20,688 research outputs found
Gamma Ray Bursts: Cosmic Rulers for the High-Redshift Universe?
The desire to extend the Hubble Diagram to higher redshifts than the range of
current Type Ia Supernovae observations has prompted investigation into
spectral correlations in Gamma Ray Bursts, in the hope that standard
candle-like properties can be identified. In this paper we discuss the
potential of these new `cosmic rulers' and highlight their limitations by
investigating the constraints that current data can place on an alternative
Cosmological model in the form of Conformal Gravity. By fitting current Type 1a
Supernovae and Gamma Ray Burst (GRB) data to the predicted luminosity distance
redshift relation of both the standard Concordance Model and Conformal Gravity,
we show that currently \emph{neither} model is strongly favoured at high
redshift. The scatter in the current GRB data testifies to the further work
required if GRBs are to cement their place as effective probes of the
cosmological distance scale.Comment: 2 pages, 1 figure (black & white, colour available). To be published
in "Phil. Trans. of the Royal Society" as proceedings from Discussion Meeting
on Gamma Ray Burst
The silicate absorption profile in the ISM towards the heavily obscured nucleus of NGC 4418
The 9.7-micron silicate absorption profile in the interstellar medium
provides important information on the physical and chemical composition of
interstellar dust grains. Measurements in the Milky Way have shown that the
profile in the diffuse interstellar medium is very similar to the amorphous
silicate profiles found in circumstellar dust shells around late M stars, and
narrower than the silicate profile in denser star-forming regions. Here, we
investigate the silicate absorption profile towards the very heavily obscured
nucleus of NGC 4418, the galaxy with the deepest known silicate absorption
feature, and compare it to the profiles seen in the Milky Way. Comparison
between the 8-13 micron spectrum obtained with TReCS on Gemini and the larger
aperture spectrum obtained from the Spitzer archive indicates that the former
isolates the nuclear emission, while Spitzer detects low surface brightness
circumnuclear diffuse emission in addition. The silicate absorption profile
towards the nucleus is very similar to that in the diffuse ISM in the Milky Way
with no evidence of spectral structure from crystalline silicates or silicon
carbide grains.Comment: 7 Pages, 3 figures. MNRAS in pres
Nanowires: A route to efficient thermoelectric devices
Miniaturization of electronic devices aims at manufacturing ever smaller
products, from mesoscopic to nanoscopic sizes. This trend is challenging
because the increased levels of dissipated power demands a better understanding
of heat transport in small volumes. A significant amount of the consumed energy
is transformed into heat and dissipated to the environment. Thermoelectric
materials offer the possibility to harness dissipated energy and make devices
less energy-demanding. Heat-to-electricity conversion requires materials with a
strongly suppressed thermal conductivity but still high electronic conduction.
Nanowires can meet nicely these two requirements because enhanced phonon
scattering at the surface and defects reduces the lattice thermal conductivity
while electric conductivity is not deteriorated, leading to an overall
remarkable thermoelectric efficiency. Therefore, nanowires are regarded as a
promising route to achieving valuable thermoelectric materials at the
nanoscale. In this paper, we present an overview of key experimental and
theoretical results concerning the thermoelectric properties of nanowires. The
focus of this review is put on the physical mechanisms by which the efficiency
of nanowires can be improved. Phonon scattering at surfaces and interfaces,
enhancement of the power factor by quantum effects and topological protection
of electron states to prevent the degradation of electrical conductivity in
nanowires are thoroughly discussed
Multiwavelength analysis of the young open cluster NGC 2362
We present a multiwavelength analysis of the young open cluster NGC 2362.
UBVRcIc CCD photometric observations, together with available data in the
Chandra data base, near infrared data from the Two Micron All Sky Survey
(2MASS), and recently published Halpha spectroscopy were used to get
information about the evolutionary stage of the cluster and the main physical
properties of its stellar content. Cluster membership is estimated for every
individual star by means of ZAMS and isochrone fitting. The cluster is
confirmed to host a richly populated pre-main sequence (PMS), and to contain a
large amount of X-ray emitting stars, which reach from the PMS members of GK
spectral type, up to the most luminous OB type main sequence (MS) members. The
PMS cluster members show no significant age spread, and the comparison to both
PMS and post-MS isochrones suggests a younger age for the more massive MS than
for lower mass PMS members. The analysis allows to asses the validity of
currently used pre-main sequence evolutionary models, and supports the
suggestion of a well defined positive correlation of the X-ray emission from
PMS stars with their bolometric luminosity. Clear differences are found on the
other hand, between the X-ray activity properties of MS and PMS cluster
members, both in the relation between X-ray luminosity and bolometric
luminosity, and in spectral properties as well.Comment: 1 gzipped file: 1 tex file with 9 pages text. 5 ps files with
figures. Submitted to Astrophysical Journa
Entanglement of two qubits mediated by one-dimensional plasmonic waveguides
We investigate qubit-qubit entanglement mediated by plasmons supported by
one-dimensional waveguides. We explore both the situation of spontaneous
formation of entanglement from an unentangled state and the emergence of driven
steady-state entanglement under continuous pumping. In both cases, we show that
large values for the concurrence are attainable for qubit-qubit distances
larger than the operating wavelength by using plasmonic waveguides that are
currently available.Comment: 4 pages, 4 figures. Minor Changes. Journal Reference added.
Highlighted in Physic
VLT and GTC observations of SDSS J0123+00: a type 2 quasar triggered in a galaxy encounter?
We present long-slit spectroscopy, continuum and [OIII]5007 imaging data
obtained with the Very Large Telescope and the Gran Telescopio Canarias of the
type 2 quasar SDSS J0123+00 at z=0.399. The quasar lies in a complex, gas-rich
environment. It appears to be physically connected by a tidal bridge to another
galaxy at a projected distance of ~100 kpc, which suggests this is an
interacting system. Ionized gas is detected to a distance of at least ~133 kpc
from the nucleus. The nebula has a total extension of ~180 kpc. This is one of
the largest ionized nebulae ever detected associated with an active galaxy.
Based on the environmental properties, we propose that the origin of the nebula
is tidal debris from a galactic encounter, which could as well be the
triggering mechanism of the nuclear activity. SDSS J0123+00 demonstrates that
giant, luminous ionized nebulae can exist associated with type 2 quasars of low
radio luminosities, contrary to expectations based on type 1 quasar studies.Comment: 6 pages, 5 figures. Accepted for publication in MNRAS Letter
Active galactic nuclei synapses: X-ray versus optical classifications using artificial neural networks
(Abridged) Many classes of active galactic nuclei (AGN) have been defined
entirely throughout optical wavelengths while the X-ray spectra have been very
useful to investigate their inner regions. However, optical and X-ray results
show many discrepancies that have not been fully understood yet. The aim of
this paper is to study the "synapses" between the X-ray and optical
classifications.
For the first time, the new EFLUXER task allowed us to analyse broad band
X-ray spectra of emission line nuclei (ELN) without any prior spectral fitting
using artificial neural networks (ANNs). Our sample comprises 162 XMM-Newton/pn
spectra of 90 local ELN in the Palomar sample. It includes starbursts (SB),
transition objects (T2), LINERs (L1.8 and L2), and Seyferts (S1, S1.8, and S2).
The ANNs are 90% efficient at classifying the trained classes S1, S1.8, and
SB. The S1 and S1.8 classes show a wide range of S1- and S1.8-like components.
We suggest that this is related to a large degree of obscuration at X-rays. The
S1, S1.8, S2, L1.8, L2/T2/SB-AGN (SB with indications of AGN), and SB classes
have similar average X-ray spectra within each class, but these average spectra
can be distinguished from class to class. The S2 (L1.8) class is linked to the
S1.8 (S1) class with larger SB-like component than the S1.8 (S1) class. The L2,
T2, and SB-AGN classes conform a class in the X-rays similar to the S2 class
albeit with larger fractions of SB-like component. This SB-like component is
the contribution of the star-formation in the host galaxy, which is large when
the AGN is weak. An AGN-like component seems to be present in the vast majority
of the ELN, attending to the non-negligible fraction of S1-like or S1.8-like
component. This trained ANN could be used to infer optical properties from
X-ray spectra in surveys like eRosita.Comment: 15 pages, 7 figures, accepted for publication in A&A. Appendix B only
in the full version of the paper here:
https://dl.dropboxusercontent.com/u/3484086/AGNSynapsis_OGM_online.pd
Quantum Renormalization Group for 1 Dimensional Fermion Systems
Inspired by the superblock method of White, we introduce a simple
modification of the standard Renormalization Group (RG) technique for the study
of quantum lattice systems. Our method which takes into account the effect of
Boundary Conditions(BC), may be regarded as a simple way for obtaining first
estimates of many properties of quantum lattice systems. By applying this
method to the 1-dimensional free and interacting fermion system, we obtain the
ground state energy with much higher accuracy than the standard RG. We also
calculate the density-density correlation function in the free-fermion case
which shows good agreement with the exact result.Comment: LaTex file, 1 PS figur
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