1,995 research outputs found
Relativistic Iron lines at high redshifts
The shape and the intensity of the 6.4 keV iron line bring unique information
on the geometrical and physical properties of the supermassive black hole and
the surrounding accreting gas at the very center of Active Galactic Nuclei.
While there are convincing evidences of a relativistically broadened iron line
in a few nearby bright objects, their properties at larger distances are
basically unknown. We have searched for the presence of iron line by fully
exploiting Chandra observations in the deep fields. The line is clearly
detected in the average spectra of about 250 sources stacked in several
redshift bins over the range z=0.5-4.0. We discuss their average properties
with particular enphasys on the presence and intensity of a broad component.Comment: 6 pages, 4 figures. To appear in the proceedings of "Relativistic
Astrophysics and Cosmology - Einstein's Legacy" (Eds.: B. Aschenbach, V.
Burwitz, G. Hasinger, and B. Leibundgut), 7 - 11 November 2005, Munich,
German
Low radiative efficiency accretion at work in active galactic nuclei: the nuclear spectral energy distribution of NGC4565
We derive the spectral energy distribution (SED) of the nucleus of the
Seyfert galaxy NGC4565. Despite its classification as a Seyfert2, the nuclear
source is substantially unabsorbed. The absorption we find from Chandra data
(N_H=2.5 X 10^21 cm^-2) is consistent with that produced by material in the
galactic disk of the host galaxy. HST images show a nuclear unresolved source
in all of the available observations, from the near-IR H band to the optical U
band. The SED is completely different from that of Seyfert galaxies and QSO, as
it appears basically ``flat'' in the IR-optical region, with a small drop-off
in the U-band. The location of the object in diagnostic planes for low
luminosity AGNs excludes a jet origin for the optical nucleus, and its
extremely low Eddington ratio L_o/L_Edd indicates that the radiation we observe
is most likely produced in a radiatively inefficient accretion flow (RIAF).
This would make NGC4565 the first AGN in which an ADAF-like process is
identified in the optical. We find that the relatively high [OIII] flux
observed from the ground cannot be all produced in the nucleus. Therefore, an
extended NLR must exist in this object. This may be interpreted in the
framework of two different scenarios: i) the radiation from ADAFs is sufficient
to give rise to high ionization emission-line regions through photoionization,
or ii) the nuclear source has recently ``turned-off'', switching from a
high-efficiency accretion regime to the present low-efficiency state.Comment: 7 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Abundance ratios of volatile vs. refractory elements in planet-harbouring stars: hints of pollution?
We present the [X/H] trends as function of the elemental condensation
temperature Tc in 88 planet host stars and in a volume-limited comparison
sample of 33 dwarfs without detected planetary companions. We gathered
homogeneous abundance results for many volatile and refractory elements
spanning a wide range of Tc, from a few dozens to several hundreds kelvin. We
investigate possible anomalous trends of planet hosts with respect to
comparison sample stars in order to detect evidence of possible pollution
events. No significant differences are found in the behaviour of stars with and
without planets. This result is in agreement with a ``primordial'' origin of
the metal excess in planet host stars. However, a subgroup of 5 planet host and
1 comparison sample stars stands out for having particularly high [X/H] vs. Tc
slopes.Comment: 10 pages, 7 figures, accepted for publication in A&A. Figures with
higher resolution are available at www.iac.es/proyect/abuntes
Using economic and financial information for stock selection
A major inconvenience of the traditional approach in portfolio choice, based upon historical information, is its inability to anticipate sudden changes of price tendencies. Introducing information about future behavior of the assets fundamentals may help to make more appropriate choices. However, the specification and parameterization of a model linking this exogenous information to the asset prices is not straightforward. Classification trees can be used to construct partitions of assets of forecasted similar behavior. We analyze the performance of this approach and apply it to different sectors of the S&P50
The high-redshift Universe with the International X-ray Observatory
We discuss some of the main open issues related to the light-up and evolution
of the first accreting sources powering high redshift luminous quasars. We
discuss the perspectives of future deep X-ray surveys with the International
X-ray Observatory and possible synergies with the Wide Field X-ray Telescope.Comment: 6 pages, 6 figures. Proceedings of "The Wide Field X-ray Telescope
Workshop", held in Bologna, Italy, Nov. 25-26 2009. To appear in Memorie
della Societ\`a Astronomica Italiana 2010 (arXiv:1010.5889
Testing current synthesis models of the X-ray background
We present synthesis models of the X-ray background where the available X-ray
observational constraints are used to derive information on the AGN population
properties. We show the need for luminous X-ray absorbed AGNs, the QSO2s, in
reproducing the 2-10 keV source counts at relatively bright fluxes. We compare
a model where the evolution of absorbed AGNs is faster than that of unabsorbed
ones, with a standard model where absorbed and unabsorbed AGNs evolve at the
same rate. It is found that an increase by a factor of ~2 from z=0 to z~1.3 in
the ratio between absorbed and unabsorbed AGNs would provide a significant
improvement in the data description. Finally, we make predictions on the AGNs
to be observed in deep X-ray surveys which contain information on the AGN space
density at high redshift.Comment: 11 pages with 8 figures, A&A accepte
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