1,589 research outputs found
Electrostatic traps for dipolar excitons
We consider the design of two-dimensional electrostatic traps for dipolar
indirect excitons. We show that the excitons dipole-dipole interaction,
combined with the in-plane electric fields that arise due to the trap geometry,
constrain the maximal density and lifetime of trapped excitons. We derive an
analytic estimate of these values and determine their dependence on the trap
geometry, thus suggesting the optimal design for high density trapping as a
route for observing excitonic Bose-Einstein condensation.Comment: 5 pages, 3 figures. This 2nd version contains a revised Fig.3 + minor
revisions to the discussion and abstrac
Medium-Term Load Forecasting in Isolated Power Systems Based on Ensemble Machine Learning Models
Over the past decades, power companies have been implementing load forecasting to determine trends in the electric power system (EPS); therefore, load forecasting is applied to solve the problems of management and development of power systems. This paper considers the issue of building a model of medium-term forecasting of load graphs for EPS with specific properties, based on the use of ensemble machine learning methods. This paper implements the approach of identification of the most significant features to apply machine learning models for medium-term load forecasting in an isolated power system. A comparative study of the following models was carried out: linear regression, support vector regression (SVR), decision tree regression, random forest (Random Forest), gradient boosting over decision trees (XGBoost), adaptive boosting over decision trees (AdaBoost), AdaBoost over linear regression. Isolation of features from a time series allows for the implementation of simpler and more overfitting-resistant models. All the above makes it possible to increase the reliability of forecasts and expand the use of information technologies in the planning, management, and operation of isolated EPSs. Calculations of the total forecast error have proved that the characteristics of the proposed models are high quality and accurate, and thus they can be used to forecast the real load of a power system. © 2021 The Author(s).The reported study was funded by RFBR, Sirius University of Science and Technology, JSC Russian Railways and Educational Fund “Talent and success”, project number 20-38-51007
Extended Emission from Short Gamma-Ray Bursts Detected with SPI-ACS/INTEGRAL
The short duration (T90 < 2 s) gamma-ray bursts (GRBs) detected in the
SPI-ACS experiment onboard the INTEGRAL observatory are investigated. Averaged
light curves have been constructed for various groups of events, including
short GRBs and unidentified short events. Extended emission has been found in
the averaged light curves of both short GRBs and unidentified short events. It
is shown that the fraction of the short GRBs in the total number of SPI-ACS
GRBs can range from 30 to 45%, which is considerably larger than has been
thought previously.Comment: 27 pages, 10 figure
Graviton Production By Two Photon and Electron-Photon Processes In Kaluza-Klein Theories With Large Extra Dimensions
We consider the production of gravitons via two photon and electron-photon
fusion in Kaluza-Klein theories which allow TeV scale gravitational
interactions. We show that at electron-positron colliders, the processes l+l-
-> l+ l- graviton, with l=e, mu, can lead to a new signal of low energy gravity
of the form l+l- -> l+l- + missing energy which is well above the Standard
Model background. For example, with two extra dimensions at the Next Linear
Collider with a center of mass energy of 500 or 1000 GeV, hundreds to thousands
such l+ l- graviton events may be produced if the scale of the gravitational
interactions, M_D, is around a few TeV. At a gamma-electron collider, more
stringent bounds may be placed on M_D via the related reaction e^-\gamma -> e^-
graviton. For instance, if a 1TeV electron positron collider is converted to an
electron-photon collider, a bound of ~10TeV may be placed on the scale M_D if
the number of extra dimensions delta=2 while a bound of ~4TeV may be placed if
delta=4.Comment: 25 pages 6 figures, minor changes made in the text and changes in
reference
Electrodynamics of Magnetars: Implications for the Persistent X-ray Emission and Spindown of the Soft Gamma Repeaters and Anomalous X-ray Pulsars
(ABBREVIATED) We consider the structure of neutron star magnetospheres
threaded by large-scale electrical currents, and the effect of resonant Compton
scattering by the charge carriers (both electrons and ions) on the emergent
X-ray spectra and pulse profiles. In the magnetar model for the SGRs and AXPs,
these currents are maintained by magnetic stresses acting deep inside the star.
We construct self-similar, force-free equilibria of the current-carrying
magnetosphere with a power-law dependence of magnetic field on radius, B ~
r^(-2-p), and show that a large-scale twist softens the radial dependence to p
< 1. The spindown torque acting on the star is thereby increased in comparison
with a vacuum dipole. We comment on the strength of the surface magnetic field
in the SGR and AXP sources, and the implications of this model for the narrow
measured distribution of spin periods. A magnetosphere with a strong twist,
B_\phi/B_\theta = O(1) at the equator, has an optical depth ~ 1 to resonant
cyclotron scattering, independent of frequency (radius), surface magnetic field
strength, or charge/mass ratio of the scattering charge. When electrons and
ions supply the current, the stellar surface is also heated by the impacting
charges at a rate comparable to the observed X-ray output of the SGR and AXP
sources, if B_{dipole} ~ 10^{14} G. Redistribution of the emerging X-ray flux
at the ion and electron cyclotron resonances will significantly modify the
emerging pulse profile and, through the Doppler effect, generate a non-thermal
tail to the X-ray spectrum. The sudden change in the pulse profile of SGR
1900+14 after the 27 August 1998 giant flare is related to an enhanced optical
depth to electron cyclotron scattering, resulting from a sudden twist imparted
to the external magnetic field.Comment: 31 January 2002, minor revisions, new section 5.4.
Resonant Cyclotron Radiation Transfer Model Fits to Spectra from Gamma-Ray Burst GRB870303
We demonstrate that models of resonant cyclotron radiation transfer in a
strong field (i.e. cyclotron scattering) can account for spectral lines seen at
two epochs, denoted S1 and S2, in the Ginga data for GRB870303. Using a
generalized version of the Monte Carlo code of Wang et al. (1988,1989b), we
model line formation by injecting continuum photons into a static
plane-parallel slab of electrons threaded by a strong neutron star magnetic
field (~ 10^12 G) which may be oriented at an arbitrary angle relative to the
slab normal. We examine two source geometries, which we denote "1-0" and "1-1,"
with the numbers representing the relative electron column densities above and
below the continuum photon source plane. We compare azimuthally symmetric
models, i.e. models in which the magnetic field is parallel to the slab normal,
with models having more general magnetic field orientations. If the bursting
source has a simple dipole field, these two model classes represent line
formation at the magnetic pole, or elsewhere on the stellar surface. We find
that the data of S1 and S2, considered individually, are consistent with both
geometries, and with all magnetic field orientations, with the exception that
the S1 data clearly favor line formation away from a polar cap in the 1-1
geometry, with the best-fit model placing the line-forming region at the
magnetic equator. Within both geometries, fits to the combined (S1+S2) data
marginally favor models which feature equatorial line formation, and in which
the observer's orientation with respect to the slab changes between the two
epochs. We interpret this change as being due to neutron star rotation, and we
place limits on the rotation period.Comment: LaTeX2e (aastex.cls included); 45 pages text, 17 figures (on 21
pages); accepted by ApJ (to be published 1 Nov 1999, v. 525
Gauge Boson - Gauge Boson Scattering in Theories with Large Extra Dimensions
We consider the scattering amplitudes of the form V_1 V_2 -> V_3 V_4, where
V_i=\gamma, Z, W or g(=gluon) are the Standard Model gauge bosons, due to
graviton exchange in Kaluza-Klein theories with large extra dimensions. This
leads to a number of experimentally viable signatures at high energy leptonic
and hadronic colliders. We discuss the observability or future limits on the
scale of the gravitational interactions (m_D), that may be obtained at an e+e-
Next Linear Collider (NLC) and at the LHC, by studying some of these type of
gauge boson scattering processes. We find that the attainable limits through
these type of processes are: m_D > 3 TeV at the NLC and m_D > 6 TeV at the LHC.Comment: 27 pages, plain latex, 7 figures embadded in the text using epsfi
The ultraluminous GRB 110918A
GRB 110918A is the brightest long GRB detected by Konus-WIND during its 19
years of continuous observations and the most luminous GRB ever observed since
the beginning of the cosmological era in 1997. We report on the final IPN
localization of this event and its detailed multiwavelength study with a number
of space-based instruments. The prompt emission is characterized by a typical
duration, a moderare of the time-integrated spectrum, and strong
hard-to-soft evolution. The high observed energy fluence yields, at z=0.984, a
huge isotropic-equivalent energy release
erg. The record-breaking energy flux observed at the peak of the short, bright,
hard initial pulse results in an unprecedented isotropic-equivalent luminosity
erg s. A tail of the soft gamma-ray
emission was detected with temporal and spectral behavior typical of that
predicted by the synchrotron forward-shock model. Swift/XRT and Swift/UVOT
observed the bright afterglow from 1.2 to 48 days after the burst and revealed
no evidence of a jet break. The post-break scenario for the afterglow is
preferred from our analysis, with a hard underlying electron spectrum and
ISM-like circumburst environment implied. We conclude that, among multiple
reasons investigated, the tight collimation of the jet must have been a key
ingredient to produce this unusually bright burst. The inferred jet opening
angle of 1.7-3.4 deg results in reasonable values of the collimation-corrected
radiated energy and the peak luminosity, which, however, are still at the top
of their distributions for such tightly collimated events. We estimate a
detection horizon for a similar ultraluminous GRB of for Konus-WIND,
and for Swift/BAT, which stresses the importance of GRBs as probes of
the early Universe.Comment: 22 pages, 20 figures, accepted for publication in Ap
Silica suspension and coating developments for Advanced LIGO
The proposed upgrade to the LIGO detectors to form the Advanced LIGO detector system is intended to incorporate a low thermal noise monolithic fused silica final stage test mass suspension based on developments of the GEO 600 suspension design. This will include fused silica suspension elements jointed to fused silica test mass substrates, to which dielectric mirror coatings are applied.
The silica fibres used for GEO 600 were pulled using a Hydrogen-Oxygen flame system. This successful system has some limitations, however, that needed to be overcome for the more demanding suspensions required for Advanced LIGO. To this end a fibre pulling machine based on a CO2 laser as the heating element is being developed in Glasgow with funding from EGO and PPARC.
At the moment a significant limitation for proposed detectors like Advanced LIGO is expected to come from the thermal noise of the mirror coatings. An investigation on mechanical losses of silica/tantala coatings was carried out by several labs involved with Advanced LIGO R&D. Doping the tantala coating layer with titania was found to reduce the coating mechanical dissipation. A review of the results is given here
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