Results for "Tim Linden"

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Cluster Mergers and the Origin of the ARCADE-2 ExcessJun 18 2015Oct 11 2016Radio observations at multiple frequencies have detected a significant isotropic emission component between 22~MHz and 10~GHz, commonly termed the ARCADE-2 Excess. The origin of this radio emission is unknown, as the intensity, spectrum and isotropy of ... More
Active Galactic Nuclei and the Origin of IceCube's Diffuse Neutrino FluxOct 05 2018The excess of neutrino candidate events detected by IceCube from the direction of TXS 0506+056 has generated a great deal of interest in blazars as sources of high-energy neutrinos. In this study, we analyze the publicly available portion of the IceCube ... More
Dark Matter and Synchrotron Emission from Galactic Center Radio FilamentsJun 27 2011Aug 24 2011The inner degrees of the Galactic center contain a large population of filamentary structures observed at radio frequencies. These so-called non-thermal radio filaments (NRFs) trace magnetic field lines and have attracted significant interest due to their ... More
Further remarks on the "Smith is Huq" conditionJan 22 2014We compare the 'Smith is Huq' condition (SH) with three commutator conditions in semi-abelian categories: first an apparently weaker condition which arose in joint work with Bourn and turns out to be equivalent with (SH), then an apparently equivalent ... More
A Robust Excess in the Cosmic-Ray Antiproton Spectrum: Implications for Annihilating Dark MatterMar 06 2019An excess of $\sim$10-20 GeV cosmic-ray antiprotons has been identified in the spectrum reported by the AMS-02 Collaboration. The systematic uncertainties associated with this signal, however, have made it difficult to interpret these results. In this ... More
Circular Polarization of Pulsar Wind Nebulae and the Cosmic-Ray Positron ExcessJun 23 2014Recent observations by the PAMELA and AMS-02 telescopes have uncovered an anomalous rise in the positron fraction at energies above 10~GeV. One possible explanation for this excess is the production of primary electron/positron pairs through electromagnetic ... More
Tying Dark Matter to Baryons with Self-interactionsNov 26 2013Jun 23 2014Self-interacting dark matter (SIDM) models have been proposed to solve the small-scale issues with the collisionless cold dark matter (CDM) paradigm. We derive equilibrium solutions in these SIDM models for the dark matter halo density profile including ... More
Resolutions, higher extensions and the relative Mal'tsev axiomJun 13 2012We study how the concept of higher-dimensional extension which comes from categorical Galois theory relates to simplicial resolutions. For instance, an augmented simplicial object is a resolution if and only if its truncation in every dimension gives ... More
A criterion for reflectiveness of normal extensionsFeb 08 2016We give a new sufficient condition for the normal extensions in an admissible Galois structure to be reflective. We then show that this condition is indeed fulfilled when X is the (protomodular) reflective subcategory of S-special objects of a Barr-exact ... More
Known Radio Pulsars Do Not Contribute to the Galactic Center Gamma-Ray ExcessSep 09 2015Observations using the Fermi Large Area Telescope (Fermi-LAT) have found a significant gamma-ray excess surrounding the center of the Milky Way (GC). One possible interpretation of this excess invokes gamma-ray emission from an undiscovered population ... More
Star-Forming Galaxies Significantly Contribute to the Isotropic Gamma-Ray BackgroundDec 09 2016The origin of the isotropic gamma-ray background (IGRB) --- the portion of the extragalactic gamma-ray sky that is not resolvable into individual point sources --- provides a powerful probe into the evolution of the high-energy universe. Star-forming ... More
On the R-Process Enrichment of Dwarf Spheroidal GalaxiesJan 25 2016Recent observations of Reticulum II have uncovered an overabundance of r-process elements, compared to similar ultra-faint dwarf spheroidal galaxies (UFDs). Because the metallicity and star formation history of Reticulum II appear consistent with all ... More
Detecting Dark Matter with Imploding Pulsars in the Galactic CenterMay 05 2014Oct 27 2014The paucity of old millisecond pulsars observed at the galactic center of the Milky Way could be the result of dark matter accumulating in and destroying neutron stars. In regions of high dark matter density, dark matter clumped in a pulsar can exceed ... More
Exploring the Nature of the Galactic Center Gamma-Ray Source with the Cherenkov Telescope ArrayJun 19 2012Nov 15 2012Observations from multiple gamma-ray telescopes have uncovered a high energy gamma-ray source spatially coincident with the Galactic center. Recently, a compelling model for the broad-band gamma-ray emission has been formulated which posits that high ... More
Systematic Effects in Extracting a "Gamma-Ray Haze" from Spatial TemplatesFeb 26 2010Apr 02 2010Recent claims of a gamma-ray excess in the diffuse galactic emission detected by the Fermi Large Area Telescope with a morphology similar to the WMAP haze were based on the assumption that spatial templates of the interstellar medium (ISM) column density ... More
Gamma Rays From The Galactic Center and the WMAP HazeNov 19 2010Mar 22 2011Recently, an analysis of data from the Fermi Gamma Ray Space Telescope has revealed a flux of gamma rays concentrated around the inner ~0.5 degrees of the Milky Way, with a spectrum that is sharply peaked at 2-4 GeV. If interpreted as the products of ... More
On the R-Process Enrichment of Dwarf Spheroidal GalaxiesJan 25 2016Nov 02 2016Recent observations of Reticulum II have uncovered an overabundance of r-process elements, compared to similar ultra-faint dwarf spheroidal galaxies (UFDs). Because the metallicity and star formation history of Reticulum II appear consistent with all ... More
Gamma-ray Lines in the Fermi Data: is it a Bubble?Apr 26 2012Jun 19 2012Recently, tentative evidence for an excess of gamma rays at energies around 130 GeV has been reported from analyses of data from the Fermi Large Area Telescope (LAT). The excess is potentially of great interest, as it could be associated with the pair-annihilation ... More
Probing the Pulsar Origin of the Anomalous Positron Fraction with AMS-02 and Atmospheric Cherenkov TelescopesApr 05 2013May 29 2013Recent observations by PAMELA, Fermi-LAT, and AMS-02 have conclusively indicated a rise in the cosmic-ray positron fraction above 10 GeV, a feature which is impossible to mimic under the paradigm of secondary positron production with self-consistent Galactic ... More
On The Gamma-Ray Emission From Reticulum II and Other Dwarf GalaxiesMar 20 2015The recent discovery of ten new dwarf galaxy candidates by the Dark Energy Survey (DES) and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) could increase the Fermi Gamma-Ray Space Telescope's sensitivity to annihilating dark matter ... More
The Anisotropy of the Extragalactic Radio Background from Dark Matter AnnihilationDec 23 2014Observations of the extragalactic radio background have uncovered a significant isotropic emission across multiple frequencies spanning from 22 MHz to 10 GHz. The intensity of this non-thermal emission component significantly exceeds the expected contribution ... More
Millisecond Pulsars, TeV Halos, and Implications For The Galactic Center Gamma-Ray ExcessMar 21 2018Observations by HAWC indicate that many young pulsars (including Geminga and Monogem) are surrounded by spatially extended, multi-TeV emitting regions. It is not currently known, however, whether TeV emission is also produced by recycled, millisecond ... More
On The Origin Of The Gamma Rays From The Galactic CenterSep 30 2011The region surrounding the center of the Milky Way is both astrophysically rich and complex, and is predicted to contain very high densities of dark matter. Utilizing three years of data from the Fermi Gamma Ray Space Telescope (and the recently available ... More
The Gamma-Ray Pulsar Population of Globular Clusters: Implications for the GeV ExcessJun 29 2016Jul 06 2016It has been suggested that the GeV excess, observed from the region surrounding the Galactic Center, might originate from a population of millisecond pulsars that formed in globular clusters. With this in mind, we employ the publicly available Fermi data ... More
Measuring the Local Diffusion Coefficient with H.E.S.S. Observations of Very High-Energy ElectronsNov 20 2017The HAWC Collaboration has recently reported the detection of bright and spatially extended multi-TeV gamma-ray emission from Geminga, Monogem, and a handful of other nearby, middle-aged pulsars. The angular profile of the emission observed from these ... More
Homology and homotopy in semi-abelian categoriesJul 04 2006The theory of abelian categories proved very useful, providing an axiomatic framework for homology and cohomology of modules over a ring and, in particular, of abelian groups. For many years, a similar categorical framework has been lacking for non-abelian ... More
Are Lines From Unassociated Gamma-Ray Sources Evidence For Dark Matter Annihilation?Aug 03 2012Very recently, it was pointed out that there exists a population of gamma-ray sources without associations at other wavelengths which exhibit spectral features consistent with mono-energetic lines at energies of approximately 111 and 129 GeV. Given recent ... More
Simplicial homotopy in semi-abelian categoriesJul 04 2006We study Quillen's model category structure for homotopy of simplicial objects in the context of Janelidze, Marki and Tholen's semi-abelian categories. This model structure exists as soon as the base category A is regular Mal'tsev and has enough regular ... More
Pulsar TeV Halos Explain the TeV Excess Observed by MilagroJul 06 2017Milagro observations have found bright, diffuse TeV emission concentrated along the galactic plane of the Milky Way. The intensity and spectrum of this emission is difficult to explain with current models where gamma-ray production is dominated by hadronic ... More
Slow continued fractions and permutative representations of $\mathcal{O}_N$Oct 14 2018We study permutative representations of the Cuntz algebras $\mathcal{O}_N$ associated to a class of branching functions systems called slow continued fraction algorithms. We show in particular that the unitary equivalence classes of irreducible permutative ... More
Improved Cosmic-Ray Injection Models and the Galactic Center Gamma-Ray ExcessMar 21 2016Fermi-LAT observations of the Galactic Center (GC) have revealed a spherically- symmetric excess of GeV gamma rays extending to at least 10 deg from the dynamical center of the Galaxy. A critical uncertainty in extracting the intensity, spectrum, and ... More
What Does The PAMELA Antiproton Spectrum Tell Us About Dark Matter?Oct 06 2014Measurements of the cosmic ray antiproton spectrum can be used to search for contributions from annihilating dark matter and to constrain the dark matter annihilation cross section. Depending on the assumptions made regarding cosmic ray propagation in ... More
The Morphology of Hadronic Emission Models for the Gamma-Ray Source at the Galactic CenterMar 15 2012Apr 14 2012Recently, detections of a high-energy gamma-ray source at the position of the Galactic center have been reported by multiple gamma-ray telescopes, spanning the energy range between 100 MeV and 100 TeV. Analysis of these signals strongly suggests the TeV ... More
Challenges in Explaining the Galactic Center Gamma-Ray Excess with Millisecond PulsarsJul 21 2014Feb 26 2016Millisecond pulsars have been discussed as a possible source of the gamma-ray excess observed from the region surrounding the Galactic Center. With this in mind, we use the observed population of bright low-mass X-ray binaries to estimate the number of ... More
On the "three subobjects lemma" and its higher-order generalisationsMar 30 2019We solve a problem mentioned in an article of Berger and Bourn: we prove that in the context of an algebraically coherent semi-abelian category, two natural definitions of the lower central series coincide. In a first, "standard" approach, nilpotency ... More
A comonadic interpretation of Baues-Ellis homology of crossed modulesMay 20 2018We introduce and study a homology theory of crossed modules with coefficients in an abelian crossed module. We discuss the basic properties of these new homology groups and give some applications. We then restrict our attention to the case of integral ... More
Categories vs. groupoids via generalised Mal'tsev propertiesJun 13 2012Aug 18 2014We study the difference between internal categories and internal groupoids in terms of generalised Mal'tsev properties---the weak Mal'tsev property on the one hand, and $n$-permutability on the other. In the first part of the article we give conditions ... More
A decomposition formula for the weighted commutatorApr 04 2013We decompose the weighted subobject commutator of M. Gran, G. Janelidze and A. Ursini as a join of a binary and a ternary commutator.
Putting Things Back Where They Belong: Tracing Cosmic-Ray Injection with H2Oct 15 2015At present, all physical models of diffuse Galactic gamma-ray emission assume that the distribution of cosmic-ray sources traces the observed populations of either OB stars, pulsars, or supernova remnants. However, since H2-rich regions host significant ... More
Is The Gamma-Ray Source 3FGL J2212.5+0703 A Dark Matter Subhalo?Feb 23 2016In a previous paper, we pointed out that the gamma-ray source 3FGL J2212.5+0703 shows evidence of being spatially extended. If a gamma-ray source without detectable emission at other wavelengths were unambiguously determined to be spatially extended, ... More
A New Determination of the Spectra and Luminosity Function of Gamma-Ray Millisecond PulsarsJul 21 2014In this article, we revisit the gamma-ray emission observed from millisecond pulsars and globular clusters. Based on 5.6 years of data from the Fermi Gamma-Ray Space Telescope, we report gamma-ray spectra for 61 millisecond pulsars, finding most to be ... More
Galois theory and commutatorsApr 04 2011We prove that the relative commutator with respect to a subvariety of a variety of Omega-groups introduced by the first author can be described in terms of categorical Galois theory. This extends the known correspondence between the Froehlich-Lue and ... More
On satellites in semi-abelian categories: Homology without projectivesAug 20 2008Mar 06 2009Working in a semi-abelian context, we use Janelidze's theory of generalised satellites to study universal properties of the Everaert long exact homology sequence. This results in a new definition of homology which does not depend on the existence of projective ... More
Radio Galaxies Dominate the High-Energy Diffuse Gamma-Ray BackgroundApr 26 2016Aug 08 2016It has been suggested that unresolved radio galaxies and radio quasars (sometimes referred to as misaligned active galactic nuclei) could be responsible for a significant fraction of the observed diffuse gamma-ray background. In this study, we use the ... More
A characterisation of Lie algebras via algebraic exponentiationNov 02 2017Oct 22 2018In this article we describe varieties of Lie algebras via algebraic exponentiation, a concept introduced by Gray in his Ph.D. thesis. For $\mathbb{K}$ an infinite field of characteristic different from $2$, we prove that the variety of Lie algebras over ... More
Approximate Hagemann-Mitschke co-operationsMay 06 2013We show that varietal techniques based on the existence of operations of a certain arity can be extended to n-permutable categories with binary coproducts. This is achieved via what we call approximate Hagemann-Mitschke co-operations, a generalisation ... More
Improving the Sensitivity of Gamma-Ray Telescopes to Dark Matter Annihilation in Dwarf Spheroidal GalaxiesSep 04 2014The Fermi-LAT collaboration has studied the gamma-ray emission from a stacked population of dwarf spheroidal galaxies and used this information to set constraints on the dark matter annihilation cross section. Interestingly, their analysis uncovered an ... More
Galactic Center Excess in Gamma Rays from Annihilation of Self-Interacting Dark MatterJan 14 2015Observations by the Fermi-LAT telescope have uncovered a significant $\gamma$-ray excess toward the Milky Way Galactic Center. There has been no detection of a similar signal in the direction of the Milky Way dwarf spheroidal galaxies. Additionally, astronomical ... More
The Morphology of the Galactic Dark Matter Synchrotron Emission with Self-Consistent Cosmic Ray Diffusion ModelsApr 22 2010Sep 02 2010A generic prediction in the paradigm of weakly interacting dark matter is the production of relativistic particles from dark matter pair-annihilation in regions of high dark matter density. Ultra-relativistic electrons and positrons produced in the center ... More
Possible Evidence for the Stochastic Acceleration of Secondary Antiprotons by Supernova RemnantsJan 16 2017Jun 15 2017The antiproton-to-proton ratio in the cosmic-ray spectrum is a sensitive probe of new physics. Using recent measurements of the cosmic-ray antiproton and proton fluxes in the energy range of 1-1000 GeV, we study the contribution to the $\bar{p}/p$ ratio ... More
A Predictive Analytic Model for the Solar Modulation of Cosmic RaysNov 04 2015Nov 25 2016An important factor limiting our ability to understand the production and propagation of cosmic rays pertains to the effects of heliospheric forces, commonly known as solar modulation. The solar wind is capable of generating time and charge-dependent ... More
Examining The Fermi-LAT Third Source Catalog In Search Of Dark Matter SubhalosApr 08 2015Dark matter annihilations taking place in nearby subhalos could appear as gamma-ray sources without detectable counterparts at other wavelengths. In this study, we consider the collection of unassociated gamma-ray sources reported by the Fermi Collaboration ... More
Searching for Dark Matter with Neutron Star Mergers and Quiet KilonovaeMay 30 2017Mar 20 2018We identify new astrophysical signatures of dark matter that implodes neutron stars (NSs), which could decisively test whether NS-imploding dark matter is responsible for missing pulsars in the Milky Way galactic center, the source of some $r$-process ... More
TeV Halos are Everywhere: Prospects for New DiscoveriesFeb 21 2019Milagro and HAWC have detected extended TeV gamma-ray emission around nearby pulsar wind nebulae (PWNe). Building on these discoveries, Linden et al. [1] identified a new source class -- TeV halos -- powered by the interactions of high-energy electrons ... More
TeV Gamma Rays From Galactic Center PulsarsMay 25 2017Dec 11 2017Measurements of the nearby pulsars Geminga and B0656+14 by the HAWC and Milagro telescopes have revealed the presence of bright TeV-emitting halos surrounding these objects. If young and middle-aged pulsars near the Galactic Center transfer a similar ... More
Higher extensions in exact Mal'tsev categories: distributivity of congruences and the $3^n$-LemmaJul 09 2018The aim of this article is to better understand the correspondence between $n$-cubic extensions and $3^n$-diagrams, which may be seen as non-abelian Yoneda extensions, useful in (co)homology of non-abelian algebraic structures. We study a higher-dimensional ... More
A comparison theorem for simplicial resolutionsJul 24 2007Oct 08 2007It is well known that Barr and Beck's definition of comonadic homology makes sense also with a functor of coefficients taking values in a semi-abelian category instead of an abelian one. The question arises whether such a homology theory has the same ... More
A relative theory of universal central extensionsAug 26 2009Oct 11 2012Basing ourselves on Janelidze and Kelly's general notion of central extension, we study universal central extensions in the context of semi-abelian categories. Thus we unify classical, recent and new results in one conceptual framework. The theory we ... More
A note on the "Smith is Huq" conditionJun 09 2010We show that two known conditions which arose naturally in commutator theory and in the theory of internal crossed modules coincide: every star-multiplicative graph is multiplicative if and only if every two effective equivalence relations commute as ... More
A characterisation of Lie algebras amongst anti-commutative algebrasJan 19 2017Jan 23 2019Let $\mathbb{K}$ be an infinite field. We prove that if a variety of anti-commutative $\mathbb{K}$-algebras - not necessarily associative, where $xx=0$ is an identity - is locally algebraically cartesian closed, then it must be a variety of Lie algebras ... More
A Predictive Analytic Model for the Solar Modulation of Cosmic RaysNov 04 2015Feb 24 2016An important factor limiting our ability to understand the production and propagation of cosmic rays pertains to the effects of heliospheric forces, commonly known as solar modulation. The solar wind is capable of generating time and charge-dependent ... More
Self-Generated Cosmic-Ray Confinement in TeV Halos: Implications for TeV Gamma-Ray Emission and the Positron ExcessJul 24 2018Sep 26 2018Recent observations have detected extended TeV gamma-ray emission surrounding young and middle-aged pulsars. The morphology of these "TeV halos" requires cosmic-ray diffusion to be locally suppressed by a factor of ~100-1000 compared to the typical interstellar ... More
A Critical Reevaluation of Radio Constraints on Annihilating Dark MatterAug 26 2014Feb 05 2016A number of groups have employed radio observations of the Galactic center to derive stringent constraints on the annihilation cross section of weakly interacting dark matter. In this paper, we show that electron energy losses in this region are likely ... More
Relative Commutator Theory in Semi-Abelian CategoriesAug 19 2011Basing ourselves on the concept of double central extension from categorical Galois theory, we study a notion of commutator which is defined relative to a Birkhoff subcategory B of a semi-abelian category A. This commutator characterises Janelidze and ... More
On the second cohomology group in semi-abelian categoriesNov 14 2005May 07 2007We develop some new aspects of cohomology in the context of semi-abelian categories: we establish a Hochschild-Serre 5-term exact sequence extending the classical one for groups and Lie algebras; we prove that an object is perfect if and only if it admits ... More
The Yoneda isomorphism commutes with homologyMay 20 2015Sep 06 2015We show that, for a right exact functor from an abelian category to abelian groups, Yoneda's isomorphism commutes with homology and, hence, with functor derivation. Then we extend this result to semiabelian domains. An interpretation in terms of satellites ... More
Universal central extensions in semi-abelian categoriesFeb 22 2013Basing ourselves on Janelidze and Kelly's general notion of central extension, we study universal central extensions in the context of semi-abelian categories. We consider a new fundamental condition on composition of central extensions and give examples ... More
The ternary commutator obstruction for internal crossed modulesJul 05 2011Jun 19 2012In finitely cocomplete homological categories, co-smash products give rise to (possibly higher-order) commutators of subobjects. We use binary and ternary co-smash products and the associated commutators to give characterisations of internal crossed modules ... More
A note on split extensions of bialgebrasJan 03 2017Mar 13 2017We prove a universal characterization of Hopf algebras among cocommutative bialgebras over a field: a cocommutative bialgebra is a Hopf algebra precisely when every split extension over it admits a join decomposition. We also explain why this result cannot ... More
Higher central extensions and cohomologyFeb 18 2015We establish a Galois-theoretic interpretation of cohomology in semi-abelian categories: cohomology with trivial coefficients classifies central extensions, also in arbitrarily high degrees. This allows us to obtain a duality, in a certain sense, between ... More
Two characterisations of groups amongst monoidsJun 28 2016The aim of this paper is to solve a problem proposed by Dominique Bourn: to provide a categorical-algebraic characterisation of groups amongst monoids and of rings amongst semirings. In the case of monoids, our solution is given by the following equivalent ... More
HAWC Observations Strongly Favor Pulsar Interpretations of the Cosmic-Ray Positron ExcessFeb 27 2017Recent measurements of the Geminga and B0656+14 pulsars by the gamma-ray telescope HAWC (along with earlier measurements by Milagro) indicate that these objects generate significant fluxes of very high-energy electrons. In this paper, we use the very ... More
Indirect Detection Constraints on s and t Channel Simplified Models of Dark MatterJun 13 2016Recent Fermi-LAT observations of dwarf spheroidal galaxies in the Milky Way have placed strong limits on the gamma-ray flux from dark matter annihilation. In order to produce the strongest limit on the dark matter annihilation cross-section, the observations ... More
IceCube and HAWC constraints on very-high-energy emission from the Fermi bubblesApr 12 2017Nov 17 2017The nature of the $\gamma$-ray emission from the \emph{Fermi} bubbles is unknown. Both hadronic and leptonic models have been formulated to explain the peculiar $\gamma$-ray signal observed by the Fermi-LAT between 0.1-500~GeV. If this emission continues ... More
Higher Hopf formulae for homology via Galois TheoryJan 29 2007May 21 2007We use Janelidze's Categorical Galois Theory to extend Brown and Ellis's higher Hopf formulae for homology of groups to arbitrary semi-abelian monadic categories. Given such a category A and a chosen Birkhoff subcategory B of A, thus we describe the Barr-Beck ... More
Do $n$-Lie algebras have universal enveloping algebrasAug 27 2015The aim of this paper is to investigate in which sense, for $n\geq 3$, $n$-Lie algebras admit universal enveloping algebras. There have been some attempts at a construction (see [10] and [5]) but after analysing those we come to the conclusion that they ... More
A Clustering Analysis of the Morphology of the 130 GeV Gamma-Ray FeatureApr 19 2013Recent observations indicating the existence of a monochromatic gamma-ray line with energy ~130 GeV in the Fermi-LAT data have attracted great interest due to the possibility that the line feature stems from the annihilation of dark matter particles. ... More
Two characterisations of groups amongst monoidsJun 28 2016Apr 26 2017The aim of this paper is to solve a problem proposed by Dominique Bourn: to provide a categorical-algebraic characterisation of groups amongst monoids and of rings amongst semirings. In the case of monoids, our solution is given by the following equivalent ... More
On the Formation of Ultraluminous X-ray Sources with Neutron Star Accretors: the Case of M82 X-2Jan 12 2015Mar 03 2015The recent discovery of a neutron star accretor in the ultra-luminous X-ray source M82 X-2 challenges our understanding of high-mass X-ray binary formation and evolution. By combining binary population synthesis and detailed mass-transfer models, however, ... More
Testing the dark matter origin of the WMAP-Planck Haze with radio observations of Spiral GalaxiesDec 23 2012Jul 19 2013If the Galactic WMAP radio haze, as recently confirmed by Planck, is produced by dark matter annihilation or decay, similar diffuse radio halos should exist around other galaxies with physical properties comparable to the Milky Way. If instead the haze ... More
Transverse Anderson localization of surface plasmon polaritonsJan 31 2017We investigate the effect of disorder on the propagation of surface plasmon polaritons in arrays of evanescently coupled dielectric loaded surface plasmon polariton waveguides. Diagonal disorder is implemented by randomly varying the heights of the waveguides. ... More
The fundamental group functor as a Kan extensionJun 29 2013Sep 17 2013We prove that the fundamental group functor from categorical Galois theory may be computed as a Kan extension.
Low Mass X-Ray Binaries in the Inner Galaxy: Implications for Millisecond Pulsars and the GeV ExcessJan 10 2017Jun 14 2017If millisecond pulsars (MSPs) are responsible for the excess gamma-ray emission observed from the region surrounding the Galactic Center, the same region should also contain a large population of low-mass X-ray binaries (LMXBs). In this study, we compile ... More
Is the Ultra-High Energy Cosmic-Ray Excess Observed by the Telescope Array Correlated with IceCube Neutrinos?Apr 24 2014Aug 19 2014The Telescope Array (TA) has observed a statistically significant excess in cosmic-rays with energies above 57 EeV in a region of approximately 1150 square degrees centered on coordinates (R.A. = 146.7, Dec. = 43.2). We note that the location of this ... More
Peri-abelian categories and the universal central extension conditionApr 11 2014Aug 08 2014We study the relation between Bourn's notion of peri-abelian category and conditions involving the coincidence of the Smith, Huq and Higgins commutators. In particular we show that a semi-abelian category is peri-abelian if and only if for each normal ... More
An observation on n-permutabilityJun 27 2013We prove that in a regular category all reflexive and transitive relations are symmetric if and only if every internal category is an internal groupoid. In particular, these conditions hold when the category is n-permutable for some n.
High Density Limit of the Fermi Polaron with Infinite MassJul 17 2018We analyze the ground state energy for $N$ identical fermions in a two-dimensional box of volume $L^2$ interacting with an external point scatterer. Since the point scatterer can be considered as an impurity particle of infinite mass, this system is a ... More
Stability of the two-dimensional Fermi polaronSep 08 2017Feb 09 2018A system composed of an ideal gas of N fermions interacting with an impurity particle in two space dimensions is considered. The interaction between impurity and fermions is given in terms of two-body point interactions whose strength is determined by ... More
On the harmonic and geometric maximal operatorsApr 22 2016Jan 11 2017We examine the harmonic and geometric maximal operators defined for a general basis of open sets in $\R^n$. We prove two weight norm inequalities for the harmonic maximal operator assuming testing conditions over characteristic functions of unions of ... More
The mildly nonlinear imprint of structure on the CMBDec 03 1999Dec 05 1999I outline some nonperturbative relativistic effects that arise from gravitational corrections to the Boltzmann equations. These may be important for the study of CMB temperature anisotropies, particularly their interpretation. These terms are not included ... More
On the equivalence of the discrete nonlinear Schrödinger equation and the discrete isotropic Heisenberg magnetJun 28 1999The equivalence of the discrete isotropic Heisenberg magnet (IHM) model and the discrete nonlinear Schr\"odinger equation (NLSE) given by Ablowitz and Ladik is shown. This is used to derive the equivalence of their discretization with the one by Izergin ... More
A Structured Variational Auto-encoder for Learning Deep Hierarchies of Sparse FeaturesFeb 28 2016In this note we present a generative model of natural images consisting of a deep hierarchy of layers of latent random variables, each of which follows a new type of distribution that we call rectified Gaussian. These rectified Gaussian units allow spike-and-slab ... More
What Bell DidAug 08 2014On the 50th anniversary of Bell's monumental 1964 paper, there is still widespread misunderstanding about exactly what Bell proved. This misunderstanding derives in turn from a failure to appreciate the earlier arguments of Einstein, Podolsky and Rosen. ... More
Reply to WernerAug 08 2014Reinhard Werner authored a comment on my paper "What Bell Did", disputing the conclusion and argumentation of the paper. This is my reply.
Euclidean-valued group cohomology is always reducedOct 11 2013Nov 25 2013If G is a countable discrete group acting linearly on a finite-dimensional vector space over any topological field, then the groups of coboundaries are closed for the product topology in all degrees, and hence the cohomology is reduced in all degrees. ... More
Correlation functions, null polygonal Wilson loops, and local operatorsOct 18 2011Jul 30 2012We consider the ratio of the correlation function of n+1 local operators over the correlator of the first n of these operators in planar N=4 super-Yang-Mills theory, and consider the limit where the first n operators become pairwise null separated. By ... More
The LSST Data Processing Software Stack: Summer 2015 ReleaseNov 20 2015The Large Synoptic Survey Telescope (LSST) is an 8-m optical ground-based telescope being constructed on Cerro Pachon in Chile. LSST will survey half the sky every few nights in six optical bands. The data will be transferred to NCSA and within 60 seconds ... More
Evolutionary Innovations and Where to Find Them: Routes to Open-Ended Evolution in Natural and Artificial SystemsJun 05 2018Feb 21 2019This paper presents a high-level conceptual framework to help orient the discussion and implementation of open-endedness in evolutionary systems. Drawing upon earlier work by Banzhaf et al., three different kinds of open-endedness are identified: exploratory, ... More
Discrete Hashimoto surfaces and a doubly discrete smokering flowJul 25 2000B\"acklund transformations for smooth and ``space discrete'' Hashimoto surfaces are discussed and a geometric interpretation is given. It is shown that the complex curvature of a discrete space curve evolves with the discrete nonlinear Schr\"odinger equation ... More
On the regularity over positively graded algebrasDec 20 2006Aug 30 2007We study the relationship between the Tor-regularity and the local-regularity over a positively graded algebra defined over a field which coincide if the algebra is a standard graded polynomial ring. In this case both are characterizations of the so-called ... More