scholarly journals Radio emission physics in the Crab pulsar

2016 ◽  
Vol 82 (3) ◽  
Author(s):  
Jean A. Eilek ◽  
Timothy H. Hankins
Keyword(s):  
High Altitude ◽  
Low Frequency ◽  
High Time Resolution ◽  
Emission Model ◽  
Crab Pulsar ◽  
Radio Observations ◽  
Wide Range ◽  
Cascade Models ◽  
Standard Picture ◽  
Radio Power

We review our high-time-resolution radio observations of the Crab pulsar and compare our data to a variety of models for the emission physics. The Main Pulse and the Low Frequency Interpulse come from regions somewhere in the high-altitude emission zones (caustics) that also produce pulsed X-ray and${\it\gamma}$-ray emission. Although no emission model can fully explain these two components, the most likely models suggest they arise from a combination of beam-driven instabilities, coherent charge bunching and strong electromagnetic turbulence. Because the radio power fluctuates on a wide range of time scales, we know the emission zones are patchy and dynamic. It is tempting to invoke unsteady pair creation in high-altitude gaps as the source of the variability, but current pair cascade models cannot explain the densities required by any of the likely models. It is harder to account for the mysterious High Frequency Interpulse. We understand neither its origin within the magnetosphere nor the striking emission bands in its dynamic spectrum. The most promising models are based on analogies with solar zebra bands, but they require unusual plasma structures which are not part of our standard picture of the magnetosphere. We argue that radio observations can reveal much about the upper magnetosphere, but work is required before the models can address all of the data.

scholarly journals LOFAR radio search for single and periodic pulses from M 31

2020 ◽  
Vol 634 ◽  
pp. A3 ◽  
Author(s):  
Joeri van Leeuwen ◽  
Klim Mikhailov ◽  
Evan Keane ◽  
Thijs Coenen ◽  
Liam Connor ◽  
...  
Keyword(s):  
Low Frequency ◽  
Crab Pulsar ◽  
Radio Bursts ◽  
Pulse Emission ◽  
Giant Pulses ◽  
Wide Range ◽  
The Galaxy ◽  
Order Of Magnitude ◽  
M 31 ◽  
Pulsar Populations

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.

scholarly journals VI. Solar Radiophysics

1985 ◽  
Vol 19 (1) ◽  
pp. 78-84
Author(s):  
R. T. Stewart
Keyword(s):  
Time Resolution ◽  
Solar Maximum ◽  
Research Effort ◽  
Solar Spectrum ◽  
Low Frequency ◽  
High Time Resolution ◽  
Two Dimensional ◽  
Radio Observations ◽  
One Dimensional ◽  
X Ray

The past 3 years since the last solar maximum have witnessed an unprecedented number (>200) of published scientific papers on many aspects of solar radiophysics. These contributions are the result of an intense research effort mounted during the first Solar Maximum Mission of 1980 and continued until the present. Excellent x-ray, EUV, and visible light observations of the disturbed corona and transition region have been obtained from the SMM, Hinotori, P78-1, and ISEE-3 spacecraft. ISEE-3 also has provided very low-frequency radio observations of solar bursts in the interplanetary medium. Ground-based radio support for space experiments has been provided by many observatories throughout the world. In particular, many collaborative studies using x-ray and radio observations of solar flares have been reported. The outstanding radio instrument during this period has been the VLA, operating at 2, 6, and 20 cm with a time resolution of 10 s and both modes of circular polarization. The two-dimensional spatial resolution of the radio images is a few seconds of arc, almost as good as the best resolution obtained so far at any wavelength in the solar spectrum. To complement the rather poor time resolution of the VLA one-dimensional arrays such as the WSRT at 6 cm wavelength and the Nobeyama interferometer at 17 GHz have been used successfully. In addition, a number of very-high-time resolution radiometers have been built at different locations. At meter wavelengths two-dimensional arrays at Clark Lake, Culgoora, and Nancay and a one-dimensional array at Nobeyama have been employed. The little known region of the solar spectrum at decimeter wavelengths is being investigated by the Zurich spectrograph. It is pleasing to see Chinese participation in solar radiophysics.

The Attenuation of Very Low Frequency Brain Oscillations in Transitions from a Rest State to Active Attention

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
Keyword(s):  
Brain Activity ◽  
Sampling Rate ◽  
Low Frequency ◽  
Future Research ◽  
Adhd Symptoms ◽  
Default Mode ◽  
Very Low Frequency ◽  
Wide Range ◽  
Interference Hypothesis ◽  
Mode Interference

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.

Low frequency radio observations: critical in revealing inhomogeneous shock structures

2019 ◽  
Author(s):  
Poonam Chandra ◽  
A. J. Nayana ◽  
Claes-Ingvar Bjornsson ◽  
Peter Lundqvist ◽  
Alak K. Ray
Keyword(s):  
Low Frequency ◽  
Radio Observations

scholarly journals Prediction of Wide Range Two-Dimensional Refractivity Using an IDW Interpolation Method from High-Altitude Refractivity Data of Multiple Meteorological Observatories

2021 ◽  
Vol 11 (4) ◽  
pp. 1431
Author(s):  
Sungsik Wang ◽  
Tae Heung Lim ◽  
Kyoungsoo Oh ◽  
Chulhun Seo ◽  
Hosung Choo
Keyword(s):  
High Altitude ◽  
Korean Peninsula ◽  
Interpolation Method ◽  
Atmospheric Condition ◽  
Propagation Path ◽  
Two Dimensional ◽  
Data Set ◽  
Wide Range ◽  
Atmospheric Data ◽  
Terrain Surface

This article proposes a method for the prediction of wide range two-dimensional refractivity for synthetic aperture radar (SAR) applications, using an inverse distance weighted (IDW) interpolation of high-altitude radio refractivity data from multiple meteorological observatories. The radio refractivity is extracted from an atmospheric data set of twenty meteorological observatories around the Korean Peninsula along a given altitude. Then, from the sparse refractive data, the two-dimensional regional radio refractivity of the entire Korean Peninsula is derived using the IDW interpolation, in consideration of the curvature of the Earth. The refractivities of the four seasons in 2019 are derived at the locations of seven meteorological observatories within the Korean Peninsula, using the refractivity data from the other nineteen observatories. The atmospheric refractivities on 15 February 2019 are then evaluated across the entire Korean Peninsula, using the atmospheric data collected from the twenty meteorological observatories. We found that the proposed IDW interpolation has the lowest average, the lowest average root-mean-square error (RMSE) of ∇M (gradient of M), and more continuous results than other methods. To compare the resulting IDW refractivity interpolation for airborne SAR applications, all the propagation path losses across Pohang and Heuksando are obtained using the standard atmospheric condition of ∇M = 118 and the observation-based interpolated atmospheric conditions on 15 February 2019. On the terrain surface ranging from 90 km to 190 km, the average path losses in the standard and derived conditions are 179.7 dB and 182.1 dB, respectively. Finally, based on the air-to-ground scenario in the SAR application, two-dimensional illuminated field intensities on the terrain surface are illustrated.

scholarly journals Temporal Variations of the Turbulence Profiles at the Sayan Solar Observatory Site

Atmosphere ◽  
2019 ◽  
Vol 10 (9) ◽  
pp. 499 ◽  
Author(s):  
Artem Shikhovtsev ◽  
Pavel Kovadlo ◽  
Vladimir Lukin
Keyword(s):  
Atmospheric Turbulence ◽  
Temporal Variability ◽  
Low Frequency ◽  
Temporal Variations ◽  
Solar Observatory ◽  
Vertical Profiles ◽  
Spatial And Temporal Scales ◽  
Atmospheric Disturbances ◽  
Wide Range ◽  
The Mean

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.

scholarly journals Low Frequency Radio Observations of GRS 1915+105 with GMRT

2005 ◽  
Vol 5 (S1) ◽  
pp. 87-92 ◽  
Author(s):  
C H Ishwara-Chandra ◽  
A Pramesh Rao ◽  
Mamta Pandey ◽  
R K Manchanda ◽  
Philippe Durouchoux
Keyword(s):  
Low Frequency ◽  
Radio Observations

scholarly journals Investigating Pulse Morphology in GX 1+4

1998 ◽  
Vol 15 (2) ◽  
pp. 217-221 ◽  
Author(s):  
Michelle C. Storey ◽  
J. G. Greenhill ◽  
T. Kotani
Keyword(s):  
Pulse Shape ◽  
Transition Probabilities ◽  
Low Frequency ◽  
Opening Angle ◽  
Emission Model ◽  
Two Photon ◽  
Maximum Value ◽  
Emission Transition ◽  
Cyclotron Emission ◽  
Theoretical Evidence

AbstractObservational and theoretical evidence points to the existence of an unusually high magnetic field on GX 1+4. The pulsar is thus an ideal laboratory for studying two-photon cyclotron emission, an important source of photons of frequency significantly less than the cyclotron frequency in X-ray pulsars. Low-frequency approximations to the two-photon cyclotron emission transition probabilities are derived. These are used to calculate the theoretical opening angle of the double-humped pulse shape predicted by the two-photon cyclotron emission model. The theoretical pulse shape, incorporating the effects of gravitational light bending, is compared with observations of GX 1+4. Observed light curves have opening angles consistent with the theoretically predicted maximum value.

scholarly journals Echocardiographic and invasive measurements of pulmonary artery pressure correlate closely at high altitude

2000 ◽  
Vol 279 (4) ◽  
pp. H2013-H2016 ◽  
Author(s):  
Yves Allemann ◽  
Claudio Sartori ◽  
Mattia Lepori ◽  
Sébastien Pierre ◽  
Christian Mélot ◽  
...  
Keyword(s):  
Pulmonary Hypertension ◽  
High Altitude ◽  
Doppler Echocardiography ◽  
Systolic Pulmonary Arterial Pressure ◽  
Wide Range ◽  
High Altitude Pulmonary Edema ◽  
Reproducible Method ◽  
Pulmonary Arterial ◽  
Spearman Test ◽  
Invasive Measurement

Exaggerated hypoxia-induced pulmonary hypertension is a hallmark of high-altitude pulmonary edema (HAPE) and plays a major role in its pathogenesis. Many studies of HAPE have estimated systolic pulmonary arterial pressure (SPAP) with Doppler echocardiography. Whereas at low altitude, Doppler echocardiographic estimation of SPAP correlates closely with its invasive measurement, no such evidence exists for estimations obtained at high altitude, where alterations of blood viscosity may invalidate the simplified Bernoulli equation. We measured SPAP by Doppler echocardiography and invasively in 14 mountaineers prone to HAPE and in 14 mountaineers resistant to this condition at 4,559 m. Mountaineers prone to HAPE had more pronounced pulmonary hypertension (57 ± 12 and 58 ± 10 mmHg for noninvasive and invasive determination, respectively; means ± SD) than subjects resistant to HAPE (37 ± 8 and 37 ± 6 mmHg, respectively), and the values measured in the two groups as a whole covered a wide range of pulmonary arterial pressures (30–83 mmHg). Spearman test showed a highly significant correlation ( r = 0.89, P < 0.0001) between estimated and invasively measured SPAP values. The mean difference between invasively measured and Doppler-estimated SPAP was 0.5 ± 8 mmHg. At high altitude, estimation of SPAP by Doppler echocardiography is an accurate and reproducible method that correlates closely with its invasive measurement.

Sign in / Sign up

Export Citation Format

Share Document