369 research outputs found
Nonlocal Conformal Theories Have State-dependent Central Charges
Using the recently developed fractional Virasoro algebra [1], we construct
the equivalent operator product expansions for nonlocal quantum field theories
in which the nonlocality is provided by the fractional Laplacian as has been
shown to be relevant in the long-range Ising model. We find that the OPE's of a
general nonlocal CFT are of the form and which naturally
results in a central charge, , that is state-dependent and hence not a
constant. In fact, our work indicates that only those theories which are truly
nonlocal have state-dependent central charges. All others can be mapped onto an
equivalent Gaussian one using a suitable field redefinition.Comment: 24 pages, one appendi
UV/Optical Detections of Candidate Tidal Disruption Events by GALEX and CFHTLS
We present two luminous UV/optical flares from the nuclei of apparently
inactive early-type galaxies at z=0.37 and 0.33 that have the radiative
properties of a flare from the tidal disruption of a star. In this paper we
report the second candidate tidal disruption event discovery in the UV by the
GALEX Deep Imaging Survey, and present simultaneous optical light curves from
the CFHTLS Deep Imaging Survey for both UV flares. The first few months of the
UV/optical light curves are well fitted with the canonical t^(-5/3) power-law
decay predicted for emission from the fallback of debris from a tidally
disrupted star. Chandra ACIS X-ray observations during the flares detect soft
X-ray sources with T_bb= (2-5) x 10^5 K or Gamma > 3 and place limits on hard
X-ray emission from an underlying AGN down to L_X (2-10 keV) <~ 10^41 ergs/s.
Blackbody fits to the UV/optical spectral energy distributions of the flares
indicate peak flare luminosities of > 10^44-10^45 ergs/s. The temperature,
luminosity, and light curves of both flares are in excellent agreement with
emission from a tidally disrupted main sequence star onto a central black hole
of several times 10^7 msun. The observed detection rate of our search over ~
2.9 deg^2 of GALEX Deep Imaging Survey data spanning from 2003 to 2007 is
consistent with tidal disruption rates calculated from dynamical models, and we
use these models to make predictions for the detection rates of the next
generation of optical synoptic surveys.Comment: 28 pages, 27 figures, 11 tables, accepted to ApJ, final corrections
from proofs adde
Supernova / Acceleration Probe: A Satellite Experiment to Study the Nature of the Dark Energy
The Supernova / Acceleration Probe (SNAP) is a proposed space-based
experiment designed to study the dark energy and alternative explanations of
the acceleration of the Universe's expansion by performing a series of
complementary systematics-controlled measurements. We describe a
self-consistent reference mission design for building a Type Ia supernova
Hubble diagram and for performing a wide-area weak gravitational lensing study.
A 2-m wide-field telescope feeds a focal plane consisting of a 0.7
square-degree imager tiled with equal areas of optical CCDs and near infrared
sensors, and a high-efficiency low-resolution integral field spectrograph. The
SNAP mission will obtain high-signal-to-noise calibrated light-curves and
spectra for several thousand supernovae at redshifts between z=0.1 and 1.7. A
wide-field survey covering one thousand square degrees resolves ~100 galaxies
per square arcminute. If we assume we live in a cosmological-constant-dominated
Universe, the matter density, dark energy density, and flatness of space can
all be measured with SNAP supernova and weak-lensing measurements to a
systematics-limited accuracy of 1%. For a flat universe, the
density-to-pressure ratio of dark energy can be similarly measured to 5% for
the present value w0 and ~0.1 for the time variation w'. The large survey area,
depth, spatial resolution, time-sampling, and nine-band optical to NIR
photometry will support additional independent and/or complementary dark-energy
measurement approaches as well as a broad range of auxiliary science programs.
(Abridged)Comment: 40 pages, 18 figures, submitted to PASP, http://snap.lbl.go
The silicon micro-strip detector plane for the LOFT/Wide Field Monitor
The main objective of the Wide Field Monitor (WFM) on the LOFT mission is to
provide unambiguous detection of the high-energy sources in a large field of
view, in order to support science operations of the LOFT primary instrument,
the LAD. The monitor will also provide by itself a large number of results on
the timing and spectral behaviour of hundreds of galactic compact objects,
Active Galactic Nuclei and Gamma-Ray Bursts. The WFM is based on the coded
aperture concept where a position sensitive detector records the shadow of a
mask projected by the celestial sources. The proposed WFM detector plane, based
on Double Sided micro-Strip Silicon Detectors (DSSD), will allow proper
2-dimensional recording of the projected shadows. Indeed the positioning of the
photon interaction in the detector with equivalent fine resolution in both
directions insures the best imaging capability compatible with the allocated
budgets for this telescope on LOFT. We will describe here the overall
configuration of this 2D-WFM and the design and characteristics of the DSSD
detector plane including its imaging and spectral performances. We will also
present a number of simulated results discussing the advantages that this
configuration offers to LOFT. A DSSD-based WFM will in particular reduce
significantly the source confusion experienced by the WFM in crowded regions of
the sky like the Galactic Center and will in general increase the observatory
science capability of the mission.Comment: Proceedings of SPIE, Vol. 8443, Paper No. 8443-89, 201
Experimental simulations of methane-oriented underground coal gasification using hydrogen - The effect of coal rank and gasification pressure on the hydrogasification process
This paper presents a series of surface experimental simulations of methane-oriented underground coal gasification using hydrogen as gasification medium. The main aim of the experiments conducted was to evaluate the feasibility of methane-rich gas production through the in situ coal hydrogasification process. Two multi-day trials were carried out using large scale gasification facilities designed for ex situ experimental simulations of the underground coal gasification (UCG) process. Two different coals were investigated: the âSix Feetâ semi-anthracite (Wales) and the âWesoĆa" hard coal (Poland). The coal samples were extracted directly from the respective coal seams in the form of large blocks. The gasification tests were conducted in the artificial coal seams (0.41 Ă 0.41 Ă 3.05 m) under two distinct pressure regimes - 20 and 40 bar. The series of experiments conducted demonstrated that the physicochemical properties of coal (coal rank) considerably affect the hydrogasification process. For both gasification pressures applied, gas from âSix Feetâ semi-anthracite was characterized by a higher content of methane. The average CH4 concentration for âSix Feetâ experiment during the H2 stage was 24.12% at 20 bar and 27.03% at 40 bar. During the hydrogasification of âWesoĆa" coal, CH4 concentration was 19.28% and 21.71% at 20 and 40 bar, respectively. The process was characterized by high stability and reproducibility of conditions favorable for methane formation in the whole sequence of gasification cycles. Although the feasibility of methane-rich gas production by underground hydrogasification was initially demonstrated, further techno-economic studies are necessary to assess the economic feasibility of methane production using this process
Background Light in Potential Sites for the ANTARES Undersea Neutrino Telescope
The ANTARES collaboration has performed a series of {\em in situ}
measurements to study the background light for a planned undersea neutrino
telescope. Such background can be caused by K decays or by biological
activity. We report on measurements at two sites in the Mediterranean Sea at
depths of 2400~m and 2700~m, respectively. Three photomultiplier tubes were
used to measure single counting rates and coincidence rates for pairs of tubes
at various distances. The background rate is seen to consist of three
components: a constant rate due to K decays, a continuum rate that
varies on a time scale of several hours simultaneously over distances up to at
least 40~m, and random bursts a few seconds long that are only correlated in
time over distances of the order of a meter. A trigger requiring coincidences
between nearby photomultiplier tubes should reduce the trigger rate for a
neutrino telescope to a manageable level with only a small loss in efficiency.Comment: 18 pages, 8 figures, accepted for publication in Astroparticle
Physic
Search for muon-neutrino emission from GeV and TeV gamma-ray flaring blazars using five years of data of the ANTARES telescope
The ANTARES telescope is well-suited for detecting astrophysical transient
neutrino sources as it can observe a full hemisphere of the sky at all times
with a high duty cycle. The background due to atmospheric particles can be
drastically reduced, and the point-source sensitivity improved, by selecting a
narrow time window around possible neutrino production periods. Blazars, being
radio-loud active galactic nuclei with their jets pointing almost directly
towards the observer, are particularly attractive potential neutrino point
sources, since they are among the most likely sources of the very high-energy
cosmic rays. Neutrinos and gamma rays may be produced in hadronic interactions
with the surrounding medium. Moreover, blazars generally show high time
variability in their light curves at different wavelengths and on various time
scales. This paper presents a time-dependent analysis applied to a selection of
flaring gamma-ray blazars observed by the FERMI/LAT experiment and by TeV
Cherenkov telescopes using five years of ANTARES data taken from 2008 to 2012.
The results are compatible with fluctuations of the background. Upper limits on
the neutrino fluence have been produced and compared to the measured gamma-ray
spectral energy distribution.Comment: 27 pages, 16 figure
All-sky Search for High-Energy Neutrinos from Gravitational Wave Event GW170104 with the ANTARES Neutrino Telescope
Advanced LIGO detected a significant gravitational wave signal (GW170104)
originating from the coalescence of two black holes during the second
observation run on January 4, 2017. An all-sky high-energy
neutrino follow-up search has been made using data from the ANTARES neutrino
telescope, including both upgoing and downgoing events in two separate
analyses. No neutrino candidates were found within s around the GW
event time nor any time clustering of events over an extended time window of
months. The non-detection is used to constrain isotropic-equivalent
high-energy neutrino emission from GW170104 to less than
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