5,601 research outputs found
Progress on stochastic background search codes for LIGO
One of the types of signals for which the LIGO interferometric gravitational
wave detectors will search is a stochastic background of gravitational
radiation. We review the technique of searching for a background using the
optimally-filtered cross-correlation statistic, and describe the state of plans
to perform such cross-correlations between the two LIGO interferometers as well
as between LIGO and other gravitational-wave detectors, in particular the
preparation of software to perform such data analysis.Comment: 7 pages, 1 encapsulated PostScript figure, uses IOP class files,
submitted to the proceedings of the 4th Amaldi meeting (which will be
published in Classical and Quantum Gravity
Effects of the integrated galactic IMF on the chemical evolution of the solar neighbourhood
The initial mass function determines the fraction of stars of different
intial mass born per stellar generation. In this paper, we test the effects of
the integrated galactic initial mass function (IGIMF) on the chemical evolution
of the solar neighbourhood. The IGIMF (Weidner & Kroupa 2005) is computed from
the combination of the stellar intial mass function (IMF), i.e. the mass
function of single star clusters, and the embedded cluster mass function, i.e.
a power law with index beta. By taking into account also the fact that the
maximum achievable stellar mass is a function of the total mass of the cluster,
the IGIMF becomes a time-varying IMF which depends on the star formation rate.
We applied this formalism to a chemical evolution model for the solar
neighbourhood and compared the results obtained by assuming three possible
values for beta with the results obtained by means of a standard, well-tested,
constant IMF. In general, a lower absolute value of beta implies a flatter
IGIMF, hence a larger number of massive stars and larger metal ejection rates.
This translates into higher type Ia and II supernova rates, higher mass
ejection rates from massive stars and a larger amount of gas available for star
formation, coupled with lower present-day stellar mass densities. (abridged) We
also discuss the importance of the present day stellar mass function (PDMF) in
providing a way to disentangle among various assumptions for beta. Our results
indicate that the model adopting the IGIMF computed with beta ~2 should be
considered the best since it allows us to reproduce the observed PDMF and to
account for most of the chemical evolution constraints considered in this work.Comment: 22 pages, 19 figure
Powerful H Emission and Star Formation on the Interacting Galaxy System Arp 143: Observations with Spitzer and GALEX
We present new mid-infrared (m) and ultraviolet (1539 -- 2316 \AA)
observations of the interacting galaxy system Arp 143 (NGC 2444/2445) from the
Spitzer Space Telescope and GALEX. In this system, the central nucleus of NGC
2445 is surrounded by knots of massive star-formation in a ring-like structure.
We find unusually strong emission from warm H associated with an expanding
shock wave between the nucleus and the western knots. At this ridge, the flux
ratio between H and PAH emission is nearly ten times higher than in the
nucleus. Arp 143 is one of the most extreme cases known in that regard. From
our multi-wavelength data we derive a narrow age range of the star-forming
knots between 2 Myr and 7.5 Myr, suggesting that the ring of knots was formed
almost simultaneously in response to the shock wave traced by the H
emission. However, the knots can be further subdivided in two age groups: those
with an age of 2--4 Myr (knots A, C, E, and F), which are associated with
m emission from PAHs, and those with an age of 7-8 Myr (knots D and G),
which show little or no m emission shells surrounding them. We attribute
this finding to an ageing effect of the massive clusters which, after about 6
Myr, no longer excite the PAHs surrounding the knots.Comment: 19 pages, 11 figures, including tables at the end; accepted by Ap
Effect of disorder on superconductivity in the boson-fermion model
We study how a randomness of either boson or fermion site energies affects
the superconducting phase of the boson fermion model. We find that, contrary to
what is expected for s-wave superconductors, the non-magnetic disorder is
detrimental to the s-wave superconductivity. However, depending in which
subsystem the disorder is located, we can observe different channels being
affected. Weak disorder of the fermion subsystem is responsible mainly for
renormalization of the single particle density of states while disorder in the
boson subsystem directly leads to fluctuation of the strength of the effective
pairing between fermions.Comment: 7 pages, 6 figures. Physical Review B (accepted for publication
Synergistic warm inflation
We consider an alternative warm inflationary scenario in which scalar
fields coupled to a dissipative matter fluid cooperate to produce power--law
inflation. The scalar fields are driven by an exponential potential and the
bulk dissipative pressure coefficient is linear in the expansion rate. We find
that the entropy of the fluid attains its asymptotic value in a characteristic
time proportional to the square of the number of fields. This scenario remains
nearly isothermal along the inflationary stage. The perturbations in energy
density and entropy are studied in the long--wavelength regime and seen to grow
roughly as the square of the scale factor. They are shown to be compatible with
COBE measurements of the fluctuations in temperature of the CMB.Comment: 13 pages, Revtex 3 To be published in Physical Review
Combining Semi-analytic Models with Simulations of Galaxy Clusters: the Need for Heating from Active Galactic Nuclei
We present hydrodynamical N-body simulations of clusters of galaxies with
feedback taken from semi-analytic models of galaxy formation. The advantage of
this technique is that the source of feedback in our simulations is a
population of galaxies that closely resembles that found in the real universe.
We demonstrate that, to achieve the high entropy levels found in clusters,
active galactic nuclei must inject a large fraction of their energy into the
intergalactic/intracluster media throughout the growth period of the central
black hole. These simulations reinforce the argument of Bower et al., who
arrived at the same conclusion on the basis of purely semi-analytic reasoning.Comment: 25 pages and 10 colour figures. Accepted by Ap
Variability and spectral energy distributions of low-luminosity active galactic nuclei: a simultaneous X-ray/UV look with Swift
We have observed four low-luminosity active galactic nuclei classified as
Type 1 LINERs with the X-ray Telescope (XRT) and the UltraViolet-Optical
Telescope (UVOT) onboard Swift, in an attempt to clarify the main powering
mechanism of this class of nearby sources. Among our targets, we detect X-ray
variability in NGC 3998 for the first time. The light curves of this object
reveal variations of up to 30% amplitude in half a day, with no significant
spectral variability on this time scale. We also observe a decrease of ~30%
over 9 days, with significant spectral softening. Moreover, the X-ray flux is
~40% lower than observed in previous years. Variability is detected in M 81 as
well, at levels comparable to those reported previously: a flux increase in the
hard X-rays (1-10 keV) of 30% in ~3 hours and variations by up to a factor of 2
within a few years. This X-ray behaviour is similar to that of
higher-luminosity, Seyfert-type, objects. Using previous
high-angular-resolution imaging data from the Hubble Space Telescope (HST), we
evaluate the diffuse UV emission due to the host galaxy and isolate the nuclear
flux in our UVOT observations. All sources are detected in the UV band, at
levels similar to those of the previous observations with HST. The XRT (0.2-10
keV) spectra are well described by single power-laws and the UV-to-X-ray flux
ratios are again consistent with those of Seyferts and radio-loud AGNs of
higher luminosity. The similarity in X-ray variability and broad-band energy
distributions suggests the presence of similar accretion and radiation
processes in low- and high-luminosity AGNs.Comment: 12 pages, 6 figures, in press in MNRA
Obsidians of Pantelleria (Strait of Sicily): A Petrographic, Geochemical and Magnetic Study of Known and New Geological Sources
This paper provides new petrochemical and paleomagnetic data from obsidian sub-sources on the island of Pantelleria, exploited since the Neolithic. Data has been obtained from 14 obsidian samples from 4 locations: Fossa della Pernice (2 sites), Salto la Vecchia and Balata dei Turchi. Here, we aim to better characterize these obsidians using a cross-disciplinary and multi-analytical approach, to further understand their archaeological significance. Major element analyses (EMP) have enabled two compositional super-groups to be distinguished: (i) Fossa della Pernice, less peralkaline and (ii) Balata dei Turchi-Salto la Vecchia, distinctly more peralkaline and having almost identical chemical patterns. Trace element analyses (LA-ICP-MS) corroborate major element groupings, with the Balata dei Turchi-Salto la Vecchia super-group being further characterized by a pronounced negative europium anomaly. Glass H2O contents (FT-IR) reveal an overlap among all the sub-sources (H2O = 0.1-0.3 wt. %). Magnetic methods have refined the petrochemical groupings, permitting further distinction between Balata dei Turchi-Salto La Vecchia and the Fossa della Pernice super-groups. The occurrence of sub-microscopic (< 1 μm) ferromagnetic minerals results in different magnetic susceptibility and Natural Remanent Magnetization values and allows the best distinction among the products from the chosen sites. When compared with obsidian tools excavated from Bronze-age settlements on the island of Ustica (230 km NE of Pantelleria), 12% are distinctly peralkaline, indicating their provenance to be from the Balata dei Turchi sub-source
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