3,258 research outputs found
Searching for Hyperbolicity
This is an expository paper, based on by a talk given at the AWM Research
Symposium 2017. It is intended as a gentle introduction to geometric group
theory with a focus on the notion of hyperbolicity, a theme that has inspired
the field from its inception to current-day research
Detection of Bursts from FRB 121102 with the Effelsberg 100-m Radio Telescope at 5 GHz and the Role of Scintillation
FRB 121102, the only repeating fast radio burst (FRB) known to date, was
discovered at 1.4 GHz and shortly after the discovery of its repeating nature,
detected up to 2.4 GHz. Here we present three bursts detected with the 100-m
Effelsberg radio telescope at 4.85 GHz. All three bursts exhibited frequency
structure on broad and narrow frequency scales. Using an autocorrelation
function analysis, we measured a characteristic bandwidth of the small-scale
structure of 6.41.6 MHz, which is consistent with the diffractive
scintillation bandwidth for this line of sight through the Galactic
interstellar medium (ISM) predicted by the NE2001 model. These were the only
detections in a campaign totaling 22 hours in 10 observing epochs spanning five
months. The observed burst detection rate within this observation was
inconsistent with a Poisson process with a constant average occurrence rate;
three bursts arrived in the final 0.3 hr of a 2 hr observation on 2016 August
20. We therefore observed a change in the rate of detectable bursts during this
observation, and we argue that boosting by diffractive interstellar
scintillations may have played a role in the detectability. Understanding
whether changes in the detection rate of bursts from FRB 121102 observed at
other radio frequencies and epochs are also a product of propagation effects,
such as scintillation boosting by the Galactic ISM or plasma lensing in the
host galaxy, or an intrinsic property of the burst emission will require
further observations.Comment: Accepted to ApJ. Minor typos correcte
Modeling the non-recycled Fermi gamma-ray pulsar population
We use Fermi Gamma-ray Space Telescope detections and upper limits on
non-recycled pulsars obtained from the Large Area Telescope (LAT) to constrain
how the gamma-ray luminosity L depends on the period P and the period
derivative \dot{P}. We use a Bayesian analysis to calculate a best-fit
luminosity law, or dependence of L on P and \dot{P}, including different
methods for modeling the beaming factor. An outer gap (OG) magnetosphere
geometry provides the best-fit model, which is L \propto P^{-a} \dot{P}^{b}
where a=1.36\pm0.03 and b=0.44\pm0.02, similar to but not identical to the
commonly assumed L \propto \sqrt{\dot{E}} \propto P^{-1.5} \dot{P}^{0.5}. Given
upper limits on gamma-ray fluxes of currently known radio pulsars and using the
OG model, we find that about 92% of the radio-detected pulsars have gamma-ray
beams that intersect our line of sight. By modeling the misalignment of radio
and gamma-ray beams of these pulsars, we find an average gamma-ray beaming
solid angle of about 3.7{\pi} for the OG model, assuming a uniform beam. Using
LAT-measured diffuse fluxes, we place a 2{\sigma} upper limit on the average
braking index and a 2{\sigma} lower limit on the average surface magnetic field
strength of the pulsar population of 3.8 and 3.2 X 10^{10} G, respectively. We
then predict the number of non-recycled pulsars detectable by the LAT based on
our population model. Using the two-year sensitivity, we find that the LAT is
capable of detecting emission from about 380 non-recycled pulsars, including
150 currently identified radio pulsars. Using the expected five-year
sensitivity, about 620 non-recycled pulsars are detectable, including about 220
currently identified radio pulsars. We note that these predictions
significantly depend on our model assumptions.Comment: 26 pages, 10 figures, Accepted by ApJ on 8 September 201
Time-Correlated Structure in Spin Fluctuations in Pulsars
We study statistical properties of stochastic variations in pulse arrival
times, timing noise, in radio pulsars using a new analysis method applied in
the time domain. The method proceeds in two steps. First, we subtract
low-frequency wander using a high-pass filter. Second, we calculate the
discrete correlation function of the filtered data. As a complementary method
for measuring correlations, we introduce a statistic that measures the
dispersion of the data with respect to the data translated in time. The
analysis methods presented here are robust and of general usefulness for
studying arrival time variations over timescales approaching the average
sampling interval. We apply these methods to timing data for 32 pulsars. In two
radio pulsars, PSRs B1133+16 and B1933+16, we find that fluctuations in arrival
times are correlated over timescales of 10 - 20 d with the distinct signature
of a relaxation process. Though this relaxation response could be
magnetospheric in origin, we argue that damping between the neutron star crust
and interior liquid is a more likely explanation. Under this interpretation,
our results provide the first evidence independent from pulsar spin glitches of
differential rotation in neutron stars. PSR B0950+08, shows evidence for
quasi-periodic oscillations that could be related to mode switching.Comment: 25 pages, Final journal version (MNRAS
The relation between the volatile and total acidity in starters and in cultures of S. Lacticus
During the last few years evidence has been presented which shows that starters are not pure cultures of S. lacticus. Hammer and Bailey, Storch and Boekhout and Ott de Vries have found that starters contain organisms other than S. lactic us that are important from the standpoint of volatile acid and odor and flavor development. More recently Hammer4has secured data that indicate the source of the volatile acidity produced by the organisms associated with S. lacticus and has also shown that starters do not contain pure lactic acid as they would be expected to if they were pure cultures of S. lacticus. The percent of the total acidity represented by volatile acid at various times during the period of rapid acid development is quite different with starters than it is with pure cultures of S. lactictus, and this difference offers further proof that starters are not pure cultures; data on this point are presented herewith .
A study of lactose-fermenting yeasts present in yeasty cream
The shipment for considerable distances of cream to be made into butter, either from receiving stations or from individual farmers, is a common practice in many sections. Much of this cream is sent without refrigeration and undergoes extensive changes as a result of the activity of the organisms contained. \u27l\u27he type of fermentation that occurs is influenced to a considerable extent by the types of organisms present, but other factors, the most important of which is apparently the temperature, cause a significant effect. One of the important fermentations occurring in cream during the warm months is the yeasty or\u27 foamy fermentation
Burnt or caramel flavor of dairy products
While many abnormal flavors present in dairy products are indefinite and difficult to name, others are sufficiently distinct so that descriptive names can be applied to them. \u27fhe so-called burnt flavor is one of the rather common flavors belong\u27ing to the latter group and is observed particularly in butter. Butter showing this flavor has often been made from raw cream without the use of starter prepared from heated milk, so it is evident that the flavor is not due to the heat used in pasteurization. Moreover, the experienced butter judge recognizes a pronounced difference between the so-called burnt flavor and the flavor that results from excessive pasteurization exposures. The burnt flavor, like all flavors, is very difficult to describe. It suggests caramel to some people and the flavor of condensed milk to others, while still others believe it is more suggestive of malt
Parallax and Kinematics of PSR B0919+06 from VLBA Astrometry and Interstellar Scintillometry
Results are presented from a long-term astrometry program on PSR B0919+06
using the NRAO Very Long Baseline Array. With ten observations (seven epochs)
between 1994--2000, we measure a proper motion of 18.35 +/- 0.06 mas/yr in RA,
86.56 +/- 0.12 mas/yr in Dec, and a parallax of 0.83 +/- 0.13 mas (68%
confidence intervals). This yields a pulsar distance of 1.21 +/- 0.19 kpc,
making PSR B0919+06 the farthest pulsar for which a trigonometric parallax has
been obtained, and the implied pulsar transverse speed is 505 +/- 80 km/s.
Combining the distance estimate with interstellar scintillation data spanning
20 years, we infer the existence of a patchy or clumpy scattering screen along
the line of sight in addition to the distributed electron density predicted by
models for the Galaxy, and constrain the location of this scattering region to
within about 250 parsecs of the Sun. Comparison with the lines of sight towards
other pulsars in the same quadrant of the Galaxy permits refinement of our
knowledge of the local interstellar matter in this direction.Comment: 12 pages, includes 4 figures and 3 tables, uses AASTeX 5 (included);
ApJ submitte
Development of the microbiological population in water miscible metal working fluids
The deterioration of metal working fluids (MWFs) due to the microbial colonization
and degradation is a considerable economic factor in the metal working industry.
Microorganisms (MO) are able to metabolize almost all components of MWFs and
thus lead to a loss of its function by the reduction or depletion of additives. Microbial
growth cannot be avoided completely, although various methods exist to reduce the
bacterial load in MWFs. This paper presents a study on the colonization of MWFs by
bacteria and fungi in an industrial environment. The cooling lubricants have been
periodically examined based on biological and chemical methods. The level of the
total bacterial load in the lubricant is considered as well as the composition of the
species community and its development over the evaluation period. With regard to
the increasing relevance of environment friendly processes, a conventional mineral
oil based MWF has been compared to a product based on renewable resources
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