3,700 research outputs found
An X-ray, Optical and Radio Search for Supernova Remnants in the Nearby Sculptor Group Sd Galaxy NGC 7793
We present a multi-wavelength study of the properties of supernova remnants
(SNRs) in the nearby Sculptor Group Sd galaxy NGC 7793. Using our own Very
Large Array radio observations at 6 and 20 cm, as well as archived ROSAT X-ray
data, previously published optical results and our own H-alpha image, we have
searched for X-ray and radio counterparts to previously known
optically-identified SNRs and for new previously unidentified SNRs at these two
wavelength regimes. Only two of the 28 optically-identified SNRs are detected
at another wavelength. The most noteworthy source in our study is N7793-S26,
which is the only SNR that is detected at all three wavelengths. It features a
long (approximately 450 pc) filamentary morphology that is clearly seen in both
the optical and radio images. N7793-S26's radio luminosity exceeds that of the
Galactic SNR Cas A, and based on equipartition calculations we determine that
an energy of at least 10^52 ergs is required to maintain this source. A second
optically identified SNR, N7793-S11, has detectable radio emission but is not
detected in the X-ray. Complementary X-ray and radio searches for SNRs have
yielded five new candidate radio SNRs, to be added to the 28 SNRs in this
galaxy that have already been detected by optical methods. We find that the
density of the ambient interstellar medium (ISM) surrounding these SNRs
significantly impacts the spectral characteristics of SNRs in this galaxy,
consistent with surveys of the SNR populations in other galaxies.Comment: 32 pages, 25 figures, to appear in the Astrophysical Journal
(February 2002
Artificial gauge fields for the Bose-Hubbard model on a checkerboard superlattice and extended Bose-Hubbard model
We study the effects of an artificial gauge field on the ground-state phases
of the Bose-Hubbard model on a checkerboard superlattice in two dimensions,
including the superfluid phase and the Mott and alternating Mott insulators.
First, we discuss the single-particle Hofstadter problem, and show that the
presence of a checkerboard superlattice gives rise to a magnetic
flux-independent energy gap in the excitation spectrum. Then, we consider the
many-particle problem, and derive an analytical mean-field expression for the
superfluid-Mott and superfluid--alternating-Mott insulator phase transition
boundaries. Finally, since the phase diagram of the Bose-Hubbard model on a
checkerboard superlattice is in many ways similar to that of the extended
Bose-Hubbard model, we comment on the effects of magnetic field on the latter
model, and derive an analytical mean-field expression for the
superfluid-insulator phase transition boundaries as well.Comment: 8 pages, 5 figures and 1 table; to appear in EPJ
Cosmic magnetization: from spontaneously emitted aperiodic turbulent to ordered equipartition fields
It is shown that an unmagnetized nonrelativistic thermal electron-proton
plasma spontaneously emits aperiodic turbulent magnetic field fluctuations of
strength G, where is the
normalized thermal electron temperature, the thermal plasma energy
density and the plasma parameter. Aperiodic modes fluctuate only in space,
but are not propagating. For the unmagnetized intergalactic medium, immediately
after the reionization onset, the field strength from this mechanism is about
G, too weak to affect the dynamics of the plasma. The shear
and/or compression of the intergalactic medium exerted by the first supernova
explosions amplify these seed fields and make them anisotropic, until the
magnetic restoring forces affect the gas dynamics at ordered plasma betas near
unity.Comment: 4 pages, 1 figur
Astrobiological Complexity with Probabilistic Cellular Automata
Search for extraterrestrial life and intelligence constitutes one of the
major endeavors in science, but has yet been quantitatively modeled only rarely
and in a cursory and superficial fashion. We argue that probabilistic cellular
automata (PCA) represent the best quantitative framework for modeling
astrobiological history of the Milky Way and its Galactic Habitable Zone. The
relevant astrobiological parameters are to be modeled as the elements of the
input probability matrix for the PCA kernel. With the underlying simplicity of
the cellular automata constructs, this approach enables a quick analysis of
large and ambiguous input parameters' space. We perform a simple clustering
analysis of typical astrobiological histories and discuss the relevant boundary
conditions of practical importance for planning and guiding actual empirical
astrobiological and SETI projects. In addition to showing how the present
framework is adaptable to more complex situations and updated observational
databases from current and near-future space missions, we demonstrate how
numerical results could offer a cautious rationale for continuation of
practical SETI searches.Comment: 37 pages, 11 figures, 2 tables; added journal reference belo
Jet-cloud collisions in the jet of the Seyfert galaxy NGC3079
We report the results from a six-year, multi-epoch very long baseline
interferomertry monitoring of the Seyfert galaxy NGC3079. We have observed
NGC3079 during eight epochs between 1999 and 2005 predominantly at 5GHz, but
covering the frequency range of 1.7GHz to 22GHz. Using our data and
observations going back to 1985, we find that the separation of two of the
three visible nuclear radio components underwent two decelerations. At the time
of these decelerations, the flux density of one of the components increased by
factors of five and two, respectively. We interpret these events as a radio jet
component undergoing compression, possibly as a result of a collision with ISM
material. This interpretation strongly supports the existence of jets
surrounded by a clumpy medium of dense clouds within the first few parsecs from
the central engine in NGC3079. Moreover, based on recently published
simulations of jet interactions with clumpy media, this scenario is able to
explain the nature of two additional regions of ageing synchrotron material
detected at the lower frequencies as by-products of such interactions, and also
the origin of the kpc-scale super bubble observed in NGC3079 as the result of
the spread of the momentum of the jets impeded from propagating freely. The
generalization of this scenario provides an explanation why jets in Seyfert
galaxies are not able to propagate to scales of kpc as do jets in radio-loud
AGN.Comment: 11 pages, 9 figures, needs aas_macros.sty and mn2e.cl
Scenario analysis report with policy recommendations: An assessment of sustainability, resilience, efficiency and fairness and effective chain relationships in VALUMICS case studies : Deliverable 8.4
This is an open access article distributed under the Creative Commons Attribution License, to view a copy of the license, see: https://creativecommons.org/licenses/by/4.0/. The final version of this report is available at https://doi.org/10.5281/zenodo.6534011The functioning of food value chains entails a complex organisation from farm to fork which is characterised by various governance forms and externalities which have shaped the overall food system. VALUMICS food value chain case studies: wheat to bread, dairy cows to milk, beef cattle to steak, farmed salmon to fillets and tomato to processed tomato were selected to enable explorative and empirical analysis to better understand the functioning of the food system and, to identify the main challenges that need to be addressed to improve sustainability, integrity, resilience, and fairness of European food chains. The VALUMICS system analysis was executed through four operational phases starting with Groundwork & analysis including mapping specific attributes and impacts of food value chains and their externalities. This was followed by Case study baseline analysis, which provided input to the third phase on Modelling and exploration of future scenarios and finally Policy and synthesis of the overall work. This report is an overall synthesis of the VALUMICS results as follows: ⢠Key findings from the VALUMICS project on the functioning of European food value chains and their impacts on more sustainable, resilient, fairer, and transparent food system are summarised through a compilation of 25 Research Findings and Policy Briefs. ⢠By highlighting the major contributions from the research activities throughout the four phases of the VALUMICS project, this report delivers an assessment of various factors influencing sustainability, resilience, efficiency and fairness and effective chain relationships of different food value chains, and their determinants. ⢠The synthesis of the outcome allows the identification of opportunities and challenges characterising the functioning of food supply chains, and thus, the prospects and potentials for strengthening the EU food sector
Combined search for the quarks of a sequential fourth generation
Results are presented from a search for a fourth generation of quarks
produced singly or in pairs in a data set corresponding to an integrated
luminosity of 5 inverse femtobarns recorded by the CMS experiment at the LHC in
2011. A novel strategy has been developed for a combined search for quarks of
the up and down type in decay channels with at least one isolated muon or
electron. Limits on the mass of the fourth-generation quarks and the relevant
Cabibbo-Kobayashi-Maskawa matrix elements are derived in the context of a
simple extension of the standard model with a sequential fourth generation of
fermions. The existence of mass-degenerate fourth-generation quarks with masses
below 685 GeV is excluded at 95% confidence level for minimal off-diagonal
mixing between the third- and the fourth-generation quarks. With a mass
difference of 25 GeV between the quark masses, the obtained limit on the masses
of the fourth-generation quarks shifts by about +/- 20 GeV. These results
significantly reduce the allowed parameter space for a fourth generation of
fermions.Comment: Replaced with published version. Added journal reference and DO
Measurement of the t t-bar production cross section in the dilepton channel in pp collisions at sqrt(s) = 7 TeV
The t t-bar production cross section (sigma[t t-bar]) is measured in
proton-proton collisions at sqrt(s) = 7 TeV in data collected by the CMS
experiment, corresponding to an integrated luminosity of 2.3 inverse
femtobarns. The measurement is performed in events with two leptons (electrons
or muons) in the final state, at least two jets identified as jets originating
from b quarks, and the presence of an imbalance in transverse momentum. The
measured value of sigma[t t-bar] for a top-quark mass of 172.5 GeV is 161.9 +/-
2.5 (stat.) +5.1/-5.0 (syst.) +/- 3.6(lumi.) pb, consistent with the prediction
of the standard model.Comment: Replaced with published version. Included journal reference and DO
Search for the standard model Higgs boson in the H to ZZ to 2l 2nu channel in pp collisions at sqrt(s) = 7 TeV
A search for the standard model Higgs boson in the H to ZZ to 2l 2nu decay
channel, where l = e or mu, in pp collisions at a center-of-mass energy of 7
TeV is presented. The data were collected at the LHC, with the CMS detector,
and correspond to an integrated luminosity of 4.6 inverse femtobarns. No
significant excess is observed above the background expectation, and upper
limits are set on the Higgs boson production cross section. The presence of the
standard model Higgs boson with a mass in the 270-440 GeV range is excluded at
95% confidence level.Comment: Submitted to JHE
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