240 research outputs found
Evidence of non-thermal X-ray emission from HH 80
Protostellar jets appear at all stages of star formation when the accretion
process is still at work. Jets travel at velocities of hundreds of km/s,
creating strong shocks when interacting with interstellar medium. Several cases
of jets have been detected in X-rays, typically showing soft emission. For the
first time, we report evidence of hard X-ray emission possibly related to
non-thermal processes not explained by previous models of the post-shock
emission predicted in the jet/ambient interaction scenario. HH 80 is located at
the south head of the jet associated to the massive protostar IRAS 18162-2048.
It shows soft and hard X-ray emission in regions that are spatially separated,
with the soft X-ray emission region situated behind the region of hard X-ray
emission. We propose a scenario for HH 80 where soft X-ray emission is
associated to thermal processes from the interaction of the jet with denser
ambient matter and the hard X-ray emission is produced by synchrotron radiation
at the front shock.Comment: Accepted for publication in ApJ
Exploiting graphic processing units parallelism to improve intelligent data acquisition system performance in JET's correlation reflectometer
The performance of intelligent data acquisition systems relies heavily on their processing capabilities and local bus bandwidth, especially in applications with high sample rates or high number of channels. This is the case of the self adaptive sampling rate data acquisition system installed as a pilot experiment in KG8B correlation reflectometer at JET. The system, which is based on the ITMS platform, continuously adapts the sample rate during the acquisition depending on the signal bandwidth. In order to do so it must transfer acquired data to a memory buffer in the host processor and run heavy computational algorithms for each data block. The processing capabilities of the host CPU and the bandwidth of the PXI bus limit the maximum sample rate that can be achieved, therefore limiting the maximum bandwidth of the phenomena that can be studied. Graphic processing units (GPU) are becoming an alternative for speeding up compute intensive kernels of scientific, imaging and simulation applications. However, integrating this technology into data acquisition systems is not a straight forward step, not to mention exploiting their parallelism efficiently. This paper discusses the use of GPUs with new high speed data bus interfaces to improve the performance of the self adaptive sampling rate data acquisition system installed on JET. Integration issues are discussed and performance evaluations are presente
Corrección del factor de potencia, sin medida de corriente, mediante implementación en FPGA de one-cycle control
En este artículo se presenta una técnica de control digital de correctores de factor de potencia (CFP) trabajando en modo de conducción continua en la que no es necesario el uso de sensor de corriente ni de convertidor analógico-digital de alta velocidad. La corriente de entrada al CFP se estima a partir de la descripción del modelo en VHDL de un convertidor elevador y de las muestras digitales de las tensiones de entrada y salida del convertidor. El objetivo final es desarrollar un controlador universal para CFP, con posibilidad de trabajar varios convertidores idénticos en paralelo y así aumentar la potencia. En este trabajo se abordan las diferentes soluciones ante los problemas que aparecen en el desarrollo del prototipo de laboratorio y se presentan resultados en diferentes condiciones, comparando el contenido armónico de la corriente con los límites establecidos por la normativa vigente más restrictiv
AE Aurigae: first detection of non-thermal X-ray emission from a bow shock produced by a runaway star
Runaway stars produce shocks when passing through interstellar medium at
supersonic velocities. Bow shocks have been detected in the mid-infrared for
several high-mass runaway stars and in radio waves for one star. Theoretical
models predict the production of high-energy photons by non-thermal radiative
processes in a number sufficiently large to be detected in X-rays. To date, no
stellar bow shock has been detected at such energies. We present the first
detection of X-ray emission from a bow shock produced by a runaway star. The
star is AE Aur, which was likely expelled from its birthplace by the encounter
of two massive binary systems and now is passing through the dense nebula IC
405. The X-ray emission from the bow shock is detected at 30" to the northeast
of the star, coinciding with an enhancement in the density of the nebula. From
the analysis of the observed X-ray spectrum of the source and our theoretical
emission model, we confirm that the X-ray emission is produced mainly by
inverse Compton upscattering of infrared photons from dust in the shock front.Comment: Accepted for publication in the Astrophysical Journal with number
ApJ, 757, L6. Four figure
On the generation of asymmetric warps in disk galaxies
The warps in many spiral galaxies are now known to asymmetric. Recent
sensitive observations have revealed that asymmetry of warps may be the norm
rather than exception. However there exists no generic mechanism to generate
these asymmetries in warps. We have derived the dispersion relation in a
compact form for the S-shaped warps(described by the m=1 mode) and the
bowl-shaped distribution(described by the m=0 mode) in galactic disk embedded
in a dark matter halo. We then performed the numerical modal analysis and used
the linear and time-dependent superposition principle to generate asymmetric
warps in the disk. On doing the modal analysis we find the frequency of the
mode is much larger than that of the mode. The linear and
time-dependent superposition of these modes with their unmodulated
amplitudes(that is, the coefficients of superposition being unity) results in
an asymmetry in warps of ~ 20 - 40 %, whereas a smaller coefficient for the m=0
mode results in a smaller asymmetry. The resulting values agree well with the
recent observations. We study the dependence of the asymmetry index on the dark
matter halo parameters. This approach can also naturally produce U-shaped warps
and L-shaped warps. We show that a rich variety of possible asymmetries in the
z-distribution of the spiral galaxies can naturally arise due to a dynamical
wave interference between the first two bending modes(i.e. m=0 and m=1) in the
disk. This is a simple but general method for generating asymmetric warps that
is independent of how the individual modes arise in the disk.Comment: Accepted for publication in A &
Comparing different methods to retrieve cloud top height from Meteosat satellite data
Cloud parameters such as the Cloud Top Height (CTH), Cloud Top Temperature (CTT), emissivity, particle size and optical depth have always been matter of interest for the atmospheric community. Particularly the CTH provides information leading to better understand the cloud radiative effects. Although there are many meteorological satellites providing the CTH, there are other sensors, not devoted to this purpose, that give some information from which this crucial parameter can be estimated. In this contribution we will describe three different methodologies to retrieve the CTH. The first technique is based on stereo-vision algorithms and requires two different views of the same scene and does not need of extra atmospheric information. In the second one, brightness temperatures in two IR spectral bands are converted to real cloud temperature by means of the proposed algorithms. From the CTT, the CTH is estimated using temperature vertical profiles (measured or modeled). The third technique retrieves the CTH from the output parameters of post event simulations performed by a Numerical Weather Prediction (NWP) model that in this work will be the mesoscale model WRF (Weather Research Forecast). This article presents a preliminary work, in which the heights retrieved by the three methodologies applied to the geostationary satellite Meteosat 10 are compared with the heights given by MODIS sensor installed on the polar satellite AQUA. This promising results show that valuable information about CTH can be retrieved from Meteosat which provide high frequency and large scale data useful for weather and climate research
Methods to Retrieve the Cloud-Top Height in the Frame of the JEM-EUSO Mission
The Japanese Experiment Module-Extreme Universe
Space Observatory (JEM-EUSO) telescope will measure
ultrahigh-energy cosmic ray properties by detecting the UV
fluorescence light generated in the interaction between cosmic
rays and the atmosphere. Therefore, information on the state of
clouds in the atmosphere is crucial for a proper interpretation of
the data. For a real-time observation of the clouds in the telescope
field of view, the JEM-EUSO will use an atmospheric monitoring
system composed of a light detection and ranging and an infrared
(IR) camera. In this paper, the focus is on the IR camera data.
To retrieve the cloud-top height (CTH) from IR images, three
different methods are considered here. The first one is based
on bispectral stereo vision algorithms and requires two different
views of the same scene in different spectral bands. For the second
one, brightness temperatures provided by the IR camera are converted
to effective cloud-top temperatures, from which the CTH is
estimated using the vertical temperature profiles. A third method
that uses the primary numerical weather prediction model output
parameters, such as the cloud fraction, has also been considered
to retrieve the CTH. This paper presents a first analysis, in which
the heights retrieved by these three methodologies are compared
with the heights given by the Moderate Resolution Imaging
Spectroradiometer sensor installed on the polar satellite Terra.
Since all these methods are suitable for the JEM-EUSO mission,
they could be used in the future in a complementary way to
improve the accuracy of the CTH retrieval
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