878 research outputs found
The energetic Universe
In this paper I review the main topics on the energetic Universe that have
been put forward as main science goals in the Cosmic Vision 2015-2025 exercise.
I discuss the study of matter under extreme conditions (both under strong
gravity and at ultra-high densities), the cosmology of baryons (assembly of
ordinary matter in dark-matter dominated structures and the creation of heavy
elements) and the co-eval growth of super-massive black holes and stars in
galaxies along cosmic history. Most of these topics can be addressed with a
large-aperture deep Universe X-ray space observatory that can be flown soon
after 2015, complemented by gravitational wave observatories (LISA), a
focussing gamma-ray observatory, a far infrared high-sensitivity observatory
and an X-ray survey telescope.Comment: 10 pages, contribution to the 39th ESLAB symposium, ESA-SP (in press
On the Influence of X-Ray Galaxy Clusters in the Fluctuations of the Cosmic Microwave Background
The negative evolution found in X--ray clusters of galaxies limits the amount
of available hot gas for the inverse Compton scattering of the Cosmic Microwave
Background (the Sunyaev--Zel'dovich effect). Using a parametrisation of the
X-ray luminosity function and its evolution in terms of a coalescence model (as
presented in the analysis of a flux limited X-ray cluster sample by Edge et al.
1990), as well as a simple virialised structure for the clusters (which
requires a gas to total mass fraction \approxgt 0.1 in order to reproduce
observed properties of nearby clusters) we show that the Compton distortion
parameter is about two orders of magnitude below the current FIRAS upper
limits. Concerning the anisotropies imprinted on arcmin scales they are
dominated by the hottest undetected objects. We show that they are negligible
({\Delta T\over T}\approxlt 10^{-7}) at wavelengths \lambda\approxgt 1~mm.
At shorter wavelengths they become more important ( at ~mm), but in fact most clusters will produce an
isolated and detectable feature in sky maps. After removal of these signals,
the fluctuations imprinted by the remaining clusters on the residual radiation
are still much smaller. The conclusion is that X-ray clusters can be ignored as
sources of Cosmic Microwave Background fluctuations.Comment: 20 pages Plain Tex, 7 figures available upon request,UCAST-94-0
A wind field downscaling strategy based on domain segmentation and transfer functions
This paper presents a novel methodology for mesoscale‐to‐microscale downscaling of near‐surface wind fields. The model chain consists on the Weather Research and Forecast mesoscale model and the Alya‐CFDWind microscale model (assuming neutral stability). The downscaling methodology combines precomputed microscale simulations with a mesoscale forecast using a domain segmentation technique and transfer functions. As a result, the downscaled wind field preserves the mesoscale pattern but, at the same time, incorporates local mesoscale subgrid terrain effects, particularly at valleys and channelling zones. The methodology has been validated over a 9‐month period on a very complex terrain site instrumented with a dense observational network of meteorological masts. With respect to mesoscale results, the global skills of the downscaled wind at masts improve for wind direction and remain similar for wind velocity. However, a substantial improvement occurs under stable and neutral conditions and for high wind velocity regimes.This work has been partially funded by the High Performance Computing for Energy (HPC4E) project (call H2020-EUB-2015, Topic: EUB-2-2015, type of action RIA, Grant Agreement number 689772) and the SEDAR ("Simulación eólica de alta resolución") project. It has also been partially supported by the Energy-oriented Centre of Excellence (EoCoE) (Grant Agreement number 676629, funded within the H2020 framework of the
EuropeanUnion). J.B. is grateful to a PhD fellowship from the IndustrialDoctorates Plan of the Government of Catalonia (Ref. eco/2497/2013). We also thank Daniel Paredes and Luis Prieto from Iberdrola Renovables S.A. for providing us access to met masts data for validation.Peer ReviewedPostprint (published version
Soft versus Hard X-ray emission in AGN: partial covering and warm plus cold absorber models
We analyse the ROSAT PSPC hardness ratio and the 0.5-2 keV to 2-10 keV flux
ratio of 65 Active Galactic Nuclei (AGN) for which there are both ROSAT
archival observations available and 2-10 keV fluxes, mostly from the HEAO-1
MC-LASS survey. We conclude that the simplest spectral model for the AGN that
can accommodate the variety of X-ray colours obtained is a standard power law
(with energy spectral index ) plus a keV black body
both partially absorbed. In our sample, type 1 AGN require an absorbing column
around with covering fractions between 20 and 100\%,
while type 2 AGN display larger columns and coverage. This simple
model also provides a good link between soft and hard AGN X-ray luminosity
functions and source counts. We also consider a warm absorber as an alternative
model to partial covering and find that the the presence of gas in two phases
(ionized and neutral) is required.Comment: 10 pages, Latex (mn.sty), 1 table, 5 figures included (epsf),
postscript version also available via anonymous ftp at
ftp://astsun1.unican.es/pub/ceballos/ . Accepted for publication in MNRA
Observando los efectos de la Relatividad General alrededor de los agujeros negros
Blog divulgativo.Observaciones astronómicas de la materia que rodea los agujeros negros ponen en evidencia algunas predicciones de la Relatividad General en condiciones de campo gravitatorio fuerte. Una de ellas, el redshift gravitatorio, no solo se observa de forma rutinaria sino que se usa para medir la rotación del agujero negro. El spin de los agujeros negros gigantes encierra la clave de su crecimiento, e indirectamente de la evolución de las galaxias.Peer Reviewe
Soft X-ray background fluctuations and large-scale structure in the Universe
We have studied the fluctuations of the soft (0.9-2 keV) X-ray background
intensity for ~10 arcmin and ~2 arcmin beam sizes, using 80 high galactic
latitude medium-deep images from the ROSAT position sensitive proportional
counter (PSPC). These fluctuations are dominated (and well reproduced) by
confusion noise produced by sources unresolved with the beam sizes we used. We
find no evidence for any excess fluctuations which could be attributed to
source clustering. The 95 per cent confidence upper limits on excess
fluctuations dIclus are: dIclus/Ixrb_10 arcmin<~ 0.12, dIclus/Ixrb_2 arcmin
<~0.07. We have checked the possibility that low surface brightness extended
objects (like groups or clusters of galaxies) may have a significant
contribution to excess fluctuations, finding that they are not necessary to fit
the distribution of fluctuations, and obtaining an upper limit on the surface
density for this type of source. Standard Cold Dark Matter models would produce
dIclus/Ixrb larger than the above limits for any value of the density of the
Universe Omega=0.1-1, unless the bias parameter of the X-ray emitting matter is
smaller than unity, or an important fraction of the sources of the soft X-ray
background (~30 per cent) is at redshifts z>1. Limits on the 2-10 keV excess
fluctuations are also considered, showing that X-ray sources in that band have
to be at redshifts z>1 unless Omega>0.4. Finally, if the spatial correlation
function of the sources that produce these excess fluctuations is instead a
power law, the density contrast drho/rho implied by the excess fluctuations
reveals that the Universe is smooth and linear on scales of tens of Mpc, while
it can be highly non-linear on scales ~1 Mpc.Comment: 10 pages, LaTeX file, epsf.sty and 7 postscript figures. To appear in
MNRAS. Fig. 7 replaced, some references improved, a few corrections to the
tex
Do nuclear starbursts obscure the X-ray background?
We propose a model for the source of the X-ray background (XRB) in which low
luminosity active nuclei (L<10^43 erg/s) are obscured by nuclear starbursts
within the inner 100pc. The obscuring material covers most of the sky as seen
from the central source, rather than being distributed in a toroidal structure,
and hardens the averaged X-ray spectrum by photoelectric absorption. The gas is
turbulent with velocity dispersion of a few 100 km/s and cloud-cloud collisions
lead to copious star formation. Although supernovae tend to produce outflows,
most of the gas is trapped in the gravity field of the starforming cluster
itself and the central black hole. A hot (T=10^6-10^7 K) virialised phase of
this gas, comprising a few per cent of the total obscuring material, feeds the
central engine of 10^7 solar masses through Bondi accretion, at a sub-Eddington
rate appropriate for the luminosity of these objects. If starburst-obscured
objects give rise to the residual XRB, then only 10 per cent of the accretion
in active galaxies occurs close to the Eddington limit in unabsorbed objects.Comment: 5 pages, 2 PS figures included in the text, MNRAS in the press. Also
at http://www.ifca.unican.es/~barcons/preprints.htm
Las ventanas abiertas por el Hubble
Blog divulgativo.El telescopio espacial Hubble ha hecho realidad el sueño del astrónomo: observar el universo sin atmósfera. El Hubble descubrió las galaxias más lejanas y localizó átomos de hidrógeno en el universo más cercano. Y continúa acercando, a través de sus imágenes, la belleza y los misterios del cosmos a la sociedad.Peer Reviewe
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