scholarly journals Scattering of the Low Frequency Pulsar Radiation

2001 ◽  
Vol 182 ◽  
pp. 53-56
Author(s):  
Arkady Kuzmin
Keyword(s):  
Frequency Dependence ◽  
Low Frequency ◽  
Dispersion Measure ◽  
Measured Dependence ◽  
Pulsar Radiation ◽  
Direct Measurements ◽  
Uniform Sample ◽  
Interstellar Scattering ◽  
Pulse Broadening

AbstractWe report the results of the measurements and analysis of the pulse broadening due to interstellar scattering on 43 pulsars at 102 MHz. This is the largest uniform sample of direct measurements of pulsar scattering τsc, which make it feasible to analyze the dependence of this value on other pulsar parameters. The measured dependence of τsc on dispersion measure τsc(DM) = 40(DM /100)2.1 is close to theoretically expected relation τsc(DM) α DM2. A frequency dependence of the scattering pulse broadening is weaker than commonly accepted τsc α v−4.4.

scholarly journals Low frequency pulsar observations with the international LOFAR station FR606

2017 ◽  
Vol 13 (S337) ◽  
pp. 313-314 ◽  
Author(s):  
Louis Bondonneau ◽  
Jean-Mathias Grießmeier ◽  
Gilles Theureau ◽  
Maciej Serylak
Keyword(s):  
Interstellar Medium ◽  
Frequency Dependence ◽  
Frequency Band ◽  
Low Frequency ◽  
Turnover Frequency ◽  
Pulsar Emission ◽  
Frequency Dependent ◽  
Monitoring Campaign ◽  
Pulse Broadening

AbstractThe pulsar’s signal passes through the interstellar medium (ISM) which leads to both chromatic dispersive delays and multipath pulse broadening. These effects have a strong frequency dependence (f−2 and f−4 respectively). Pulse profiles of pulsars are also frequency-dependent leading to some degeneracy with the ISM imprint. Furthermore, many pulsars show a turnover of their spectrum around ~100 MHz. For all these reasons, the frequency band below 100 MHz contains a lot of information about both the pulsar emission and the ISM. Our study is based on a LOw Frequency ARray (LOFAR) monitoring campaign using the international station FR606. Firstly, we demonstrate the importance of a monitoring campaign. Secondly, we calculate median spectra and locate the turnover frequency for 3 pulsars (B0809+74, B1133+16, B1508+55).

scholarly journals The Relation of the Low Frequency Source to the Crab Pulsar

1971 ◽  
Vol 46 ◽  
pp. 91-96
Author(s):  
Kenneth R. Lang
Keyword(s):  
Angular Size ◽  
Low Frequency ◽  
Crab Pulsar ◽  
Compact Source ◽  
General Properties ◽  
Interstellar Scattering ◽  
Frequency Source ◽  
Pulse Broadening ◽  
The Continuum

The view that the compact low frequency source and the pulsar NP 0532 are the same object is substantiated by an examination of the general properties of interstellar scattering. This scattering accounts for the observed angular size of the compact source, the observed pulse broadening of NP 0532, the continuum nature of the compact source, and the observed spectrum of both the pulsar and the compact source.

scholarly journals First detection of frequency-dependent, time-variable dispersion measures

2019 ◽  
Vol 624 ◽  
pp. A22 ◽  
Author(s):  
J. Y. Donner ◽  
J. P. W. Verbiest ◽  
C. Tiburzi ◽  
S. Osłowski ◽  
D. Michilli ◽  
...  
Keyword(s):  
Frequency Dependence ◽  
High Precision ◽  
Low Frequency ◽  
Time Variable ◽  
Small Scale ◽  
Dispersion Measure ◽  
Electron Content ◽  
Frequency Dependent ◽  
Pulsar Timing

Context. High-precision pulsar-timing experiments are affected by temporal variations of the dispersion measure (DM), which are related to spatial variations in the interstellar electron content and the varying line of sight to the source. Correcting for DM variations relies on the cold-plasma dispersion law which states that the dispersive delay varies with the squared inverse of the observing frequency. This may, however, give incorrect measurements if the probed electron content (and therefore the DM) varies with observing frequency, as is predicted theoretically due to the different refraction angles at different frequencies. Aims. We study small-scale density variations in the ionised interstellar medium. These structures may lead to frequency-dependent DMs in pulsar signals. Such an effect could inhibit the use of lower-frequency pulsar observations as tools to correct time-variable interstellar dispersion in higher-frequency pulsar-timing data. Methods. We used high-cadence, low-frequency observations with three stations from the German LOng-Wavelength (GLOW) consortium, which are part of the LOw-Frequency ARray (LOFAR). Specifically, 3.5 yr of weekly observations of PSR J2219+4754 are presented. Results. We present the first detection of frequency-dependent DMs towards any interstellar object and a precise multi-year time-series of the time- and frequency-dependence of the measured DMs. The observed DM variability is significant and may be caused by extreme scattering events. Potential causes for frequency-dependent DMs are quantified and evaluated. Conclusions. We conclude that frequency dependence of DMs has been reliably detected and is indeed caused by small-scale (up to tens of AUs) but steep density variations in the interstellar electron content. We find that long-term trends in DM variability equally affect DMs measured at both ends of our frequency band and hence the negative impact on long-term high-precision timing projects is expected to be limited.

Low-frequency respiratory mechanical impedance in the rat

1987 ◽  
Vol 63 (1) ◽  
pp. 36-43 ◽  
Author(s):  
Z. Hantos ◽  
B. Daroczy ◽  
B. Suki ◽  
S. Nagy
Keyword(s):  
Frequency Dependence ◽  
Chest Wall ◽  
Low Frequency ◽  
Mechanical Impedance ◽  
Total Resistance ◽  
Low Frequencies ◽  
Chest Wall Compliance ◽  
Before And After ◽  
Airway Opening ◽  
Wall Compliance

modified forced oscillatory technique was used to determine the respiratory mechanical impedances in anesthetized, paralyzed rats between 0.25 and 10 Hz. From the total respiratory (Zrs) and pulmonary impedance (ZL), measured with pseudorandom oscillations applied at the airway opening before and after thoracotomy, respectively, the chest wall impedance (ZW) was calculated as ZW = Zrs - ZL. The pulmonary (RL) and chest wall resistances were both markedly frequency dependent: between 0.25 and 2 Hz they contributed equally to the total resistance falling from 81.4 +/- 18.3 (SD) at 0.25 Hz to 27.1 +/- 1.7 kPa.l–1 X s at 2 Hz. The pulmonary compliance (CL) decreased mildly, from 2.78 +/- 0.44 at 0.25 Hz to 2.36 +/- 0.39 ml/kPa at 2 Hz, and then increased at higher frequencies, whereas the chest wall compliance declined monotonously from 4.19 +/- 0.88 at 0.25 Hz to 1.93 +/- 0.14 ml/kPa at 10 Hz. Although the frequency dependence of ZW can be interpreted on the basis of parallel inhomogeneities alone, the sharp fall in RL together with the relatively constant CL suggests that at low frequencies significant losses are imposed by the non-Newtonian resistive properties of the lung tissue.

scholarly journals Pulsar Astrometry

1984 ◽  
Vol 110 ◽  
pp. 347-353
Author(s):  
Joseph H. Taylor ◽  
Carl R. Gwinn ◽  
Joel M. Weisberg ◽  
Lloyd A. Rawley
Keyword(s):  
Electron Density ◽  
Reference Frames ◽  
Neutral Hydrogen ◽  
Space Velocity ◽  
Solar Neighborhood ◽  
Dispersion Measure ◽  
Direct Measurements ◽  
Pulse Timing ◽  
Local Space ◽  
Dispersion Measures

High precision measurements of the celestial coordinates of pulsars are desirable for a number of reasons. If carried out at several epochs, the measurements can yield angular proper motions; together with distance estimates based on dispersion measure, the proper motion of a pulsar reveals two of three components of its space velocity, and consequently provides important kinematic information on pulsar ages (see, for example, Manchester, Taylor and Van 1974; Lyne, Anderson and Salter 1982; and references therein). Direct measurements of annual parallaxes are also possible in principle, and are marginally feasible with present techniques for a few of the closest pulsars. Model independent distances obtained from parallax measurements, together with observed pulsar dispersion measures, yield the electron density along the line of sight to the pulsar. Knowledge of the interstellar electron density in the solar neighborhood provides a calibration of the dispersion-based distance scale that is complementary to the calibration derived from neutral hydrogen absorption measurements of more distant pulsars (Weisberg et al. 1980), and permits appropriate statistical analyses to be made of the local space density of pulsars and their birthrate (e.g. Taylor and Manchester 1977). Finally, pulsar astrometry can be expected to yield important information on the relative orientations of fundamental reference frames. In particular, pulse timing observations yield positions in a reference frame based on motions of the planets, while interferometric position measurements are based on an Earth-equatorial system. At present the relative orientation of these two coordinate systems is known to only accuracy, though the potential precision of both types of measurements is much higher.

Low Frequency (327 MHz) VLBI Survey: First Results

1998 ◽  
Vol 164 ◽  
pp. 141-142 ◽  
Author(s):  
V. Altunin ◽  
A. Beasley ◽  
E. Lűdke ◽  
V. Alekseev ◽  
B. Lipatov ◽  
...  
Keyword(s):  
Preliminary Data ◽  
Low Frequency ◽  
Potential Candidate ◽  
Scattering Medium ◽  
Interstellar Scattering ◽  
The Future ◽  
First Results

AbstractIn this paper we report the first results of the 327 MHz VLBI survey which has been carried out in the last two years to select potential candidate sources for the future SVLBI missions and to search for directions of small scattering-“holes” in the interstellar scattering medium. During the three VLBI sessions conducted so far, we observed about 80 sources. Preliminary data analyzed from the first part of the survey shows that at least 17 out of 50 sources have compact components with a size smaller than 30 mas.

scholarly journals CT2A Cell Viability Modulated by Electromagnetic Fields at Extremely Low Frequency under No Thermal Effects

2019 ◽  
Vol 21 (1) ◽  
pp. 152 ◽  
Author(s):  
Olga García-Minguillán ◽  
Raquel Prous ◽  
Maria del Carmen Ramirez-Castillejo ◽  
Ceferino Maestú
Keyword(s):  
Cell Viability ◽  
Frequency Dependence ◽  
Electromagnetic Fields ◽  
Thermal Effects ◽  
Low Frequency ◽  
Human Beings ◽  
Cellular Mechanisms ◽  
Low Frequencies ◽  
Hsp90 Expression

The effects produced by electromagnetic fields (EMFs) on human beings at extremely low frequencies (ELFs) have being investigated in the past years, across in vitro studies, using different cell lines. Nevertheless, the effects produced on cells are not clarified, and the cellular mechanisms and cell-signaling processes involved are still unknown. This situation has resulted in a division among the scientific community about the adequacy of the recommended level of exposure. In this sense, we consider that it is necessary to develop long-term exposure studies and check if the recommended levels of EMFs are under thermal effects. Hence, we exposed CT2A cells to different EMFs at different ELFs at short and long times. Our results showed frequency dependence in CT2A exposed during 24 h to a small EMF of 30 μT equal to those originated by the Earth and frequency dependence after the exposure during seven days to an EMF of 100 µT at different ELFs. Particularly, our results showed a remarkable cell viability decrease of CT2A cells exposed to EMFs of 30 Hz. Nevertheless, after analyzing the thermal effects in terms of HSP90 expression, we did not find thermal damages related to the differences in cell viability, so other crucial cellular mechanism should be involved.

Low-frequency ac electroporation shows strong frequency dependence and yields comparable transfection results to dc electroporation

2012 ◽  
Vol 160 (3) ◽  
pp. 570-576 ◽  
Author(s):  
Yihong Zhan ◽  
Zhenning Cao ◽  
Ning Bao ◽  
Jianbo Li ◽  
Jun Wang ◽  
...  
Keyword(s):  
Frequency Dependence ◽  
Low Frequency
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