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G. Brown

Publications and source records attributed to G. Brown.

At least 19 recordsLinked to original sources
arXiv2022

The fall of CSS100217: a tidal disruption-induced low state in an apparently hostless active galactic nucleus

CSS100217 was a nuclear, rapid and luminous flare in a Narrow-Line Seyfert 1 galaxy, whose initial interpretation as a supernova is now debated between variability of the active galactic nucleus (AGN) or a tidal disruption event (TDE). In this paper, we present and discuss new evidence in favour of a TDE or extreme flaring episode scenario. After the decay of the flare, the galaxy entered a long-term low luminosity state, 0.4 magnitudes lower than the pre-outburst emission in the V band. We attribute this to the creation of a cavity in the accretion disk after the tidal disruption of a star in a retrograde orbit with respect to the accretion disk rotation, making a TDE our favoured interpretation of the flare. We also show how the host galaxy shows a point-like, compact profile, no evidence for an extended component and a relatively low mass, unlike what expected from an AGN host galaxy at z=0.147. A compact host galaxy may result in an increased TDE rate, strengthening our interpretation of the event.

astro-ph.HE↗
arXiv2020

Classifying Permutations under Context-Directed Swaps and the \textbf{cds} game

A special sorting operation called Context Directed Swap, and denoted \textbf{cds}, performs certain types of block interchanges on permutations. When a permutation is sortable by \textbf{cds}, then \textbf{cds} sorts it using the fewest possible block interchanges of any kind. This work introduces a classification of permutations based on their number of \textbf{cds}-eligible contexts. In prior work an object called the strategic pile of a permutation was discovered and shown to provide an efficient measure of the non-\textbf{cds}-sortability of a permutation. Focusing on the classification of permutations with maximal strategic pile, a complete characterization is given when the number of \textbf{cds}-eligible contexts is close to maximal as well as when the number of eligible contexts is minimal. A group action that preserves the number of \textbf{cds}-eligible contexts of a permutation provides, via the orbit-stabilizer theorem, enumerative results regarding the number of permutations with maximal strategic pile and a given number of \textbf{cds}-eligible contexts. Prior work introduced a natural two-person game on permutations that are not \textbf{cds}-sortable. The decision problem of which player has a winning strategy in a particular instance of the game appears to be of high computational complexity. Extending prior results, this work presents new conditions for player ONE to have a winning strategy in this combinatorial game.

math.CO↗
arXiv2020

Study of scintillation light collection, production and propagation in a 4 tonne dual-phase LArTPC

The $3 \times 1 \times 1$ m$^3$ demonstrator is a dual phase liquid argon time projection chamber that has recorded cosmic rays events in 2017 at CERN. The light signal in these detectors is crucial to provide precise timing capabilities. The performances of the photon detection system, composed of five PMTs, are discussed. The collected scintillation and electroluminescence light created by passing particles has been studied in various detector conditions. In particular, the scintillation light production and propagation processes have been analyzed and compared to simulations, improving the understanding of some liquid argon properties.

physics.ins-det↗
arXiv2018

A 4 tonne demonstrator for large-scale dual-phase liquid argon time projection chambers

A 10 kilo-tonne dual-phase liquid argon TPC is one of the detector options considered for the Deep Underground Neutrino Experiment (DUNE). The detector technology relies on amplification of the ionisation charge in ultra-pure argon vapour and oers several advantages compared to the traditional single-phase liquid argon TPCs. A 4.2 tonne dual-phase liquid argon TPC prototype, the largest of its kind, with an active volume of 3x1x1 $m^3$ has been constructed and operated at CERN. In this paper we describe in detail the experimental setup and detector components as well as report on the operation experience. We also present the first results on the achieved charge amplification, prompt scintillation and electroluminescence detection, and purity of the liquid argon from analyses of a collected sample of cosmic ray muons.

physics.ins-det↗
arXiv2018

Spin constrained orbital angular momentum control in high-harmonic generation

The interplay between spin and orbital angular momentum in the up-conversion process allows us to control the macroscopic wave front of high harmonics by manipulating the microscopic polarizations of the driving field. We demonstrate control of orbital angular momentum in high harmonic generation from both solid and gas phase targets using the selection rules of spin angular momentum. The gas phase harmonics extend the control of angular momentum to extreme-ultraviolet wavelength. We also propose a bi-color scheme to produce spectrally separated extreme-ultraviolet radiation carrying orbital angular momentum.

physics.optics↗
arXiv2018

Muon (g-2) Technical Design Report

The Muon (g-2) Experiment, E989 at Fermilab, will measure the muon anomalous magnetic moment a factor-of-four more precisely than was done in E821 at the Brookhaven National Laboratory AGS. The E821 result appears to be greater than the Standard-Model prediction by more than three standard deviations. When combined with expected improvement in the Standard-Model hadronic contributions, E989 should be able to determine definitively whether or not the E821 result is evidence for physics beyond the Standard Model. After a review of the physics motivation and the basic technique, which will use the muon storage ring built at BNL and now relocated to Fermilab, the design of the new experiment is presented. This document was created in partial fulfillment of the requirements necessary to obtain DOE CD-2/3 approval.

physics.ins-det↗
arXiv2017

The Single-Phase ProtoDUNE Technical Design Report

ProtoDUNE-SP is the single-phase DUNE Far Detector prototype that is under construction and will be operated at the CERN Neutrino Platform (NP) starting in 2018. ProtoDUNE-SP, a crucial part of the DUNE effort towards the construction of the first DUNE 10-kt fiducial mass far detector module (17 kt total LAr mass), is a significant experiment in its own right. With a total liquid argon (LAr) mass of 0.77 kt, it represents the largest monolithic single-phase LArTPC detector to be built to date. It's technical design is given in this report.

physics.ins-det↗
arXiv2016

Equilibrium, Metastability, and Hysteresis in a Model Spin-crossover Material with Nearest-neighbor Antiferromagnetic-like and Long-range Ferromagnetic-like Interactions

Phase diagrams and hysteresis loops were obtained by Monte Carlo simulations and a mean-field method for a simplified model of a spin-crossover material with a two-step transition between the high-spin and low-spin states. This model is a mapping onto a square-lattice $S=1/2$ Ising model with antiferromagnetic nearest-neighbor and ferromagnetic Husimi-Temperley (equivalent-neighbor) long-range interactions. Phase diagrams obtained by the two methods for weak and strong long-range interactions are found to be similar. However, for intermediate-strength long-range interactions, the Monte Carlo simulations show that tricritical points decompose into pairs of critical endpoints and mean-field critical points surrounded by horn-shaped regions of metastability. Hysteresis loops along paths traversing the horn regions are strongly reminiscent of thermal two-step transition loops with hysteresis, recently observed experimentally in several spin-crossover materials. We believe analogous phenomena should be observable in experiments and simulations for many systems that exhibit competition between local antiferromagnetic-like interactions and long-range ferromagnetic-like interactions caused by elastic distortions.

cond-mat.mtrl-sci↗
arXiv2016

Long-Baseline Neutrino Facility (LBNF) and Deep Underground Neutrino Experiment (DUNE) Conceptual Design Report Volume 1: The LBNF and DUNE Projects

This document presents the Conceptual Design Report (CDR) put forward by an international neutrino community to pursue the Deep Underground Neutrino Experiment at the Long-Baseline Neutrino Facility (LBNF/DUNE), a groundbreaking science experiment for long-baseline neutrino oscillation studies and for neutrino astrophysics and nucleon decay searches. The DUNE far detector will be a very large modular liquid argon time-projection chamber (LArTPC) located deep underground, coupled to the LBNF multi-megawatt wide-band neutrino beam. DUNE will also have a high-resolution and high-precision near detector.

physics.ins-det↗
arXiv2015

Mu2e Technical Design Report

The Mu2e experiment at Fermilab will search for charged lepton flavor violation via the coherent conversion process mu- N --> e- N with a sensitivity approximately four orders of magnitude better than the current world's best limits for this process. The experiment's sensitivity offers discovery potential over a wide array of new physics models and probes mass scales well beyond the reach of the LHC. We describe herein the preliminary design of the proposed Mu2e experiment. This document was created in partial fulfillment of the requirements necessary to obtain DOE CD-2 approval.

physics.ins-det↗
arXiv2014

ASTRO-H White Paper - AGN Winds

In this white paper we describe the prospects for ASTRO-H for the study of outflows from active galactic nuclei. The most important breakthroughs in this field are expected to arise from the high spectral resolution and sensitivity in the Fe-K band, combined with broad-band sensitivity over the full X-ray band and spectral capabilities also at lower energies. The sensitivity in the Fe-K region allows to extend the absorption measure distribution of the outflow out to the highest ionisation states accessible, where observations with current X-ray missions indicate that most of the outflowing gas is to be found. Due to the high-resolution and sensitivity it will also be able to give the definitive proof for the existence of ultra-fast outflows, and if so, characterise their physical properties in great detail. These ultra-fast outflows carry very large amounts of energy and momentum, and are of fundamental importance for feedback studies. We show how the ASTRO-H observations in general can help to constrain numerical models for outflows. The link to reflection and emission processes is also discussed, as well as the possible relation between outflows and relativistic emission lines. Finally, we discuss the prospects for other related categories of objects like BAL quasars, partially covered sources and Compton thick outflows.

astro-ph.HE↗
arXiv2013

Placing Limits On The Transit Timing Variations Of Circumbinary Exoplanets

We present an efficient analytical method to predict the maximum transit timing variations of a circumbinary exoplanet, given some basic parameters of the host binary. We derive an analytical model giving limits on the potential location of transits for coplanar planets orbiting eclipsing binaries, then test it against numerical N-body simulations of a distribution of binaries and planets. We also show the application of the analytic model to Kepler-16b, -34b and -35b. The resulting method is fast, efficient and is accurate to approximately 1% in predicting limits on possible times of transits over a three-year observing campaign. The model can easily be used to, for example, place constraints on transit timing while performing circumbinary planet searches on large datasets. It is adaptable to use in situations where some or many of the planet and binary parameters are unknown.

astro-ph.EP↗
arXiv2012

1.5 GHz Pulse Generation From a Monolithic Waveguide Laser With a Graphene-Film Saturable Output Coupler

We fabricate a saturable absorber mirror by coating a graphene film on an output coupler mirror. This is then used to obtain Q-switched mode-locking from a diode pumped linear cavity waveguide laser inscribed in Ytterbium-doped Bismuthate Glass, with high slope and optical conversion efficiencies. The laser produces mode-locked pulses at 1039nm, with 1.5GHz repetition rate at an average 202mW output power. This performance is due to the combination of the graphene saturable absorber with the high quality laser glass.

cond-mat.mes-hall↗
arXiv2012

Ultrafast Laser Inscription of a 121-Waveguide Fan-Out for Astrophotonics

Using ultrafast laser inscription, we report the fabrication of a prototype three-dimensional 121-waveguide fan-out device capable of reformatting the output of a 120 core multicore fiber (MCF) into a one-dimensional linear array. When used in conjunction with an actual MCF, we demonstrate that the reformatting function using this prototype would result in an overall throughput loss of approximately 7.0 dB. However, if perfect coupling from the MCF into the fan-out could be achieved, the reformatting function would result in an overall loss of only approximately 1.7 dB. With adequate development, similar devices could efficiently reformat the output of so-called "photonic lanterns" fabricated using highly multicore fibers.

physics.optics↗
arXiv2011

Kinetic Monte Carlo Simulations of a Model for Heat-assisted Magnetization Reversal in Ultrathin Films

To develop practically useful systems for ultra-high-density information recording with densities above terabits/cm$^2$, it is necessary to simultaneously achieve high thermal stability at room temperature and high recording rates. One method that has been proposed to reach this goal is heat-assisted magnetization reversal (HAMR). In this method, one applies a high-coercivity material, whose coercivity is temporarily lowered during the writing process through localized heating. Here we present kinetic Monte Carlo simulations of a model of HAMR for ultrathin films, in which the temperature in the central part of the film is momentarily increased above the critical temperature, for example by a laser pulse. We observe that the speed-up achieved by this method, relative to the switching time at a constant, subcritical temperature, is optimal for an intermediate strength of the writing field. This effect is explained using the theory of nucleation-induced magnetization switching in finite systems. Our results should be particularly relevant to recording media with strong perpendicular anisotropy, such as ultrathin Co/Pt or Co/Pd multilayers.

cond-mat.mtrl-sci↗
arXiv2008

Resolution-dependent mechanisms for bimodal switching-time distributions in simulated Fe nanopillars

We study the magnetization-switching statistics following reversal of the applied field for three separate computational models representing the same physical system, an iron nanopillar. The primary difference between the models is the resolution of the computational lattice and, consequently, the intrinsic parameters that must be rescaled to retain similarity to the physical system. Considering the first-passage time to zero for the magnetization component in the longitudinal (easy-axis) direction, we look for applied fields that result in bimodal distributions of this time for each system and compare the results to the experimental system. We observe that the relevant fluctuations leading to bimodal distributions are different for each lattice resolution and result in magnetization-switching behavior that is unique to each computational model. Correct model resolution is thus essential for obtaining reliable numerical results for the system dynamics.

cond-mat.mtrl-sci↗