143 research outputs found
Selective factors involved in oil flotation isolation of black yeasts from the environment
The oil flotation isolation technique has been successfully applied to
recover chaetothyrialean black yeasts and relatives from the environment. The
selective mechanisms playing a role in isolation are unknown. The fungi
concerned are supposed to occupy specialized microniches in nature, taking
advantage of (1) oligotrophism. Mineral oil as a main selective agent may be
based on (2) hydrophobicity or on (3) assimilation. All three hypotheses are
tested in this paper. Results show that cell wall hydrophobicity is unlikely
to be a selective factor. Incubation under poor nutrient conditions provides
competitive advantage for black yeasts, especially for Exophiala
strains, which are subsequently enriched by mineral oil which enhances growth
in this group of fungi. Incubation under mineral media and mineral oil can be
used as selective factor
Unfolding of differential energy spectra in the MAGIC experiment
The paper describes the different methods, used in the MAGIC experiment, to
unfold experimental energy distributions of cosmic ray particles (gamma-rays).
Questions and problems related to the unfolding are discussed. Various
procedures are proposed which can help to make the unfolding robust and
reliable. The different methods and procedures are implemented in the MAGIC
software and are used in most of the analyses.Comment: Submitted to NIM
Implementation of the Random Forest Method for the Imaging Atmospheric Cherenkov Telescope MAGIC
The paper describes an application of the tree classification method Random
Forest (RF), as used in the analysis of data from the ground-based gamma
telescope MAGIC. In such telescopes, cosmic gamma-rays are observed and have to
be discriminated against a dominating background of hadronic cosmic-ray
particles. We describe the application of RF for this gamma/hadron separation.
The RF method often shows superior performance in comparison with traditional
semi-empirical techniques. Critical issues of the method and its implementation
are discussed. An application of the RF method for estimation of a continuous
parameter from related variables, rather than discrete classes, is also
discussed.Comment: 16 pages, 8 figure
Constraints on Dark Matter Annihilation in Clusters of Galaxies with the Fermi Large Area Telescope
Nearby clusters and groups of galaxies are potentially bright sources of
high-energy gamma-ray emission resulting from the pair-annihilation of dark
matter particles. However, no significant gamma-ray emission has been detected
so far from clusters in the first 11 months of observations with the Fermi
Large Area Telescope. We interpret this non-detection in terms of constraints
on dark matter particle properties. In particular for leptonic annihilation
final states and particle masses greater than ~200 GeV, gamma-ray emission from
inverse Compton scattering of CMB photons is expected to dominate the dark
matter annihilation signal from clusters, and our gamma-ray limits exclude
large regions of the parameter space that would give a good fit to the recent
anomalous Pamela and Fermi-LAT electron-positron measurements. We also present
constraints on the annihilation of more standard dark matter candidates, such
as the lightest neutralino of supersymmetric models. The constraints are
particularly strong when including the fact that clusters are known to contain
substructure at least on galaxy scales, increasing the expected gamma-ray flux
by a factor of ~5 over a smooth-halo assumption. We also explore the effect of
uncertainties in cluster dark matter density profiles, finding a systematic
uncertainty in the constraints of roughly a factor of two, but similar overall
conclusions. In this work, we focus on deriving limits on dark matter models; a
more general consideration of the Fermi-LAT data on clusters and clusters as
gamma-ray sources is forthcoming.Comment: accepted to JCAP, Corresponding authors: T.E. Jeltema and S. Profumo,
minor revisions to be consistent with accepted versio
Probing quantum gravity using photons from a flare of the active galactic nucleus Markarian 501 observed by the MAGIC telescope
We analyze the timing of photons observed by the MAGIC telescope during a
flare of the active galactic nucleus Mkn 501 for a possible correlation with
energy, as suggested by some models of quantum gravity (QG), which predict a
vacuum refractive index \simeq 1 + (E/M_{QGn})^n, n = 1,2. Parametrizing the
delay between gamma-rays of different energies as \Delta t =\pm\tau_l E or
\Delta t =\pm\tau_q E^2, we find \tau_l=(0.030\pm0.012) s/GeV at the 2.5-sigma
level, and \tau_q=(3.71\pm2.57)x10^{-6} s/GeV^2, respectively. We use these
results to establish lower limits M_{QG1} > 0.21x10^{18} GeV and M_{QG2} >
0.26x10^{11} GeV at the 95% C.L. Monte Carlo studies confirm the MAGIC
sensitivity to propagation effects at these levels. Thermal plasma effects in
the source are negligible, but we cannot exclude the importance of some other
source effect.Comment: 12 pages, 3 figures, Phys. Lett. B, reflects published versio
DEVELOPMENT of the MODEL of GALACTIC INTERSTELLAR EMISSION for STANDARD POINT-SOURCE ANALYSIS of FERMI LARGE AREA TELESCOPE DATA
Most of the celestial \u3b3 rays detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope originate from the interstellar medium when energetic cosmic rays interact with interstellar nucleons and photons. Conventional point-source and extended-source studies rely on the modeling of this diffuse emission for accurate characterization. Here, we describe the development of the Galactic Interstellar Emission Model (GIEM), which is the standard adopted by the LAT Collaboration and is publicly available. This model is based on a linear combination of maps for interstellar gas column density in Galactocentric annuli and for the inverse-Compton emission produced in the Galaxy. In the GIEM, we also include large-scale structures like Loop I and the Fermi bubbles. The measured gas emissivity spectra confirm that the cosmic-ray proton density decreases with Galactocentric distance beyond 5 kpc from the Galactic Center. The measurements also suggest a softening of the proton spectrum with Galactocentric distance. We observe that the Fermi bubbles have boundaries with a shape similar to a catenary at latitudes below 20\ub0 and we observe an enhanced emission toward their base extending in the north and south Galactic directions and located within \u2dc4\ub0 of the Galactic Center
The On-orbit Calibrations for the Fermi Large Area Telescope
The Large Area Telescope (LAT) on--board the Fermi Gamma ray Space Telescope
began its on--orbit operations on June 23, 2008. Calibrations, defined in a
generic sense, correspond to synchronization of trigger signals, optimization
of delays for latching data, determination of detector thresholds, gains and
responses, evaluation of the perimeter of the South Atlantic Anomaly (SAA),
measurements of live time, of absolute time, and internal and spacecraft
boresight alignments. Here we describe on orbit calibration results obtained
using known astrophysical sources, galactic cosmic rays, and charge injection
into the front-end electronics of each detector. Instrument response functions
will be described in a separate publication. This paper demonstrates the
stability of calibrations and describes minor changes observed since launch.
These results have been used to calibrate the LAT datasets to be publicly
released in August 2009.Comment: 60 pages, 34 figures, submitted to Astroparticle Physic
Velocity-space sensitivity of the time-of-flight neutron spectrometer at JET
The velocity-space sensitivities of fast-ion diagnostics are often described by so-called weight functions. Recently, we formulated weight functions showing the velocity-space sensitivity of the often dominant beam-target part of neutron energy spectra. These weight functions for neutron emission spectrometry (NES) are independent of the particular NES diagnostic. Here we apply these NES weight functions to the time-of-flight spectrometer TOFOR at JET. By taking the instrumental response function of TOFOR into account, we calculate time-of-flight NES weight functions that enable us to directly determine the velocity-space sensitivity of a given part of a measured time-of-flight spectrum from TOFOR
Relationship of edge localized mode burst times with divertor flux loop signal phase in JET
A phase relationship is identified between sequential edge localized modes (ELMs) occurrence times in a set of H-mode tokamak plasmas to the voltage measured in full flux azimuthal loops in the divertor region. We focus on plasmas in the Joint European Torus where a steady H-mode is sustained over several seconds, during which ELMs are observed in the Be II emission at the divertor. The ELMs analysed arise from intrinsic ELMing, in that there is no deliberate intent to control the ELMing process by external means. We use ELM timings derived from the Be II signal to perform direct time domain analysis of the full flux loop VLD2 and VLD3 signals, which provide a high cadence global measurement proportional to the voltage induced by changes in poloidal magnetic flux. Specifically, we examine how the time interval between pairs of successive ELMs is linked to the time-evolving phase of the full flux loop signals. Each ELM produces a clear early pulse in the full flux loop signals, whose peak time is used to condition our analysis. The arrival time of the following ELM, relative to this pulse, is found to fall into one of two categories: (i) prompt ELMs, which are directly paced by the initial response seen in the flux loop signals; and (ii) all other ELMs, which occur after the initial response of the full flux loop signals has decayed in amplitude. The times at which ELMs in category (ii) occur, relative to the first ELM of the pair, are clustered at times when the instantaneous phase of the full flux loop signal is close to its value at the time of the first ELM
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