Angular structure of extragalactic radio sources at low frequencies
The low frequency VLBI of URAN network operated in the decameter range has been designed in Ukraine to study cosmic radio sources. The network consists of five radio telescopes making up of four interferometers with baselines range from 42 to 913 km with UTR-2 radio telescope operated as the main a...
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| Опубліковано в: : | Кинематика и физика небесных тел |
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| Дата: | 2005 |
| Автори: | , , , , , |
| Формат: | Стаття |
| Мова: | Англійська |
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Головна астрономічна обсерваторія НАН України
2005
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| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Цитувати: | Angular structure of extragalactic radio sources at low frequencies / A.I. Brazhenko, V.V. Koshovy, A.R. Lozynsky, A.V. Megn, S.L. Rashkovsky, V.A. Shepelev // Кинематика и физика небесных тел. — 2005. — Т. 21, № 5-додаток. — С. 47-50. — Бібліогр.: 2 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1859652376859770880 |
|---|---|
| author | Brazhenko, A.I. Koshovy, V.V. Lozynsky, A.R. Megn, A.V. Rashkovsky, S.L. Shepelev, V.A. |
| author_facet | Brazhenko, A.I. Koshovy, V.V. Lozynsky, A.R. Megn, A.V. Rashkovsky, S.L. Shepelev, V.A. |
| citation_txt | Angular structure of extragalactic radio sources at low frequencies / A.I. Brazhenko, V.V. Koshovy, A.R. Lozynsky, A.V. Megn, S.L. Rashkovsky, V.A. Shepelev // Кинематика и физика небесных тел. — 2005. — Т. 21, № 5-додаток. — С. 47-50. — Бібліогр.: 2 назв. — англ. |
| collection | DSpace DC |
| container_title | Кинематика и физика небесных тел |
| description | The low frequency VLBI of URAN network operated in the decameter range has been designed in Ukraine to study cosmic radio sources. The network consists of five radio telescopes making up of four interferometers with baselines range from 42 to 913 km with UTR-2 radio telescope operated as the main antenna of the interferometers. The angular resolution of the network amount to 1 arcsec at the highest frequency of the range, and its sensitivity is about 20 Jy. Regular observations of galactic and extragalactic radio sources are performed with the network. Some results of studies are presented here.
|
| first_indexed | 2025-12-07T13:35:15Z |
| format | Article |
| fulltext |
ANGULAR STRUCTURE OF EXTRAGALACTIC RADIO SOURCES
AT LOW FREQUENCIES
A. I. Brazhenko2, V. V. Koshovy3, A. R. Lozynsky3,
A. V. Megn1, S. L. Rashkovsky1, V. A. Shepelev1
1Institute of Radio Astronomy, NAS of Ukraine
4 Chervonopraporna Str., 61002 Kharkiv, Ukraine
e-mail: shep@ira.kharkov.ua
2Poltava Gravimetrical Observatory, Institute of Geophysics, NAS of Ukraine
27/29 Myasoedova Str., 36029 Poltava, Ukraine
3Institute of Physics and Mechanics, NAS of Ukraine
5 Naukova Str., 79053 Lviv, Ukraine
The low frequency VLBI of URAN network operated in the decameter range has been designed in
Ukraine to study cosmic radio sources. The network consists of five radio telescopes making up of
four interferometers with baselines range from 42 to 913 km with UTR-2 radio telescope operated as
the main antenna of the interferometers. The angular resolution of the network amount to 1 arcsec
at the highest frequency of the range, and its sensitivity is about 20 Jy. Regular observations of
galactic and extragalactic radio sources are performed with the network. Some results of studies
are presented here.
INTRODUCTION
Studies of angular structure of radio sources are very important for progress of modern astronomy and they are
carried out in a wide frequency range. Investigations at very low frequencies were restrained for a long time
because of specific peculiarities of the range. At the same time various mechanisms of radio wave emission,
absorption and scattering prove themselves particularly brightly at longer wavelengths. Being “linear” at high
frequencies, the synchrotron spectra of radio sources often reach maximum at lower frequencies and then their
radiation decreases smoothly. Such distortion of “linear” frequency characteristics can result from various
physical processes in a space plasma, such as a reabsorption in the source, a radiation absorption in the space
plasma both in the source and along the path of radio wave propagation, and also the effect of Razin–Tsytovich.
In some cases when the spectral characteristics of the source or its component are well determined at high as
well as at low frequencies, it is possible to evaluate such important characteristics of the space radio sources as
magnetic field power, electron temperature, emission measure, electron concentration and other.
Therefore, the determination of the angular structure at decameter wavelengths is very important to clarify
the mechanisms of generation and propagation of energy in the space. The low frequency VLBI of URAN
network with resolution up to 1 arcsec operated at decameter wavelengths has been used to study of the space
radio sources and influence the mechanisms of generation and propagation of radio waves at their angular
structure.
APPARATUS AND METHODS
The decameter wavelength VLBI of URAN network consists of five antenna arrays: the world biggest decame-
ter radio telescope UTR-2 and four smaller instruments, i.e., URAN, making up four interferometers with
baselines range from 42 to 913 km. The central instrument of the interferometers is the North–South arm
of the UTR-2 radio telescope. Multiplying the signals received by the UTR-2 with the signals of the URAN
antenna arrays forms the interference fringes. The UTR-2 antenna receives only single linearly polarized signals,
while the URAN antennas receives simultaneously two orthogonal linear polarizations. This enables us to correct
for the Faraday rotation effect in the Earth’s ionosphere and the cosmic plasma, which is very substantial at
decameter wavelengths. As a rule, the measurements are performed simultaneously at 25 and 20 MHz during
nighttime. The measurements produce the amplitude of the visibility functions in an interval up to ±3 hr on
either side of the source culmination. Its values are obtained at various hour angles in 20 min steps. When
processing the recordings, we determine the mean visibility function amplitudes for each 20-minute interval.
The data obtained for each hour angle are averaged over all days of the observations, and the mean-weighted
c© A. I. Brazhenko, V. V. Koshovy, A. R. Lozynsky, A. V. Megn, S. L. Rashkovsky, V. A. Shepelev, 2004
47
observational values are determined with their errors. We use a method described in [2] to calibrate the inter-
ferometer data, using special digital noise generators to imitate the response of the interferometer to a point
source. The calibration generators and local time scales are synchronized at all interferometer points using GPS
receivers.
As the URAN interferometers did not provide good coverage of the UV plane, and also due to an absence
of phase measurements, the most suitable method for deriving the angular structure of the source is the model
fitting. The image of a source is presented as a number of elliptical components with Gaussian brightness
distribution, position, intensity, position angle, magnitude and axis ratio of which are adjusted when the model
is fitted. The χ2-criterion is used to test a consistency of the models and the observational data.
It is well known that the determination of models using only the visibility function amplitudes can lead
to ambiguities in solutions and requires the large amounts of computing time, especially if the structure of
the object is complex and the UV plane coverage is not very complete. In addition, comparison of the resulting
models with higher frequency maps of the radio source in order to determine variations in the structure with
a frequency can be only qualitative. We use the following algorithm to simplify the model fitting procedure and
to determine quantitative properties of the frequency dependent variations in the source structure. At the first
stage we analyze a high frequency map of a studied source with an angular resolution comparable with the URAN
network resolution by calculation of hour angle dependencies adequate to tracks of the source at the UV-plane,
as it would be observed with URAN interferometers. Then, we match a simple model consisted of the elliptical
details with the same hour angle dependencies at the URAN’s baselines which adequately describes the real
source image. This model is used as the first approximation in the fitting procedure to determine low frequency
models of the source. The number of the solutions obtained are then analyzed by cluster analysis. The low
frequency image and high frequency one represented its models can be compared quantitatively and frequency
dependences of its parameters can be defined.
So, the analysis of the experimental hour angle dependences of the visibility function modulus for different
baselines and frequencies combined with the high frequency data allowed us to determine the models of bright-
ness distribution of investigated sources at the decameter wavelengths and their quantitative and qualitative
variations depending on frequency.
STRUCTURE OF THE SOURCES
The objects studied with VLBI of URAN network were galactic and extragalactic radio sources. At the decimeter
and centimeter wavelengths extragalactic radio sources usually possess compact component and a total size
of the source is of the order or less than resolving power at the shortest baseline of the URAN network.
At the decameter waves quasars and radio galaxies structures are shown to change essentially as compared
with their images obtained at higher frequencies. The most compact details disappear due to reabsorption,
other are enlarged in dimensions. The most interesting feature of some studied sources is extended components,
which are not observed at higher frequencies due to their steep spectrum and low surface brightness, and their
diameters noticeably exceed the full size of the source measured at higher frequencies. Often these extended
components contribute most of the decameter emission of the studied sources. Total spectra of the sources
and their components are measured at the decameter wavelengths too. The obtained dependencies allow us to
estimate physical conditions in the radio sources plasma and in the intergalactic medium.
Let us illustrate that by the example of the quasar 3C196. At centimeter and decimeter wavelengths
the angular structure of the source is well known owing to maps and interferometer data available at longer
wavelengths. The angular structure of the radio source is constant in the wide frequency range. At the fre-
quencies above 500 MHz the simplest model consisting of two details (the angular diameter of each is about
2.5 arcsec, separation is about 5.5 arcsec and the spectral indices are equal to 0.83) describes the source bright-
ness distribution with a satisfactory accuracy. Observations with the URAN interferometers have shown that
the source low frequency structure differs from known one at higher frequencies. Only one compact detail
remains and an extended steep spectrum component with angular diameter of about 25 arcsec at 50% level
of Gaussian brightness distribution has been detected. The latter has not been observed earlier in the source.
Further analysis of flux measurements and interferometer data in the wide frequency range allows us to deter-
mine the angular dimensions and spectra of all the components in the source brightness distribution model at
the frequencies range from 20 to 5000 MHz. In the general case, the source model contains three components:
two compact features and the extended one. Their spectra (Ss−w , Sn−e, Se) and source total spectrum (S0) are
shown in Fig. 1.
One can see that the model has two components at the frequencies above 500 MHz, because the extended
feature has steep spectrum. At the frequencies lower than 150 MHz radio emission of the compact detail
(Sn−e) decreases dramatically with a spectral index of 2.5 because of reabsorption in the component and
at the frequencies less than 45 MHz the model possess the single compact detail (Ss−w) and the extended
region of the emission (Se). Calculations based on this model were compared with the results of interferometer
48
Figure 1. Spectral properties of 3C 196; S0 (v) is the frequency dependence of the spectral flux density of the quasar
total emission; Ss.w(v) and Sn−e(v) are the spectra of the southwestern and northeastern components of the quasar;
and Se(v) is the frequency dependence of the spectral flux density of the emission of the quasar extended component
measurements and 3C196 maps obtained at higher frequencies. Part of the total source flux density, which could
not be explained by the integrated radiation of the compact components, generally coincides with the model
estimation of the extended component flux density up to 5000 MHz. Hence, the reasons of the angular structure
changes in the 3C196 are: a) reabsorption in the compact detail; b) existence of the extended component
with steep spectrum (spectral index is equal to 1.4); c) absorption in the interstellar medium which distorts
the spectrum of the component Ss−w at the frequencies less than 40 MHz. In the component with reabsorption
the magnetic field can be evaluated when spectrum and angular size are measured [1]. In this case, it is equal
to 170 μG. The determination of the magnetic field gives the possibility to estimate such valuable physical
parameters as energy of magnetic field and relativistic electrons, their density.
The studied 3C216 and 3C254 quasars have the same core-halo structure at the decameters with halo
dimensions about 20 arcsec and spectral index equal to 1.0. In the case of the 3C 154 and 3C380 quasars
the more extended ∼40 arcsec halos have been detected with the spectral indexes of 1.5 and 1.0, respectively.
Physical sizes of such extended components in studied quasars are range from 150 to 280 kpc. For those compact
components, which are visible at higher frequencies and disappear at lower ones the physical parameters of
space plasma have been evaluated if the physical mechanisms responsible for falling of their flux density were
determined.
In contrary to the previous radio sources two details with angular distance between them about 54 arcsec have
been found at decameter wavelengths in the quasar 3C 47. The location of the details are close to the position of
known high frequency extended components of the source but their angular sizes at the decameter wavelengths
are about 20 arcsec at 50% level of Gaussian brightness distribution versus 10–15 arcsec at zero level at decimeter
wavelengths maps.
Another class of studied extragalactic radio sources is represented by radio galaxies. At decimeter and
centimeter wavelengths these ones often are the FRII radio sources with two well separated radio lobes. The same
structure they have at decameters, however their components are enlarged essentially as in the case of the 3C 47
quasar. Such structural changes have been detected in the 3C111, 3C338, and 3C234 radio galaxies. Unlike
them the 3C295 radio galaxy has the angular brightness distribution at low frequencies similar to the quasars
with halo-like structure.
3C 295 is a compact steep spectrum radio source associated with a giant elliptical galaxy. At centimeter
wavelengths it consists of two radio lobes about 2 arcsec in size separated by 4 arcsec. The low frequency
model of the source consists of a single component with Gaussian brightness distribution which has dimensions
7.4×8.7 arcsec at 25 MHz and 9.1×10.6 arcsec at 20 MHz. They are shown in Fig. 2 with the correspondent
VLA maps at 8.7 GHz (the diameters of the decameter components are shown at the 50% intensity level).
An interesting feature of the source total spectrum is a sharp turnover at low frequencies that can be explained
by absorption of radiation in the source plasma. This feature allows us to determine such important charac-
teristics of the source plasma as electron temperature Te = 160 K, emission measure ME = 16.1 cm−6pc,
and, when the dimension of the radio galaxy at the decameter waves is about 70 kpc, electron concentration
Ne ≈ 0.015 cm−3.
49
Figure 2. The most probable one-component model of the brightness distribution of the 3C 295 radio galaxy in the de-
cameter wavelengths range (solid line is 25 MHz, dashed line is 20 MHz) using the VLA map obtained at the frequency
of 8711 MHz
From the galactic radio sources the supernova remnants Crab Nebula (3C144) and Cassiopeia A (3C 461)
have been studied in detail using URAN network. The flux density and the angular size of the Crab shell and its
well-known low frequency compact component coinciding with the pulsar were measured separately. The shell
parameters do not differ essentially from those at shorter wavelengths, but the angular diameter of the compact
detail is determined by scattering of the radio waves on the inhomogeneities of the interstellar medium.
We have shown that the structure of the Cassiopeia A does not differ from the high frequency image of
the source up to 25 MHz. The experimental hour angle dependency of source visibility function modulus
obtained in the ranges of ±240 min the source culmination was in a good agreement with calculated curve of
the VLA map of the source. However, distortion of the total spectrum is accompanied by a change of the radio
image at lower frequencies. The possible reason of both modifications is the influence of the interstellar medium
along the line of sight.
CONCLUSION
The results obtained allow us to affirm that the structure of the studied extragalactic radio sources changes at
the decameter wavelengths. The reason of the changes is as a rule a combination of various effects of radio wave
generation and propagation. The peculiarities of the brightness distribution in the range are:
1. The compact details (hot spots) in the radio galaxies and quasars are usually less prominent at the de-
cameter wavelengths due to self-absorption in the source and absorption in the interstellar medium. Their
angular diameters are equal to those at higher frequencies or slightly enlarged by scattering.
2. The lobes dimensions are enlarged.
3. The characteristic feature of a quasars structure at low frequencies is the extended components with
steep spectra producing the main part of the source fluxes at the decameter wavelengths. Their angular
diameters exceed the total size of the source as measured at higher frequencies. Such halos can be revealed
in some radio galaxies too.
The galactic supernova remnants studied using URAN network mainly possess the same features of their
structure as at higher frequencies. The most prominent changes in their low frequency structure are weak
increasing of their compact details size due to scattering. The form of the shells can be evaluated from the high
frequency maps of the source with respect to spectral indexes of source parts.
[1] Megn A. V., Braude S. Ya. Several studies of physical parameters determination of radio sources with reabsorp-
tion // Radio Phys. and Radio Astron.–1997.–2, N 1.–P. 16–18.
[2] Megn A. V., Rashkovskiy S. L., Shepelev V. A., et al. URAN system of decametric interferometers (II). Equipment
and observation methods // Radio Phys. and Radio Astron.–1998.–3, N 3.–P. 284–293.
50
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| id | nasplib_isofts_kiev_ua-123456789-79601 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 0233-7665 |
| language | English |
| last_indexed | 2025-12-07T13:35:15Z |
| publishDate | 2005 |
| publisher | Головна астрономічна обсерваторія НАН України |
| record_format | dspace |
| spelling | Brazhenko, A.I. Koshovy, V.V. Lozynsky, A.R. Megn, A.V. Rashkovsky, S.L. Shepelev, V.A. 2015-04-03T15:01:46Z 2015-04-03T15:01:46Z 2005 Angular structure of extragalactic radio sources at low frequencies / A.I. Brazhenko, V.V. Koshovy, A.R. Lozynsky, A.V. Megn, S.L. Rashkovsky, V.A. Shepelev // Кинематика и физика небесных тел. — 2005. — Т. 21, № 5-додаток. — С. 47-50. — Бібліогр.: 2 назв. — англ. 0233-7665 https://nasplib.isofts.kiev.ua/handle/123456789/79601 The low frequency VLBI of URAN network operated in the decameter range has been designed in Ukraine to study cosmic radio sources. The network consists of five radio telescopes making up of four interferometers with baselines range from 42 to 913 km with UTR-2 radio telescope operated as the main antenna of the interferometers. The angular resolution of the network amount to 1 arcsec at the highest frequency of the range, and its sensitivity is about 20 Jy. Regular observations of galactic and extragalactic radio sources are performed with the network. Some results of studies are presented here. en Головна астрономічна обсерваторія НАН України Кинематика и физика небесных тел MS1: Decameter Radioastronomy Angular structure of extragalactic radio sources at low frequencies Article published earlier |
| spellingShingle | Angular structure of extragalactic radio sources at low frequencies Brazhenko, A.I. Koshovy, V.V. Lozynsky, A.R. Megn, A.V. Rashkovsky, S.L. Shepelev, V.A. MS1: Decameter Radioastronomy |
| title | Angular structure of extragalactic radio sources at low frequencies |
| title_full | Angular structure of extragalactic radio sources at low frequencies |
| title_fullStr | Angular structure of extragalactic radio sources at low frequencies |
| title_full_unstemmed | Angular structure of extragalactic radio sources at low frequencies |
| title_short | Angular structure of extragalactic radio sources at low frequencies |
| title_sort | angular structure of extragalactic radio sources at low frequencies |
| topic | MS1: Decameter Radioastronomy |
| topic_facet | MS1: Decameter Radioastronomy |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/79601 |
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