scholarly journals A Statistical Investigation of Microwave Burst Spectra for the Determination of Source Inhomogeneities

1980 ◽  
Vol 86 ◽  
pp. 183-185
Author(s):  
W. Schoechlin ◽  
A. Magun
Keyword(s):  
Cluster Analysis ◽  
Flux Density ◽  
Spectral Index ◽  
Low Frequency ◽  
Statistical Investigation ◽  
Geophysical Data ◽  
Triangular Shape ◽  
Most Frequent Value ◽  
Microwave Bursts

SummaryThe spectra of a set of 77 microwave bursts of July 1971 to September 1974 as published in Solar Geophysical Data have been fitted with an idealized spectrum of triangular shape. The spectral index of the low frequency part α, the maximum flux density and the associated frequency have been determined. A cluster-analysis carried out on these parameters showed that the sample consists of only one class of events. In the histogram of the spectral index we found a most frequent value of 1.4. Such a low value cannot be explained by the mechanisms normally assumed to account for the low frequency attenuation, as gyroresonance absorption and the Razin effect.

scholarly journals Superluminal Motion in CTA 102

1989 ◽  
Vol 134 ◽  
pp. 529-530
Author(s):  
Ann E. Wehrle
Keyword(s):  
Flux Density ◽  
High Speed ◽  
Low Frequency ◽  
Precise Determination ◽  
Flux Variation ◽  
Low Frequencies ◽  
Superluminal Motion ◽  
Low Frequency Variability ◽  
Expansion Speed

Sholomitskii (1965) discovered that the flux density of the quasar CTA 102 varies at low frequencies on a timescale of a few months. Low-frequency variability can be explained by “superluminal flux variation” (Romney et al. 1984): If the intrinsic brightness of a component moving in a relativistically beamed source varies by only a few percent, the observer sees its flux density change by a much larger factor δ3-α when the optically thin blob moves almost directly toward the observer. Such a relativistically beamed source is likely to exhibit superluminal motion if studied with sufficient resolution and sensitivity. Superluminal motion in CTA 102 was discovered by Bååth (1987) who concluded on the basis of maps made at three epochs at a frequency of 932 MHz that two components were separating at a rate of 0.65 milliarcseconds (mas) per year. Using a redshift z = 1.037 and H0 = 100 km s−1 Mpc−1, q0 = 0.5, this expansion speed corresponds to (18 ± 4)h−1c. The extraordinarily high speed led us to make VLBI images of the source at a higher frequency in order to increase the resolution and make a more precise determination of the speed.

scholarly journals Spectral Index - Redshift Relation for Radio Galaxies and Quasars

1996 ◽  
Vol 175 ◽  
pp. 569-570
Author(s):  
R.D. Dagkesamanskii
Keyword(s):  
Flux Density ◽  
Spectral Index ◽  
Low Frequency ◽  
Radio Galaxies ◽  
Cosmological Evolution ◽  
Radio Sources ◽  
Spectral Indices ◽  
Extragalactic Radio Sources

Cosmological evolution of synchrotron spectra of the powerful extragalactic radio sources was studied by many authors. Some indications of such an evolution had been found firstly by analysis of ‘spectral index - flux density’ (α – S) relation for the sample of relatively strong radio sources. Later Gopal-Krishna and Steppe extended the analysis to weaker sources and found that the slope of αmed(S) curve changes dramatically at intermediate flux densities. Gopal-Krishna and Steppe pointed out that the maxima of the αmed(S) curve and of differential source counts are at almost the same flux density ranges (see, Fig. 2). It has to be noticed that the all mentioned results were obtained using the low-frequency spectral indices and on the basis of low frequency samples.

scholarly journals The MRC 1-Jy Sample of Radio Galaxies

1996 ◽  
Vol 175 ◽  
pp. 517-518 ◽  
Author(s):  
V. K. Kapahi ◽  
P. J. Mccarthy ◽  
R. M. Athreya ◽  
W. Van BREUGEL ◽  
C. R. Subrahmanya
Keyword(s):  
Flux Density ◽  
Spectral Index ◽  
Systematic Study ◽  
Low Frequency ◽  
Radio Galaxies ◽  
Complete Sample

Most known galaxies at high redshifts have been found by concentrating on ultra steep spectrum (α≳l) sources in low frequency radio surveys (eg. Rottgering et al., 1994; McCarthy et al., 1990). We have been carrying out a systematic study of a large and complete sample from the 408 MHz Molonglo Reference Catalogue which is not biased by any spectral index criterion. Containing 558 sources, defined by S408 ≥ 0.95 Jy; −30° < δ < −20° and the RA ranges of 20h 20m to 06h00m and 09h20m to 14h00m, our sample is about 3 times larger in size and 5 times deeper in flux density than the well studied 3CRR sample.

scholarly journals Number Counts and Spectral Distribution of Radio Sources at Centimeter Wavelengths

1972 ◽  
Vol 44 ◽  
pp. 444-452 ◽  
Author(s):  
I. I. K. Pauliny-Toth ◽  
K. I. Kellermann ◽  
M. M. Davis
Keyword(s):  
Flux Density ◽  
Spectral Index ◽  
Excellent Agreement ◽  
Spectral Distribution ◽  
Low Frequency ◽  
Systematic Change ◽  
Radio Sources ◽  
Significant Dependence ◽  
5 Ghz ◽  
Number Counts

The results of a survey of selected regions of sky made at a frequency of 5 GHz, and of measurements of the spectral index distributions for sources selected from low frequency surveys are presented. The source counts and spectral distribution at 5 GHz are in excellent agreement with those expected from surveys made at lower frequencies. There is no significant dependence of the spectral distribution on flux density in any of the surveys we have investigated, implying either a low redshift for the sources, or a systematic change in their properties with redshift.

scholarly journals A wide-area GMRT 610-MHz survey of ELAIS N1 field

2020 ◽  
Vol 497 (4) ◽  
pp. 5383-5394 ◽  
Author(s):  
C H Ishwara-Chandra ◽  
A R Taylor ◽  
D A Green ◽  
J M Stil ◽  
M Vaccari ◽  
...  
Keyword(s):  
Flux Density ◽  
Spectral Index ◽  
Luminosity Function ◽  
Low Frequency ◽  
Wide Area ◽  
Radio Sources ◽  
Ancillary Data ◽  
Photometric Redshift ◽  
Radio Jets ◽  
Source Populations

ABSTRACT In this paper, we present a wide-area 610-MHz survey of the ELAIS N1 field with the GMRT, covering an area of 12.8 deg2 at a resolution of 6 arcsec and with an rms noise of ∼40 $\mu$Jy beam−1. This is equivalent to ∼20 $\mu$Jy beam−1 rms noise at 1.4 GHz for a spectral index of −0.75. The primary goal of the survey was to study the polarized sky at sub-mJy flux densities at &lt; GHz frequencies, alongwith a range of other science goals such as investigations into the nature of the low-frequency $\mu$Jy source populations and alignments of radio jets. A total of 6400 sources were found in this region, the vast majority of them compact. The sample jointly detected by GMRT at 610 MHz and by VLA FIRST at 1.4 GHz has a median spectral index of −0.85 ± 0.05 and a median 610-MHz flux density of 4.5 mJy. This region has a wealth of ancillary data, which is useful to characterize the detected sources. The multiwavelength crossmatching resulted optical/IR counterparts to ∼90 per cent of the radio sources, with a significant fraction having at least photometric redshift. Due to the improved sensitivity of this survey over preceding ones, we have discovered six giant radio sources (GRSs), with three of them at z ∼ 1 or higher. This implies that the population of GRS may be more abundant and common than known to date and if true, this has implications for the luminosity function and the evolution of radio sources. We have also identified several candidate-extended relic sources.

scholarly journals Improved well logs clustering algorithm for shale gas identification and formation evaluation

2021 ◽  
Author(s):  
N. P. Szabó ◽  
B. A. Braun ◽  
M. M. G. Abdelrahman ◽  
M. Dobróka
Keyword(s):  
Cluster Analysis ◽  
Shale Gas ◽  
Clustering Algorithm ◽  
Traditional Approach ◽  
Harmful Effect ◽  
Weighted Average ◽  
Well Logs ◽  
Organic Carbon Content ◽  
Wireline Logs ◽  
Most Frequent Value

AbstractThe identification of lithology, fluid types, and total organic carbon content are of great priority in the exploration of unconventional hydrocarbons. As a new alternative, a further developed K-means type clustering method is suggested for the evaluation of shale gas formations. The traditional approach of cluster analysis is mainly based on the use of the Euclidean distance for grouping the objects of multivariate observations into different clusters. The high sensitivity of the L2 norm applied to non-Gaussian distributed measurement noises is well-known, which can be reduced by selecting a more suitable norm as distance metrics. To suppress the harmful effect of non-systematic errors and outlying data, the Most Frequent Value method as a robust statistical estimator is combined with the K-means clustering algorithm. The Cauchy-Steiner weights calculated by the Most Frequent Value procedure is applied to measure the weighted distance between the objects, which improves the performance of cluster analysis compared to the Euclidean norm. At the same time, the centroids are also calculated as a weighted average (using the Most Frequent Value method), instead of applying arithmetic mean. The suggested statistical method is tested using synthetic datasets as well as observed wireline logs, mud-logging data and core samples collected from the Barnett Shale Formation, USA. The synthetic experiment using extremely noisy well logs demonstrates that the newly developed robust clustering procedure is able to separate the geological-lithological units in hydrocarbon formations and provide additional information to standard well log analysis. It is also shown that the Cauchy-Steiner weighted cluster analysis is affected less by outliers, which allows a more efficient processing of poor-quality wireline logs and an improved evaluation of shale gas reservoirs.

Sign in / Sign up

Export Citation Format

Share Document