371 research outputs found
GLAST Large Area Telescope Multiwavelength Planning
Gamma-ray astrophysics depends in many ways on multiwavelength studies. The Gamma-ray Large Area Space Telescope (GLAST) Large Area Telescope (LAT) Collaboration has started multiwavelength planning well before the scheduled 2007 launch of the observatory. Some of the high-priority multiwavelength needs include: (1) availability of contemporaneous radio and X-ray timing of pulsars; (2) expansion of blazar catalogs, including redshift measurements; (3) improved observations of molecular clouds, especially at high galactic latitudes; (4) simultaneous broad-spectrum blazar monitoring; (5) characterization of gamma-ray transients, including gamma ray bursts; (6) radio, optical, X-ray and TeV counterpart searches for reliable and effective sources identification and characterization. Several of these activities are needed to be in place before launch
Unidentified EGRET Sources and the Extragalactic Gamma-Ray Background
The large majority of EGRET point sources remain to this day without an
identified low-energy counterpart. Whatever the nature of the EGRET
unidentified sources, faint unresolved objects of the same class must have a
contribution to the diffuse gamma-ray background: if most unidentified objects
are extragalactic, faint unresolved sources of the same class contribute to the
background, as a distinct extragalactic population; on the other hand, if most
unidentified sources are Galactic, their counterparts in external galaxies will
contribute to the unresolved emission from these systems. Understanding this
component of the gamma-ray background, along with other guaranteed
contributions from known sources, is essential in any attempt to use gamma-ray
observations to constrain exotic high-energy physics. Here, we follow an
empirical approach to estimate whether a potential contribution of unidentified
sources to the extragalactic gamma-ray background is likely to be important,
and we find that it is. Additionally, we comment on how the anticipated GLAST
measurement of the diffuse gamma-ray background will change, depending on the
nature of the majority of these sources.Comment: 6 pages, 3 figures, to appear in proceedings of "The Multi-Messenger
Approach to High Energy Gamma-Ray Sources", Barcelona, 4-7 July 2006;
comments welcom
From M-ary Query to Bit Query: a new strategy for efficient large-scale RFID identification
The tag collision avoidance has been viewed as one of the most important research problems in RFID communications and bit tracking technology has been widely embedded in query tree (QT) based algorithms to tackle such challenge. Existing solutions show further opportunity to greatly improve the reading performance because collision queries and empty queries are not fully explored. In this paper, a bit query (BQ) strategy based Mary query tree protocol (BQMT) is presented, which can not only eliminate idle queries but also separate collided tags into many small subsets and make full use of the collided bits. To further optimize the reading performance, a modified dual prefixes matching (MDPM) mechanism is presented to allow multiple tags to respond in the same slot and thus significantly reduce the number of queries. Theoretical analysis and simulations are supplemented to validate the effectiveness of the proposed BQMT and MDPM, which outperform the existing QT-based algorithms. Also, the BQMT and MDPM can be combined to BQMDPM to improve the reading performance in system efficiency, total identification time, communication complexity and average energy cost
Diffuse inverse Compton and synchrotron emission from dark matter annihilations in galactic satellites
Annihilating dark matter particles produce roughly as much power in electrons
and positrons as in gamma ray photons. The charged particles lose essentially
all of their energy to inverse Compton and synchrotron processes in the
galactic environment. We discuss the diffuse signature of dark matter
annihilations in satellites of the Milky Way (which may be optically dark with
few or no stars), providing a tail of emission trailing the satellite in its
orbit. Inverse Compton processes provide X-rays and gamma rays, and synchrotron
emission at radio wavelengths might be seen. We discuss the possibility of
detecting these signals with current and future observations, in particular
EGRET and GLAST for the gamma rays.Comment: 13 pages, 5 figure
Gamma-Ray Luminosity Function of Blazars and the Cosmic Gamma-Ray Background: Evidence for the Luminosity Dependent Density Evolution
We present a comprehensive study for the gamma-ray luminosity function (GLF)
of blazars and their contribution to the extragalactic diffuse gamma-ray
background (EGRB). Radio and gamma-ray luminosity correlation is introduced to
take into account the radio detectability which is important for the blazar
identification. Previous studies considered only pure luminosity evolution
(PLE) or pure density evolution, but we introduce the luminosity dependent
density evolution (LDDE) model, which is favored from the evolution of X-ray
luminosity function (XLF) of AGNs. The model parameters are constrained by
likelihood analyses about the observed redshift and gamma-ray flux
distributions of the EGRET blazars. We find that the LDDE model gives a better
fit to the observed distributions than the PLE model, indicating that the LDDE
model is also appropriate for gamma-ray blazars, and that the jet activity is
universally correlated with the accretion history of AGNs. The normalization
between the GLF and XLF is consistent with the unified picture of AGNs, when
the beaming and a reasonable duty cycle of jet activity are taken into account.
We then find that only 25--50% of the EGRB can be explained by unresolved
blazars with the best-fit LDDE parameters. Unresolved blazars can account for
all the EGRB only with a steeper index of the faint-end slope of the GLF, which
is marginally consistent with the EGRET data but inconsistent with that of the
XLF. Therefore unresolved AGNs cannot be the dominant source of the EGRB,
unless there is a new population of gamma-ray emitting AGNs that evolves
differently from the XLF of AGNs. Predictions for the GLAST mission are made,
and we find that the best-fit LDDE model predicts about 3000 blazars in the
entire sky, which is considerably fewer than a previous estimate.Comment: 13 pages, 12 figures, accepted by ApJ; minor typos corrected and some
figures revised, main conclusions essentially unchange
Extragalactic Inverse Compton Light from Dark Matter Annihilation and the Pamela Positron Excess
We calculate the extragalactic diffuse emission originating from the
up-scattering of cosmic microwave photons by energetic electrons and positrons
produced in particle dark matter annihilation events at all redshifts and in
all halos. We outline the observational constraints on this emission and we
study its dependence on both the particle dark matter model (including the
particle mass and its dominant annihilation final state) and on assumptions on
structure formation and on the density profile of halos. We find that for
low-mass dark matter models, data in the X-ray band provide the most stringent
constraints, while the gamma-ray energy range probes models featuring large
masses and pair-annihilation rates, and a hard spectrum for the injected
electrons and positrons. Specifically, we point out that the all-redshift,
all-halo inverse Compton emission from many dark matter models that might
provide an explanation to the anomalous positron fraction measured by the
Pamela payload severely overproduces the observed extragalactic gamma-ray
background.Comment: Version accepted for publication in JCAP, one new figure and text
added; 19 pages, 5 figure
Magnetic Fields in the Milky Way
This chapter presents a review of observational studies to determine the
magnetic field in the Milky Way, both in the disk and in the halo, focused on
recent developments and on magnetic fields in the diffuse interstellar medium.
I discuss some terminology which is confusingly or inconsistently used and try
to summarize current status of our knowledge on magnetic field configurations
and strengths in the Milky Way. Although many open questions still exist, more
and more conclusions can be drawn on the large-scale and small-scale components
of the Galactic magnetic field. The chapter is concluded with a brief outlook
to observational projects in the near future.Comment: 22 pages, 5 figures, to appear in "Magnetic Fields in Diffuse Media",
eds. E.M. de Gouveia Dal Pino and A. Lazaria
Introducing CASCADEPOP: an open-source sociodemographic simulation platform for US health policy appraisal
Largescale individual-level and agent-based models are gaining importance in health policy appraisal and evaluation. Such models require the accurate depiction of the jurisdictionâs population over extended time periods to enable modeling of the development of non-communicable diseases under consideration of historical, sociodemographic developments. We developed CASCADEPOP to provide a readily available sociodemographic micro-synthesis and microsimulation platform for US populations. The micro-synthesis method used iterative proportional fitting to integrate data from the US Census, the American Community Survey, the Panel Study of Income Dynamics, Multiple Cause of Death Files, and several national surveys to produce a synthetic population aged 12 to 80 years on 01/01/1980 for five states (California, Minnesota, New York, Tennessee, and Texas) and the US. Characteristics include individualsâ age, sex, race/ethnicity, marital/employment/parental status, education, income and patterns of alcohol use as an exemplar health behavior. The microsimulation simulates individualsâ sociodemographic life trajectories over 35 years to 31/12/2015 accounting for population developments including births, deaths, and migration. Results comparing the 1980 micro-synthesis against observed data shows a successful depiction of state and US population characteristics and of drinking. Comparing the microsimulation over 30 years with Census data also showed the successful simulation of sociodemographic developments. The CASCADEPOP platform enables modelling of health behaviors across individualsâ life courses and at a population level. As it contains a large number of relevant sociodemographic characteristics it can be further developed by researchers to build US agent-based models and microsimulations to examine health behaviors, interventions, and policies
Olber's Paradox for Superluminal Neutrinos: Constraining Extreme Neutrino Speeds at TeV-ZeV Energies with the Diffuse Neutrino Background
The only invariant speed in special relativity is c; therefore, if some
neutrinos travel at even tiny speeds above c, normal special relativity is
incomplete and any superluminal speed may be possible. I derive a limit on
superluminal neutrino speeds v >> c at high energies by noting that such speeds
would increase the size of the neutrino horizon. The increased volume of the
Universe visible leads to a brighter astrophysical neutrino background. The
nondetection of "guaranteed" neutrino backgrounds from star-forming galaxies
and ultrahigh energy cosmic rays (UHECRs) constrains v/c at TeV--ZeV energies.
I find that v/c <= 820 at 60 TeV from the nondetection of neutrinos from
star-forming galaxies. The nondetection of neutrinos from UHECRs constrains v/c
to be less than 2500 at 0.1 EeV in a pessimistic model and less than 4.6 at 4
EeV in an optimistic model. The UHECR neutrino background nondetection is
strongly inconsistent with a naive quadratic extrapolation of the OPERA results
to EeV energies. The limits apply subject to some caveats, particularly that
the expected pionic neutrino backgrounds exist and that neutrinos travel faster
than c when they pass the detector. They could be improved substantially as the
expected neutrino backgrounds are better understood and with new experimental
neutrino background limits. I also point out that extremely subluminal speeds
would result in a much smaller neutrino background intensity than expected.Comment: 13 pages, 2 figures, fixed titl
Measurement of the polarisation of W bosons produced with large transverse momentum in pp collisions at sqrt(s) = 7 TeV with the ATLAS experiment
This paper describes an analysis of the angular distribution of W->enu and
W->munu decays, using data from pp collisions at sqrt(s) = 7 TeV recorded with
the ATLAS detector at the LHC in 2010, corresponding to an integrated
luminosity of about 35 pb^-1. Using the decay lepton transverse momentum and
the missing transverse energy, the W decay angular distribution projected onto
the transverse plane is obtained and analysed in terms of helicity fractions
f0, fL and fR over two ranges of W transverse momentum (ptw): 35 < ptw < 50 GeV
and ptw > 50 GeV. Good agreement is found with theoretical predictions. For ptw
> 50 GeV, the values of f0 and fL-fR, averaged over charge and lepton flavour,
are measured to be : f0 = 0.127 +/- 0.030 +/- 0.108 and fL-fR = 0.252 +/- 0.017
+/- 0.030, where the first uncertainties are statistical, and the second
include all systematic effects.Comment: 19 pages plus author list (34 pages total), 9 figures, 11 tables,
revised author list, matches European Journal of Physics C versio
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