scholarly journals Study of spider pulsar binary eclipses and discovery of an eclipse mechanism transition

2020 ◽  
Vol 494 (2) ◽  
pp. 2948-2968 ◽  
Author(s):  
E J Polzin ◽  
R P Breton ◽  
B Bhattacharyya ◽  
D Scholte ◽  
C Sobey ◽  
...  

ABSTRACT We present a comparative study of the low-frequency eclipses of spider (compact and irradiating binary) PSRs B1957+20 and J1816+4510. Combining these data with those of three other eclipsing systems we study the frequency dependence of the eclipse duration. PSRs B1957+20 and J1816+4510 have similar orbital properties, but the companions to the pulsars have masses that differ by an order of magnitude. A dedicated campaign to simultaneously observe the pulsed and imaged continuum flux densities throughout the eclipses reveals many similarities between the excess material within the two binaries, irrespective of the companion star properties. The observations show that the pulsar fluxes are removed from the line of sight throughout the main body of the eclipses. For PSR J1816 + 4510, we present the first direct evidence of an eclipse mechanism that transitions from one that removes the pulsar flux from the line of sight to one that merely smears out pulsations, and claim that this is a consequence of scattering in a tail of material flowing behind the companion. Inferred mass-loss rates from the companion stars are found to be $\dot{M}_{\text{C}} \sim 10^{-12}$ and $\sim 2 \times 10^{-13}\,\mathrm{M}_\odot$ yr−1 for PSR B1957+20 and PSR J1816 + 4510, respectively; seemingly too low to evaporate the stars within Hubble time. Measurements of eclipse durations over a wide range of radio frequencies show a significant dependence of eclipse duration on frequency for all pulsars, with wider eclipses at lower frequencies. These results provide a marked improvement in the observational constraints available for theoretical studies of the eclipse mechanisms.

2020 ◽  
Vol 634 ◽  
pp. A3 ◽  
Author(s):  
Joeri van Leeuwen ◽  
Klim Mikhailov ◽  
Evan Keane ◽  
Thijs Coenen ◽  
Liam Connor ◽  
...  

Bright short radio bursts are emitted by sources at a wide range of distances: from the nearby Crab pulsar to remote fast radio bursts (FRBs). FRBs are likely to originate from distant neutron stars, but our knowledge of the radio pulsar population has been limited to the Galaxy and the Magellanic Clouds. In an attempt to increase our understanding of extragalactic pulsar populations and their giant-pulse emission, we employed the low-frequency radio telescope LOFAR to search the Andromeda galaxy (M 31) for radio bursts emitted by young Crab-like pulsars. For direct comparison we also present a LOFAR study on the low-frequency giant pulses from the Crab pulsar; their fluence distribution follows a power law with slope 3.04 ± 0.03. A number of candidate signals were detected from M 31, but none proved persistent. FRBs are sometimes thought of as Crab-like pulsars with exceedingly bright giant pulses; based on our sensitivity, we can rule out that M 31 hosts pulsars that are more than an order of magnitude brighter than the Crab pulsar if their pulse scattering follows that of the known FRBs.


2009 ◽  
Vol 23 (4) ◽  
pp. 191-198 ◽  
Author(s):  
Suzannah K. Helps ◽  
Samantha J. Broyd ◽  
Christopher J. James ◽  
Anke Karl ◽  
Edmund J. S. Sonuga-Barke

Background: The default mode interference hypothesis ( Sonuga-Barke & Castellanos, 2007 ) predicts (1) the attenuation of very low frequency oscillations (VLFO; e.g., .05 Hz) in brain activity within the default mode network during the transition from rest to task, and (2) that failures to attenuate in this way will lead to an increased likelihood of periodic attention lapses that are synchronized to the VLFO pattern. Here, we tested these predictions using DC-EEG recordings within and outside of a previously identified network of electrode locations hypothesized to reflect DMN activity (i.e., S3 network; Helps et al., 2008 ). Method: 24 young adults (mean age 22.3 years; 8 male), sampled to include a wide range of ADHD symptoms, took part in a study of rest to task transitions. Two conditions were compared: 5 min of rest (eyes open) and a 10-min simple 2-choice RT task with a relatively high sampling rate (ISI 1 s). DC-EEG was recorded during both conditions, and the low-frequency spectrum was decomposed and measures of the power within specific bands extracted. Results: Shift from rest to task led to an attenuation of VLFO activity within the S3 network which was inversely associated with ADHD symptoms. RT during task also showed a VLFO signature. During task there was a small but significant degree of synchronization between EEG and RT in the VLFO band. Attenuators showed a lower degree of synchrony than nonattenuators. Discussion: The results provide some initial EEG-based support for the default mode interference hypothesis and suggest that failure to attenuate VLFO in the S3 network is associated with higher synchrony between low-frequency brain activity and RT fluctuations during a simple RT task. Although significant, the effects were small and future research should employ tasks with a higher sampling rate to increase the possibility of extracting robust and stable signals.


2020 ◽  
Author(s):  
Thomas Louis-Goff ◽  
Huu Vinh Trinh ◽  
Eileen Chen ◽  
Arnold L. Rheingold ◽  
Christian Ehm ◽  
...  

A new, efficient, catalytic difluorocarbenation of olefins to give 1,1-difluorocyclopropanes is presented. The catalyst, an organobismuth complex, uses TMSCF<sub>3</sub> as a stoichiometric difluorocarbene source. We demonstrate both the viability and robustness of this reaction over a wide range of alkenes and alkynes, including electron-poor alkenes, to generate the corresponding 1,1-difluorocyclopropanes and 1,1-difluorocyclopropenes. Ease of catalyst recovery from the reaction mixture is another attractive feature of this method. In depth experimental and theoretical studies showed that the key difluorocarbene-generating step proceeds through a bismuth non-redox synchronous mechanism generating a highly reactive free CF<sub>2</sub> in an endergonic pre-equilibrium. It is the reversibility when generating the difluorocarbene that accounts for the high selectivity, while minimizing CF<sub>2</sub>-recombination side-reactions.


Atmosphere ◽  
2021 ◽  
Vol 12 (6) ◽  
pp. 680
Author(s):  
Chris D. Boone ◽  
Johnathan Steffen ◽  
Jeff Crouse ◽  
Peter F. Bernath

Line-of-sight wind profiles are derived from Doppler shifts in infrared solar occultation measurements from the Atmospheric Chemistry Experiment Fourier transform spectrometers (ACE-FTS), the primary instrument on SCISAT, a satellite-based mission for monitoring the Earth’s atmosphere. Comparisons suggest a possible eastward bias from 20 m/s to 30 m/s in ACE-FTS results above 80 km relative to some datasets but no persistent bias relative to other datasets. For instruments operating in a limb geometry, looking through a wide range of altitudes, smearing of the Doppler effect along the line of sight can impact the measured signal, particularly for saturated absorption lines. Implications of Doppler effect smearing are investigated for forward model calculations and volume mixing ratio retrievals. Effects are generally small enough to be safely ignored, except for molecules having a large overhang in their volume mixing ratio profile, such as carbon monoxide.


2021 ◽  
Vol 16 (1) ◽  
Author(s):  
Roland Stengl ◽  
Bence Ágg ◽  
Miklós Pólos ◽  
Gábor Mátyás ◽  
Gábor Szabó ◽  
...  

Abstract Background Marfan syndrome (MFS) is a genetically determined systemic connective tissue disorder, caused by a mutation in the FBN1 gene. In MFS mainly the cardiovascular, musculoskeletal and ocular systems are affected. The most dangerous manifestation of MFS is aortic dissection, which needs to be prevented by a prophylactic aortic root replacement. Main body The indication criteria for the prophylactic procedure is currently based on aortic diameter, however aortic dissections below the threshold defined in the guidelines have been reported, highlighting the need for a more accurate risk stratification system to predict the occurrence of aortic complications. The aim of this review is to present the current knowledge on the possible predictors of severe cardiovascular manifestations in MFS patients, demonstrating the wide range of molecular and radiological differences between people with MFS and healthy individuals, and more importantly between MFS patients with and without advanced aortic manifestations. These differences originating from the underlying common molecular pathological processes can be assessed by laboratory (e.g. genetic testing) and imaging techniques to serve as biomarkers of severe aortic involvement. In this review we paid special attention to the rapidly expanding field of genotype–phenotype correlations for aortic features as by collecting and presenting the ever growing number of correlations, future perspectives for risk stratification can be outlined. Conclusions Data on promising biomarkers of severe aortic complications of MFS have been accumulating steadily. However, more unifying studies are required to further evaluate the applicability of the discussed predictors with the aim of improving the risk stratification and therefore the life expectancy and quality of life of MFS patients.


Atmosphere ◽  
2019 ◽  
Vol 10 (9) ◽  
pp. 499 ◽  
Author(s):  
Artem Shikhovtsev ◽  
Pavel Kovadlo ◽  
Vladimir Lukin

The paper focuses on the development of the method to estimate the mean characteristics of the atmospheric turbulence. Using an approach based on the shape of the energy spectrum of atmospheric turbulence over a wide range of spatial and temporal scales, the vertical profiles of optical turbulence are calculated. The temporal variability of the vertical profiles of turbulence under different low-frequency atmospheric disturbances is considered.


Author(s):  
Walter Anderson ◽  
Constantine Ciocanel ◽  
Mohammad Elahinia

Engine vibration has caused a great deal of research for isolation to be performed. Traditionally, isolation was achieved through the use of pure elastomeric (rubber) mounts. However, with advances in vehicle technology, these types of mounts have become inadequate. The inadequacy stems from the vibration profile associated with the engine, i.e. high displacement at low frequency and small displacement at high frequency. Ideal isolation would be achieved through a stiff mount for low frequency and a soft mount for high frequency. This is contradictory to the performance of the elastomeric mounts. Hydraulic mounts were then developed to address this problem. A hydraulic mount has variable stiffness and damping due to the use of a decoupler and an inertia track. However, further advances in vehicle technology have rendered these mounts inadequate as well. Examples of these advances are hybridization (electric and hydraulic) and cylinder on demand (VCM, MDS & ACC). With these technologies, the vibration excitation has a significantly different profile, occurs over a wide range of frequencies, and calls for a new technology that can address this need. Magnetorheological (MR) fluid is a smart material that is able to change viscosity in the presence of a magnetic field. With the use of MR fluid, variable damping and stiffness can be achieved. An MR mount has been developed and tested. The performance of the mount depends on the geometry of the rubber part as well as the behavior of the MR fluid. The rubber top of the mount is the topic of this study due to its major impact on the isolation characteristics of the MR mount. To develop a design methodology to address the isolation needs of different hybrid vehicles, a geometric parametric finite element analysis has been completed and presented in this paper.


2018 ◽  
Vol 7 (3) ◽  
pp. 24-46
Author(s):  
Sourav Paul ◽  
Provas Roy

In this article, an Oppositional Differential search algorithm (ODSA) is comprehensively developed and successfully applied for the optimal design of power system stabilizer (PSS) parameters which are added to the excitation system to dampen low frequency oscillation as it pertains to large power system. The effectiveness of the proposed method is examined and validated on a single machine infinite bus (SMIB) using the Heffron-Phillips model. The most important advantage of the proposed method is as it reaches toward the optimal solution without the optimal tuning of input parameters of the ODSA algorithm. In order to verify the effectiveness, the simulation was made for a wide range of loading conditions. The simulation results of the proposed ODSA are compared with those obtained by other techniques available in the recent literature to demonstrate the feasibility of the proposed algorithm.


1979 ◽  
Vol 42 (6) ◽  
pp. 1538-1556 ◽  
Author(s):  
J. E. Kanz ◽  
L. B. Eberly ◽  
J. S. Cobbs ◽  
H. M. Pinsker

1. Central neuronal mechanisms of siphon withdrawal in Aplysia were studied for the first time in intact, freely behaving animals by means of population recordings from implanted whole-nerve cuff electrodes. Intracellular follow-up studies were then conducted when the same animal was reduced to a semi-intact preparation. 2. Background spontaneous activity in the siphon nerve consisted of low-frequency firing of a population of efferent units containing identified siphon motoneurons. 3. Spontaneous patterned bursts of efferent activity occurred irregularly and were associated with all-or-nothing contractions of the parapodia, gill, and siphon. Spontaneous bursts were due to centrally generated activity in the interneuron II (INT II) network, an oscillatory network with endogenous pacemaker properties. 4. In intact animals, even weak tactile stimuli to the siphon typically triggered an INTII burst shortly after the stimulus-locked efferent activity. Thus, the stimulus can phase-advance the INT II oscillator. In semi-intact preparations, short-latency INT II bursts were triggered less less frequently and required more intense stimuli. 5. With weak to moderate-intensity stimuli in intact animals, the presence of short-latency triggered INT II bursts largely determined the duration of the siphon component and amplitude of the gill component of the withdrawal reflex. 6. When stimuli were repeated over a range of interstimulus intervals (from 60 to 1 min), the likelihood of triggering a short-latency INT II burst die not change systematically. Thus, the ability of the siphon stimulus to stably entrain the all-or-none INT II component over a wide range of intervals will interact behaviorally with the decrement of the monosynaptic component of the reflex with repetition.


1999 ◽  
Vol 09 (12) ◽  
pp. 2295-2303 ◽  
Author(s):  
S. RIPOLL MASSANÉS ◽  
C. J. PÉREZ VICENTE

We have studied the stochastic behavior of Fitzhugh–Nagumo neuron-like model (FN) induced by subthreshold external stimuli. Our analysis based on three standard measures: the power spectrum, interspike interval distribution (ISI) and autocorrelation function shows that it is possible to define a characteristic time scale which can be identified in the response of the system for a wide range of frequencies. In contrast to previous studies we have focused our attention on high frequency signals which could be of interest for real systems such as nervous fibers in the auditory system. We report behaviors which resemble those of classical deterministic oscillators but never the stochastic resonance phenomenon typical of low frequency signals.


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