7,573 research outputs found
Exploring jet-launching conditions for SFXTs
In the magneto-centrifugal mechanism for jet formation, accreting neutron
stars are assumed to produce relativistic jets only if their surface magnetic
field is weak enough ( G). However, the most common manifestation
of neutron stars are pulsars, whose magnetic field distribution peaks at G. If the neutron star magnetic field has at least this strength
at birth, it must decay considerably before jets can be launched in binary
systems. We study the magnetic field evolution of a neutron star that accretes
matter from the wind of a high-mass stellar companion so that we can constrain
the accretion rate and the impurities in the crust, which are necessary
conditions for jet formation. We solved the induction equation for the
diffusion and convection of the neutron star magnetic field confined to the
crust, assuming spherical accretion in a simpliflied one-dimensional treatment.
We incorporated state-of-the-art microphysics, including consistent thermal
evolution profiles, and assumed two different neutron star cooling scenarios
based on the superfluidity conditions at the core. We find that in this
scenario, magnetic field decay at long timescales is governed mainly by the
accretion rate, while the impurity content and thermal evolution of the neutron
star play a secondary role. For accretion rates
M yr, surface magnetic fields can decay up to four orders of
magnitude in 10 yr, which is the timescale imposed by the evolution
of the high-mass stellar companion in these systems. Based on these results, we
discuss the possibility of transient jet-launching in strong wind-accreting
high-mass binary systems like supergiant fast X-ray transients.Comment: 8 pages, 8 figures. Accepted for publication in A&
On the origin of two unidentified radio/X-ray sources discovered with XMM-Newton
We aim at clarifying the nature of the emission of two spatially related
unidentified X-ray sources detected with XMM-Newton telescope at
intermediate-low Galactic latitude. Observations reveal a point-like source
aligned with elongated diffuse emission. The X-ray spectra are best-fitted by
absorbed power laws with photon indices ~1.7 for the point-like and ~2.0 for
the extended one. Both sources show nonthermal radio-continuum counterparts
that might indicate a physical association. From the available data, we did not
detect variability on the point-like source in several timescales. Two possible
scenarios are analyzed: first, based on HI line absorption, assuming a Galactic
origin, we infer a distance upper bound of <2 kpc, which poses a constraint on
the height over the Galactic plane of <200 pc and on the linear size of the
system of 10^32 erg/s and
>7.5 x 10^32 erg/s, for the point-like and extended sources, respectively;
second, an extra-Galactic nature is discussed, where the point-like source
might be the core of a radio galaxy and the extended source its lobe. In this
case, we compare derived fluxes, spectral indices, and spatial correlation with
those typical from the radio galaxy population, showing the feasibility of this
alternative astrophysical scenario. From the available observational evidence,
we suggest that the most promising scenario to explain the nature of these
sources is a system consisting of a one-sided radio galaxy, where the
point-like source is an active galactic nucleus and the extended source
corresponds to the emission from its lobe. Other possibilities include a
PSR/PWN origin, where the radio/X-ray emission originates from the synchrotron
cooling of relativistic particles in the PSR magnetic field or a casual
alignment between two unrelated sources, such as an AGN core and a Galactic
X-ray blob.Comment: 6 pages, 2 figures. Accepted for publication in Astronomy and
Astrophysics (A&A
Microwave Photon Detector in Circuit QED
Quantum optical photodetection has occupied a central role in understanding
radiation-matter interactions. It has also contributed to the development of
atomic physics and quantum optics, including applications to metrology,
spectroscopy, and quantum information processing. The quantum microwave regime,
originally explored using cavities and atoms, is seeing a novel boost with the
generation of nonclassical propagating fields in circuit quantum
electrodynamics (QED). This promising field, involving potential developments
in quantum information with microwave photons, suffers from the absence of
photodetectors. Here, we design a metamaterial composed of discrete
superconducting elements that implements a high-efficiency microwave photon
detector. Our design consists of a microwave guide coupled to an array of
metastable quantum circuits, whose internal states are irreversibly changed due
to the absorption of photons. This proposal can be widely applied to different
physical systems and can be generalized to implement a microwave photon
counter.Comment: accepted in Phys. Rev. Let
Estudio mineralógico de las arcillas cerámicas de Pereruela (Zamora)
Trabajo financiado por el Instituto de Estudios Zamoranos Florián de
Ocampo.Peer reviewe
IEO Studies on discards and By-catch of the Shrimper Industrial Fleet in West Africa
Studies on discards and by-catch on the shrimper industrial fleet in West Africa (Mauritania and Guinea Bissau)(IEO Program of Scientific observations onboard. 2010 and 2011
Effect of nutrient input frequency on the structure and dynamics of the marine pelagic community: A modeling approach
33 páginas, 9 figuras, 2 tablas.An analysis of the induced changes in both total productivity and community structure as a consequence of different pulsed nutrient inputs has been made by using two different ecosystem modeling tools. One was a modified version of the N-based model proposed by Fasham et al. (1990) for pelagic ecosystems and the other the plankton functional type model by Vichi et al. (2007). Both models lead to higher total biomass production with a pulsed nutrient input compared to a continuous supply, affecting both phytoplankton and zooplankton. Detailed responses are, however, different among plankton groups depending on the frequency of pulsed nutrient input and the complexity of the model. The differences in biomass production were higher for an oscillation period of 150-170 days following a sinusoidal supply pattern. A tidal mixing-induced nutrient supply was also simulated by using both models and a larger biomass increment was observed compared to that obtained with the sinusoidal pattern. Finally, a theoretical application of this approach to the pelagic ecosystem of the North Western Alboran Sea shows a preferential selection of diatoms and larger zooplankton under discontinuous nutrient inputs.This work was funded by the Spanish National Research Program, Project;
CTM2005–08142-C03–01. D.M. was supported by the Spanish Postdoc Fellowship Program and
E.R. by a grant from the Spanish FPU fellowship.Peer reviewe
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