347 research outputs found
Determining electron column density fluctuations in a dominant scattering region using pulsar scintillation
Density fluctuations in the ionised interstellar medium have a profound
effect on radio pulsar observations, through angular scattering, intensity
scintillations, and small changes in time delays from dispersion. Here we show
that it is possible to recover the variations in dispersive delays that
originate from a dominant scattering region using measurements of the dynamic
spectrum of intensity scintillations, provided that the pulsar velocity and
scattering region location are known. We provide a theoretical framework for
the technique, which involves estimating the phase gradient from the dynamic
spectra and integrating that gradient to obtain phase variations. It can be
used to search for "extreme scattering events" (ESEs) in pulsars for which
precision dispersion delay measurements are not otherwise possible, or to
separate true dispersion variations from apparent variability caused by
frequency-dependent pulse shape changes. We demonstrate that it works in
practice by recovering an ESE in PSR J16037202, which is known from
precision dispersion delay measurements from pulsar timing. For this pulsar, we
find that the phase gradients also track the long-term variations in electron
column density observed by pulsar timing, indicating that the column density
variations and the scattering are dominated by the same thin scattering screen.
We identify a sudden increase in the scintillation strength and magnitude of
phase gradients over days in 2010, indicating a compact structure. A
decrease in the electron density in 2012 was associated with persistent phase
gradients and preceded a period of decreased scintillation strength and an
absence of scintillation arcs.Comment: 9 pages, 7 figures, Accepted for publication in MNRA
Theory of Parabolic Arcs in Interstellar Scintillation Spectra
Our theory relates the secondary spectrum, the 2D power spectrum of the radio
dynamic spectrum, to the scattered pulsar image in a thin scattering screen
geometry. Recently discovered parabolic arcs in secondary spectra are generic
features for media that scatter radiation at angles much larger than the rms
scattering angle. Each point in the secondary spectrum maps particular values
of differential arrival-time delay and fringe rate (or differential Doppler
frequency) between pairs of components in the scattered image. Arcs correspond
to a parabolic relation between these quantities through their common
dependence on the angle of arrival of scattered components. Arcs appear even
without consideration of the dispersive nature of the plasma. Arcs are more
prominent in media with negligible inner scale and with shallow wavenumber
spectra, such as the Kolmogorov spectrum, and when the scattered image is
elongated along the velocity direction. The arc phenomenon can be used,
therefore, to constrain the inner scale and the anisotropy of scattering
irregularities for directions to nearby pulsars. Arcs are truncated by finite
source size and thus provide sub micro arc sec resolution for probing emission
regions in pulsars and compact active galactic nuclei. Multiple arcs sometimes
seen signify two or more discrete scattering screens along the propagation
path, and small arclets oriented oppositely to the main arc persisting for long
durations indicate the occurrence of long-term multiple images from the
scattering screen.Comment: 22 pages, 11 figures, submitted to the Astrophysical Journa
Simulation of wave propagation in three-dimensional random media
Quantitative error analysis for simulation of wave propagation in three dimensional random media assuming narrow angular scattering are presented for the plane wave and spherical wave geometry. This includes the errors resulting from finite grid size, finite simulation dimensions, and the separation of the two-dimensional screens along the propagation direction. Simple error scalings are determined for power-law spectra of the random refractive index of the media. The effects of a finite inner scale are also considered. The spatial spectra of the intensity errors are calculated and compared to the spatial spectra of intensity. The numerical requirements for a simulation of given accuracy are determined for realizations of the field. The numerical requirements for accurate estimation of higher moments of the field are less stringent
Behavior Therapy and Callous-Unemotional Traits: Effects of a Pilot Study Examining Modified Behavioral Contingencies on Child Behavior
The conduct problems of children with callous-unemotional traits (i.e., lack of empathy, guilt/lack of caring behaviors) (CU) are particularly resistant to current behavioral interventions, and it is possible that differential sensitivities to punishment and reward may underlie this resistance. Children with conduct problems and CU (CPCU) are less responsive to behavioral punishment techniques (e.g., time-out), however reward techniques (e.g., earning points for prizes or activities) are effective for reducing conduct problems. This study examined the efficacy of modified behavioral interventions, which de-emphasized punishment (condition B) and emphasized reward techniques (condition C), compared to a standard behavioral intervention (condition A). Interventions were delivered through a Summer Treatment Program over seven weeks with an A-B-A-C-A-BC-A design to a group of eleven children (7–11 years; 91% male). All children were diagnosed with either oppositional defiant disorder or conduct disorder, in addition to attention-deficit/hyperactivity disorder. Results revealed the best treatment response occurred during the low punishment condition, with rates of negative behavior (e.g., aggression, teasing, stealing) increasing over the seven weeks. However, there was substantial individual variability in treatment response, and several children demonstrated improvement during the modified intervention conditions. Future research is necessary to disentangle treatment effects from order effects, and implications of group treatment of CPCU children (i.e., deviancy training) are discussed
Subaortic stenosis in the spectrum of atrioventricular septal defects Solutions may be complex and palliative
AbstractFrom July 1982 through September 1994, 19 children had operative treatment of subaortic stenosis associated with an atrioventricular septal defect. Specific diagnosis were septum primum defects in 7, Rastelli type A defects in 6, transitional defects in 4, inlet ventricular septal defect with malattached chordae in 1, and tetralogy of Fallot with Rastelli type C defect in 1. Twenty-seven operations for subaortic stenosis were performed. Surgical treatment of the outlet lesion was performed at initial atrioventricular septal defect repair in 3 children and in the remaining 16 from 1.2 to 13.1 years (mean 4.9 years, median 3.9 years) after repair. Eighteen of the 19 children had fibrous resection and myectomy for relief of obstruction. Seven children had an associated left atrioventricular valve procedure. One child received an apicoaortic conduit. Seven children (36.8%) required 8 reoperations for previously treated subaortic stenosis. Time to the second procedure was 2.8 to 7.4 years (mean 4.9 years). Follow-up is 0.4 to 14.0 years (median 5.6 years). Six-year actuarial freedom from reoperation is 66% ±15%. The angle between the plane of the outlet septum and the plane of the septal crest was measured in 10 normal hearts (86.4 ±13.7) and 10 hearts with atrioventricular septal defects (22.2 ±26.0; p <0.01). The outflow tract can be effectively shortened, widened, and the angle increased toward normal by augmenting the left side of the superior bridging leaflet and performing a fibromyectomy. Conclusion: Standard fibromyectomy for subaortic stenosis in children with atrioventricular septal defects leads to a high rate of reoperation. Leaflet augmentation and fibromyectomy may decrease the likelihood of reoperation. (J THORAC CARDIOVASC SURG 1995;110:1534-42
The medical student
The Medical Student was published from 1888-1921 by the students of Boston University School of Medicine
Conventional and sutureless techniques for management of the pulmonary veins: Evolution of indications from postrepair pulmonary vein stenosis to primary pulmonary vein anomalies
ObjectiveWe have previously reported a limited but favorable experience with a novel sutureless technique for surgical management of postoperative pulmonary vein stenosis occurring after repair of total anomalous pulmonary venous drainage. Because this technique requires integrity of the retrocardiac space for hemostasis, extension of the technique to the primary repair of pulmonary vein anomalies requires evaluation. This analysis reviews our experience with the sutureless technique in patients with postrepair pulmonary vein stenosis, as well as our extension of the technique into primary repair of pulmonary vein anomalies.MethodsRetrospective univariable-multivariable analysis of all pulmonary vein stenosis procedures and sutureless pulmonary vein procedures over a 20-year period was performed. Cox proportional hazards modeling was used to identify variables associated with freedom from reoperation or death.ResultsSixty patients underwent 73 procedures, with pulmonary vein stenosis present in 65 procedures. The sutureless technique was used in 40 procedures. Freedom from reoperation or death at 5 years after the initial procedure was 49%. Unadjusted freedom from reoperation or death was greater with the sutureless technique for patients with postrepair pulmonary vein stenosis (P = .04). By using multivariable analysis, a higher pulmonary vein stenosis score was associated with greater risk of reoperation or death. After adjustment, the sutureless repair was associated with a nonsignificant trend toward greater freedom from reoperation or death (P = .12). Despite the absence of retrocardiac adhesions, operative mortality was not increased with the sutureless technique (P = .64). Techniques to control bleeding (intrapleural hilar reapproximation) and improve exposure (inferior vena cava division) were identified.ConclusionThe sutureless technique for postrepair pulmonary vein stenosis is associated with encouraging midterm results. Extension of the indications for the technique to primary repair appears safe with the development of simple intraoperative maneuvers
Precision orbital dynamics from interstellar scintillation arcs for PSR J0437-4715
Intensity scintillations of radio pulsars are known to originate from
interference between waves scattered by the electron density irregularities of
interstellar plasma, often leading to parabolic arcs in the two-dimensional
power spectrum of the recorded dynamic spectrum. The degree of arc curvature
depends on the distance to the scattering plasma and its transverse velocity
with respect to the line-of-sight. We report the observation of annual and
orbital variations in the curvature of scintillation arcs over a period of 16
years for the bright millisecond pulsar, PSR J0437-4715. These variations are
the signature of the relative transverse motions of the Earth, pulsar, and
scattering medium, which we model to obtain precise measurements of parameters
of the pulsar's binary orbit and the scattering medium itself. We observe two
clear scintillation arcs in most of our 5000 observations and we show that
they originate from scattering by thin screens located at distances pc and pc from Earth. The best-fit scattering model
we derive for the brightest arc yields the pulsar's orbital inclination angle
, and longitude of ascending node,
. Using scintillation arcs for precise astrometry and
orbital dynamics can be superior to modelling variations in the diffractive
scintillation timescale, because the arc curvature is independent of variations
in the level of turbulence of interstellar plasma. This technique can be used
in combination with pulsar timing to determine the full three-dimensional
orbital geometries of binary pulsars, and provides parameters essential for
testing theories of gravity and constraining neutron star masses.Comment: 19 pages, 10 figures. Accepted for publication in Ap
PSR J1926-0652: A Pulsar with Interesting Emission Properties Discovered at FAST
We describe PSR J1926-0652, a pulsar recently discovered with the
Five-hundred-meter Aperture Spherical radio Telescope (FAST). Using sensitive
single-pulse detections from FAST and long-term timing observations from the
Parkes 64-m radio telescope, we probed phenomena on both long and short time
scales. The FAST observations covered a wide frequency range from 270 to 800
MHz, enabling individual pulses to be studied in detail. The pulsar exhibits at
least four profile components, short-term nulling lasting from 4 to 450 pulses,
complex subpulse drifting behaviours and intermittency on scales of tens of
minutes. While the average band spacing P3 is relatively constant across
different bursts and components, significant variations in the separation of
adjacent bands are seen, especially near the beginning and end of a burst. Band
shapes and slopes are quite variable, especially for the trailing components
and for the shorter bursts. We show that for each burst the last detectable
pulse prior to emission ceasing has different properties compared to other
pulses. These complexities pose challenges for the classic carousel-type
models.Comment: 13pages with 12 figure
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