190 research outputs found
The Feasibility of Magnetic Reconnection Powered Blazar Flares from Synchrotron Self-Compton Emission
Order of magnitude variability has been observed in the blazar sub-class of
Active Galactic Nuclei on minute timescales. These high-energy flares are often
difficult to explain with shock acceleration models due to the small size of
the inferred emitting region, with recent particle-in-cell (PIC) simulations
showing that magnetic reconnection is a promising alternative mechanism. Here,
we present a macroscopic emission model physically motivated by PIC
simulations, where the energy for particle acceleration originates from the
reconnecting magnetic field. We track the radial growth and relative velocity
of a reconnecting plasmoid, modelling particle acceleration and radiative
losses from synchrotron and synchrotron self-Compton (SSC) emission. To test
the viability of magnetic reconnection as the mechanism behind rapid blazar
flares we simultaneously fit our model to the observed light-curve and SED from
the 2016 TeV flare of BL Lacertae. We find generally that, without considering
external photons, reconnecting plasmoids are unable to produce Compton-dominant
TeV flares and so cannot reproduce the observations due to overproduction of
synchrotron emission. Additionally, problematically large plasmoids, comparable
in size to the entire jet radius, are required to emit sufficient SSC
gamma-rays to be observable. However, our plasmoid model can reproduce the
rapid TeV lightcurve of the flare, demonstrating that reconnection is able to
produce rapid, powerful TeV flares on observed timescales. We conclude that
while reconnection can produce SSC flares on the correct timescales, the
primary source of TeV emission cannot be SSC and the size of plasmoids required
may be implausibly large.Comment: Replaced with accepted version. Contains additional figures and
considers the effect of a magnetic guide fiel
Gamma-ray Novae: Rare or Nearby?
Classical Novae were revealed as a surprise source of gamma-rays in Fermi LAT
observations. During the first 8 years since the LAT was launched, 6 novae in
total have been detected to > 5 sigma in gamma-rays, in contrast to the 69
discovered optically in the same period. We attempt to resolve this discrepancy
by assuming all novae are gamma-ray emitters, and assigning peak one-day fluxes
based on a flat distribution of the known emitters to a simulated population.
To determine optical parameters, the spatial distribution and magnitudes of
bulge and disc novae in M31 are scaled to the Milky Way, which we approximate
as a disc with a 20 kpc radius and elliptical bulge with semi major axis 3 kpc
and axis ratios 2:1 in the xy plane. We approximate Galactic reddening using a
double exponential disc with vertical and radial scale heights of r_d = 5 kpc
and z_d = 0.2 kpc, and demonstrate that even such a rudimentary model can
easily reproduce the observed fraction of gamma-ray novae, implying that these
apparently rare sources are in fact nearby and not intrinsically rare. We
conclude that classical novae with m_R < 12 and within ~8 kpc are likely to be
discovered in gamma-rays using the Fermi LAT.Comment: Accepted by MNRAS, 10 pages, 7 figure
Bacterial communities in the rumen of Holstein heifers differ when fed orchardgrass as pasture vs. hay
The rich and diverse microbiota of the rumen provides ruminant animals the capacity to utilize highly fibrous feedstuffs as their energy source, but there is surprisingly little information on the composition of the microbiome of ruminants fed all-forage diets, despite the importance of such agricultural production systems worldwide. In three 28-d periods, three ruminally-cannulated Holstein heifers sequentially grazed orchardgrass pasture (OP), then were fed orchardgrass hay (OH), then returned to OP. These heifers displayed greater shifts in ruminal bacterial community composition (determined by automated ribosomal intergenic spacer analysis and by pyrotag sequencing of 16S rRNA genes) than did two other heifers maintained 84 d on the same OP. Phyla Firmicutes and Bacteroidetes dominated all ruminal samples, and quantitative PCR indicated that members of the genus Prevotella averaged 23 % of the 16S rRNA gene copies, well below levels previously reported with cows fed total mixed rations. Differences in bacterial community composition and ruminal volatile fatty acid (VFA) profiles were observed between the OP and OH despite similarities in gross chemical composition. Compared to OP, feeding OH increased the molar proportion of ruminal acetate (P = 0.02) and decreased the proportion of ruminal butyrate (P < 0.01), branched-chain VFA (P < 0.01) and the relative population size of the abundant genus Butyrivibrio (P < 0.001), as determined by pyrotag sequencing. Despite the low numbers of animals examined, the observed changes in VFA profile in the rumens of heifers on OP vs. OH are consistent with the shifts in Butyrivibrio abundance and its known physiology as a butyrate producer that ferments both carbohydrates and proteins
Sub-Arcsecond Resolution Mid-IR Imaging of The Starburst NGC 520
Abstract We present sub-arcsecond resolution mid-infrared images of the nearby starburst NGC 520 in four narrow filters (7.9, 9.7 , 11.6 and 12.5 µm). These data, together with matched-resolution VLA data at four frequencies and Brγ data, have enabled us to calculate, in a selfconsistent way, the properties of the starburst. This model agrees with our estimate of extinction from the 9.7 µm silicate absorption feature, which we have also used to produce a map of the column density of the obscuring material
Dopamine Operates as a Subsecond Modulator of Food Seeking
The dopamine projection to the nucleus accumbens has been implicated in behaviors directed toward the acquisition and consumption of natural rewards. The neurochemical studies that established this link made time-averaged measurements over minutes, and so the precise temporal relationship between dopamine changes and these behaviors is not known. To resolve this, we sampled dopamine every 100 msec using fast-scan cyclic voltammetry at carbon-fiber microelectrodes in the nucleus accumbens of rats trained to press a lever for sucrose. Cues that signal the opportunity to respond for sucrose evoked dopamine release (67 +/- 20 nm) with short latency (0.2 +/- 0.1 sec onset). When the same cues were presented to rats naive to the cue-sucrose pairing, similar dopamine signals were not observed. Thus, cue-evoked increases in dopamine in trained rats reflected a learned association between the cues and sucrose availability. Lever presses for sucrose occurred at the peak of the dopamine surges. After lever presses, and while sucrose was delivered and consumed, no further increases in dopamine were detected. Rather, dopamine returned to baseline levels. Together, the results strongly implicate subsecond dopamine signaling in the nucleus accumbens as a real-time modulator of food-seeking behavior
Dynamic Gain Control of Dopamine Delivery in Freely Moving Animals
Activity changes in a large subset of midbrain dopamine neurons fulfill numerous assumptions of learning theory by encoding a prediction error between actual and predicted reward. This computational interpretation of dopaminergic spike activity invites the important question of how changes in spike rate are translated into changes in dopamine delivery at target neural structures. Using electrochemical detection of rapid dopamine release in the striatum of freely moving rats, we established that a single dynamic model can capture all the measured fluctuations in dopamine delivery. This model revealed three independent short-term adaptive processes acting to control dopamine release. These short-term components generalized well across animals and stimulation patterns and were preserved under anesthesia. The model has implications for the dynamic filtering interposed between changes in spike production and forebrain dopamine release
Unified time analysis of photon and (nonrelativistic) particle Tunnelling, and the Superluminal group-velocity problem
A unified approach to the time analysis of tunnelling of nonrelativistic
particles is presented, in which Time is regarded as a quantum-mechanical
observable, canonically conjugated to Energy. The validity of the Hartman
effect (independence of the Tunnelling Time of the opaque barrier width, with
Superluminal group velocities as a consequence) is verified for ALL the known
expressions of the mean tunnelling time. Moreover, the analogy between particle
and photon tunnelling is suitably exploited. On the basis of such an analogy,
an explanation of some recent microwave and optics experimental results on
tunnelling times is proposed. Attention is devoted to some aspects of the
causality problem for particle and photon tunnelling.Comment: plain (old) LaTeX; 42 pages; plus figures 1, 2, 3, 4a, 4b, and
Reference Array and Design Consideration for the next-generation Event Horizon Telescope
We describe the process to design, architect, and implement a transformative
enhancement of the Event Horizon Telescope (ngEHT). This program - the
next-generation Event Horizon Telescope (ngEHT) - will form a networked global
array of radio dishes capable of making high-fidelity real-time movies of
supermassive black holes (SMBH) and their emanating jets. This builds upon the
EHT principally by deploying additional modest-diameter dishes to optimized
geographic locations to enhance the current global mm/submm wavelength Very
Long Baseline Interferometric (VLBI) array, which has, to date, utilized mostly
pre-existing radio telescopes. The ngEHT program further focuses on observing
at three frequencies simultaneously for increased sensitivity and Fourier
spatial frequency coverage. Here, the concept, science goals, design
considerations, station siting and instrument prototyping are discussed, and a
preliminary reference array to be implemented in phases is described.Comment: Submitted to the journal Galaxie
Cheating on Their Taxes: When are Tax Limitations Effective at Limiting State Taxes, Expenditures, and Budgets?
Use of procalcitonin for the diagnosis of pneumonia in patients presenting with a chief complaint of dyspnoea: results from the BACH (Biomarkers in Acute Heart Failure) trial
Biomarkers have proven their ability in the evaluation of cardiopulmonary diseases.We investigated the utility of concentrations
of the biomarker procalcitonin (PCT) alone and with clinical variables for the diagnosis of pneumonia in
patients presenting to emergency departments (EDs) with a chief complaint of shortness of breath.
The BACH trial was a prospective, international, study of 1641 patients presenting to EDs with dyspnoea. Blood
samples were analysed for PCT and other biomarkers. Relevant clinical data were also captured. Patient outcomes
were assessed at 90 days. The diagnosis of pneumonia was made using strictly validated guidelines. A model using
PCT was more accurate [area under the curve (AUC) 72.3%] than any other individual clinical variable for the diagnosis
of pneumonia in all patients, in those with obstructive lung disease, and in those with acute heart failure (AHF).
Combining physician estimates of the probability of pneumonia with PCT values increased the accuracy to .86% for
the diagnosis of pneumonia in all patients. Patients with a diagnosis of AHF and an elevated PCT concentration
(.0.21 ng/mL) had a worse outcome if not treated with antibiotics (P ¼ 0.046), while patients with low PCT
values (,0.05 ng/mL) had a better outcome if they did not receive antibiotic therapy (P ¼ 0.049).
Procalcitonin may aid in the diagnosis of pneumonia, particularly in cases with high diagnostic uncertainty. Importantly,
PCT may aid in the decision to administer antibiotic therapy to patients presenting with AHF in which clinical uncertainty
exists regarding a superimposed bacterial infection
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