50 research outputs found
Extreme TeV Blazars and Lower Limits on Intergalactic Magnetic Fields
The intergalactic magnetic field (IGMF) in cosmic voids can be indirectly
probed through its effect on electromagnetic cascades initiated by a source of
TeV gamma rays, such as blazars, a subclass of active galactic nuclei. Blazars
that are sufficiently luminous at TeV energies, "extreme TeV blazars", can
produce detectable levels of secondary radiation from inverse Compton
scattering of the electrons in the cascade, provided that the IGMF is not too
large. We reveiw recent work in the literature which utilizes this idea to
derive constraints on the IGMF for three TeV-detected blazars-1ES 0229+200, 1ES
1218+304, and RGB J0710+591, and we also investigate four other hard-spectrum
TeV blazars in the same framework. Through a recently developed detailed 3D
particle tracking Monte Carlo simulation code, incorporating all major effects
of QED and cosmological expansion, we research effects of major uncertainties
such as the spectral properties of the source, uncertainty in the intensity of
the UV - far IR extragalactic background light (EBL), under-sampled Very High
Energy (VHE; energy > 100 GeV) coverage, past history of gamma-ray emission,
source vs. observer geometry, and jet AGN Doppler factor. The implications of
these effects on the recently reported lower limits of the IGMF are thoroughly
examined to conclude that presently available data are compatible with a zero
IGMF hypothesis.Comment: 2012 Fermi Symposium proceedings - eConf C12102
Toward a general theory of linking invariants
Let N_1, N_2, M be smooth manifolds with dim N_1 + dim N_2 +1 = dim M$ and
let phi_i, for i=1,2, be smooth mappings of N_i to M with Im phi_1 and Im phi_2
disjoint. The classical linking number lk(phi_1,phi_2) is defined only when
phi_1*[N_1] = phi_2*[N_2] = 0 in H_*(M).
The affine linking invariant alk is a generalization of lk to the case where
phi_1*[N_1] or phi_2*[N_2] are not zero-homologous. In arXiv:math.GT/0207219 we
constructed the first examples of affine linking invariants of
nonzero-homologous spheres in the spherical tangent bundle of a manifold, and
showed that alk is intimately related to the causality relation of wave fronts
on manifolds.
In this paper we develop the general theory. The invariant alk appears to be
a universal Vassiliev-Goussarov invariant of order < 2. In the case where
phi_1*[N_1] and phi_2*[N_2] are 0 in homology it is a splitting of the
classical linking number into a collection of independent invariants.
To construct alk we introduce a new pairing mu on the bordism groups of
spaces of mappings of N_1 and N_2 into M, not necessarily under the restriction
dim N_1 + dim N_2 +1 = dim M. For the zero-dimensional bordism groups, mu can
be related to the Hatcher-Quinn invariant. In the case N_1=N_2=S^1, it is
related to the Chas-Sullivan string homology super Lie bracket, and to the
Goldman Lie bracket of free loops on surfaces.Comment: Published by Geometry and Topology at
http://www.maths.warwick.ac.uk/gt/GTVol9/paper42.abs.htm
Microtorus: a High Finesse Microcavity with Whispering-Gallery Modes
We have demonstrated a 165 micron oblate spheroidal microcavity with free
spectral range 383.7 GHz (3.06nm), resonance bandwidth 25 MHz (Q ~ 10^7) at
1550nm, and finesse F > 10^4. The highly oblate spheroidal dielectric
microcavity combines very high Q-factor, typical of microspheres, with vastly
reduced number of excited whispering-gallery (WG) modes (by two orders of
magnitude). The very large free spectral range in the novel microcavity - few
hundred instead of few GigaHertz in typical microspheres - is desirable for
applications in spectral analysis, narrow-linewidth optical and RF oscillators,
and cavity QED.Comment: 11 pages, 3 figure
Intensity correlations in resonance nonlinear magneto-optical rotation
We have studied the intensity correlations between two orthogonally linearly
polarized components of a laser field propagating through a resonant atomic
medium. These experiments have been performed in a Rubidium atomic vapor. We
observe that the correlations between the orthogonally polarized components of
the laser beam are maximal in the absence of a magnetic field. The magnitude of
the correlations depends on the applied magnetic field, and the magnitude first
decreases and then increases with increasing magnetic field. Minimal
correlations and maximal rotation angles are observed at the same magnetic
fields. The width of the correlation function is directly proportional to the
excited state lifetime and inversely proportional to the Rabi frequency of
laser field. These results can be useful for improving optical magnetometers
and for optical field or atomic spin squeezing.Comment: 8 pages, 4 figure
Challenges in QCD matter physics - The Compressed Baryonic Matter experiment at FAIR
Substantial experimental and theoretical efforts worldwide are devoted to
explore the phase diagram of strongly interacting matter. At LHC and top RHIC
energies, QCD matter is studied at very high temperatures and nearly vanishing
net-baryon densities. There is evidence that a Quark-Gluon-Plasma (QGP) was
created at experiments at RHIC and LHC. The transition from the QGP back to the
hadron gas is found to be a smooth cross over. For larger net-baryon densities
and lower temperatures, it is expected that the QCD phase diagram exhibits a
rich structure, such as a first-order phase transition between hadronic and
partonic matter which terminates in a critical point, or exotic phases like
quarkyonic matter. The discovery of these landmarks would be a breakthrough in
our understanding of the strong interaction and is therefore in the focus of
various high-energy heavy-ion research programs. The Compressed Baryonic Matter
(CBM) experiment at FAIR will play a unique role in the exploration of the QCD
phase diagram in the region of high net-baryon densities, because it is
designed to run at unprecedented interaction rates. High-rate operation is the
key prerequisite for high-precision measurements of multi-differential
observables and of rare diagnostic probes which are sensitive to the dense
phase of the nuclear fireball. The goal of the CBM experiment at SIS100
(sqrt(s_NN) = 2.7 - 4.9 GeV) is to discover fundamental properties of QCD
matter: the phase structure at large baryon-chemical potentials (mu_B > 500
MeV), effects of chiral symmetry, and the equation-of-state at high density as
it is expected to occur in the core of neutron stars. In this article, we
review the motivation for and the physics programme of CBM, including
activities before the start of data taking in 2022, in the context of the
worldwide efforts to explore high-density QCD matter.Comment: 15 pages, 11 figures. Published in European Physical Journal
VERITAS discovery of very high energy gamma-ray emission from S3 1227+25 and multiwavelength observations
We report the detection of very high energy gamma-ray emission from the
blazar S3 1227+25 (VER J1230+253) with the Very Energetic Radiation Imaging
Telescope Array System (VERITAS). VERITAS observations of the source were
triggered by the detection of a hard-spectrum GeV flare on May 15, 2015 with
the Fermi-Large Area Telescope (LAT). A combined five-hour VERITAS exposure on
May 16th and May 18th resulted in a strong 13 detection with a
differential photon spectral index, = 3.8 0.4, and a flux level
at 9% of the Crab Nebula above 120 GeV. This also triggered target of
opportunity observations with Swift, optical photometry, polarimetry and radio
measurements, also presented in this work, in addition to the VERITAS and
Fermi-LAT data. A temporal analysis of the gamma-ray flux during this period
finds evidence of a shortest variability timescale of = 6.2
0.9 hours, indicating emission from compact regions within the jet, and the
combined gamma-ray spectrum shows no strong evidence of a spectral cut-off. An
investigation into correlations between the multiwavelength observations found
evidence of optical and gamma-ray correlations, suggesting a single-zone model
of emission. Finally, the multiwavelength spectral energy distribution is well
described by a simple one-zone leptonic synchrotron self-Compton radiation
model.Comment: 18 pages, 6 figures. Accepted for publication in the Astrophysical
Journal (ApJ
Finishing the euchromatic sequence of the human genome
The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead