共查询到20条相似文献,搜索用时 31 毫秒
1.
Ch. V. Sastry K. S. Dwarakanath R. K. Shevgaonkar 《Journal of Astrophysics and Astronomy》1981,2(3):339-347
We have observed the large supernova remnant Cygnus Loop at 34.5 MHz with the low frequency radio telescope at Gauribi-danur,
India. A radio map of the region with a resolution of 26 arcmin × 40 arcmin (α × δ) is presented. The integrated flux density
of the Cygnus Loop at this frequency is 1245 ± 195 Jy. The radio fluxes of different parts of the nebula at this frequency
were also measured and used to construct their spectra. It is found that the spectrum of the region associated with the optical
nebulosity NGC 6992/5 is not flat at low frequencies, and also exhibits a break at a frequency around 400 MHz. The spectrum
of the region associated with NGC 6960 also shows a break but around 1000 MHz, while the spectrum of the region associated
with NGC 6974 is straight in the entire frequency range 25 to 5000 MHz. The implication of these results on the basis of existing
theories of the origin of radio emission from supernova remnants is discussed. 相似文献
2.
Rakesh K. Malik Pradeep Gothoskar P. K. Manohran G. Swarup K. Subramanian V. Balasubramanian 《Journal of Astrophysics and Astronomy》1995,16(3-4):393-398
Jupiter flux at 327 MHz was monitored using the Ooty radio telescope from July 12th to July 29th during the collision of comet
Shoemaker-Levi 9 with Jupiter. Flux was found to increase steadily from July 17th to July 26th by ∼ 2–5 Jy, after which it
declined to its pre-event value. The comparison of 327 MHz observations with those at 840 MHz and 2240 MHz indicates that
the enhancement was mainly due to the increased synchrotron emission and the contribution of thermal emission was very small
at metric-decimetric frequencies. The enhancement in radio emission was found to be more at 840 MHz than at 327 or 2240 MHz.
The steepening of the spectrum between 327 and 840 MHz as well as between 2240 and 840 MHz was also noted. 相似文献
3.
Three supernova remnants (SNR) have been mapped in the galaxy M33 with the Very Large Array* (VLA) at 20 cm. The angular resolution
is ∼1.3 arcsec or ∼4 pc at a distance of 600 kpc and the rms noise is ∼0.04 mJy /beam. One of the radio sources shows evidence
for a shell structure with a size of ∼15 pc, confirming the SNR nature of this source. The second object is extended and may
well be a thick-shell SNR of size ∼12 pc. The third object is a small, presumably young SNR with a size of ∼4 pc. 相似文献
4.
We have reinvestigated the reported tendency for the extended radio structures associated with bright elliptical galaxies
to be oriented preferentially along the optical minor axes. It is found that such a tendency exists only for those galaxies
in which the compact radio cores coincident with their nuclei are quite prominent. If the galaxies are divided into two groups
according to whether their cores account for less than or greater than 10 per cent of the total flux density at 2.7 GHz, the
angle Φ (between the radio axis and the optical minor axis) appears to be uniformly distributed between 0‡ and 90‡ for the
former, but is nearly always < 30‡ for the latter group. One possible explanation is that the radio emission from compact
cores suffers thermal absorption by ionized gas that is distributed differently in the two groups. 相似文献
5.
K. Golap N. Udaya Shankar S. Sachdev R. Dodson Ch. V. Sastry 《Journal of Astrophysics and Astronomy》1998,19(1-2):35-53
A new, meter-wave radio telescope has been built in the north-east of Mauritius, an island in the Indian Ocean, at a latitude
of -20.14‡. The Mauritius Radio Telescope (MRT) is a Fourier Synthesis T-shaped array, consisting of a 2048 m long East-West
arm and an 880 m long South arm. In the East-West arm 1024 fixed helices are arranged in 32 groups and in the South arm 16
trolleys, with four helices on each, which move on a rail are used. A 512-channel digital complex correlation receiver is
used to measure the visibility function. At least 60 days of observing are required for obtaining the visibilities up to 880
m spacing. The Fourier transform of the calibrated visibilities produces a map of the area of the sky under observation with
a synthesized beam width 4′ × 4.6′ sec(δ + 20.14‡) at 151.5 MHz.
The primary objective of the telescope is to produce a sky survey in the declination range –70‡ to –10‡ with a point source
sensitivity of about 200 mJy (3a level). This will be the southern sky equivalent of the Cambridge 6C survey. In this paper
we describe the telescope, discuss the array design and the calibration techniques used, and present a map made using the
telescope. 相似文献
6.
M. Jamrozy C. Konar D. J. Saikia . Stawarz K.-H. Mack A. Siemiginowska 《Monthly notices of the Royal Astronomical Society》2007,378(2):581-593
We present radio observations at frequencies ranging from 240 to 8460 MHz of the radio galaxy 4C 29.30 (J0840+2949) using the Giant Metrewave Radio Telescope (GMRT), the Very Large Array (VLA) and the Effelsberg telescope. We report the existence of weak extended emission with an angular size of ∼520 arcsec (639 kpc) within which a compact edge-brightened double-lobed source with a size of 29 arcsec (36 kpc) is embedded. We determine the spectrum of the inner double from 240 to 8460 MHz and show that it has a single power-law spectrum with a spectral index of ∼0.8. Its spectral age is estimated to be ≲33 Myr. The extended diffuse emission has a steep spectrum with a spectral index of ∼1.3 and a break frequency ≲240 MHz. The spectral age is ≳200 Myr, suggesting that the extended diffuse emission is due to an earlier cycle of activity. We re-analyse archival X-ray data from Chandra and suggest that the X-ray emission from the hotspots consists of a mixture of non-thermal and thermal components, the latter being possibly due to gas which is shock heated by the jets from the host galaxy. 相似文献
7.
Using the recently completed Giant Meterwave Radio Telescope, we have detected the HI 21 cm-line absorption from the peculiar
galaxy C153 in the galaxy cluster Abell 2125. The HI absorption is at a redshift of 0.2533, with a peak optical depth of 0.36.
The full width at half minimum of the absorption line is 100 km s−1. The estimated column density of atomic Hydrogen is 0.7×1022(T
s
/100) cm−2. The HI absorption is redshifted by ∼400km s−1 compared to the [OIII] emission line from this system. We attribute this to an infalling cold gas or to an out-flowing ionised
gas, or to a combination of both as a consequence of tidal interactions of C153 with either a cluster galaxy or the cluster
potential. 相似文献
8.
J. W. Broderick J. J. Bryant R. W. Hunstead E. M. Sadler T. Murphy 《Monthly notices of the Royal Astronomical Society》2007,381(1):341-366
This paper introduces a new program to find high-redshift radio galaxies in the Southern hemisphere through ultrasteep spectrum (USS) selection. We define a sample of 234 USS radio sources with spectral indices α843 408 ≤−1.0 ( S ν ∝να ) and flux densities S 408 ≥ 200 mJy in a region of 0.35 sr, chosen by cross-correlating the revised 408 MHz Molonglo Reference Catalogue, the 843 MHz Sydney University Molonglo Sky Survey and the 1400 MHz NRAO VLA Sky Survey in the overlap region −40° < δ < −30°. We present Australia Telescope Compact Array (ATCA) high-resolution 1384 and 2368 MHz radio data for each source, which we use to analyse the morphological, spectral index and polarization properties of our sample. We find that 85 per cent of the sources have observed-frame spectral energy distributions that are straight over the frequency range 408–2368 MHz, and that, on average, sources with smaller angular sizes have slightly steeper spectral indices and lower fractional linear polarization. Fractional polarization is anticorrelated with flux density at both 1400 and 2368 MHz. We also use the ATCA data to determine observed-frame Faraday rotation measures for half of the sample. 相似文献
9.
T. J. Cornwell D. J. Saikia P. Shastri L. Feretti G. Giovannini P. Parma C. J. Salter 《Journal of Astrophysics and Astronomy》1986,7(2):119-129
We present VLA A-array observations at λ20, 6 and 2 cm and B-array observations at λ20 and 6 cm of the quasar B2 1320 + 299,
which has a very unusual radio structure. In addition to a component, A, coincident with the quasar, there are two lobes of
radio emission, B and C, on the same side of A. These are located at distances of -25 and 50 arcsec respectively from A. The
present observations show that A has a flat-spectrum component coincident with the quasar and a weak outer component at a
distance of-4 arcsec along PA - 100°. The morphology of B resembles a head-tail type of structure with its tail towards the
north-east. The magnetic field lines in component B appear to follow the bend in the tail. Component C exhibits some extension
towards the north-west. We discuss the possible nature of B2 1320 + 299 and suggest that while A appears to be an independent
source, the relation between B and C, if they are associated at all, is unclear. Deep optical observations are essential to
help clarify the situation. 相似文献
10.
We present diagrams depicting the expected inter-dependences of two key kinematical parameters of radio knots in the parsec-scale
jets of blazars, deduced from VLBI observations. The two parameters are the apparent speed (υ
app = cβ
app) and the effective Doppler boosting factor (δ
eff) of the relativistically moving radio knot. A novel aspect of these analytical computations of β-δ diagrams is that they are made for parsec-scale jets having a conical shape, with modest opening angles (ω up to 10°), in accord with the VLBI observations of the nuclei of the nearest radio galaxies. Another motivating factor is
the recent finding that consideration of a conical geometry can have important implications for the interpretation of a variety
of radio observations of blazar jets. In addition to uniform jet flows (i.e., those having a uniform bulk Lorentz factor,
Γ), computational results are also presented for stratified jets where an ultra-relativistic central spine along the jet axis
is surrounded by a slower moving sheath, possibly arising from a velocity shear. 相似文献
11.
R. G. Gratton G. Bonanno P. Bruno A. CalÍ R. U. Claudi R. Cosentino S. Desidera F. Diego G. Farisato G. Martorana M. Rebeschini S. Scuderi 《Experimental Astronomy》2001,12(2):107-143
SARG is a cross dispersed echelle spectrograph in operation since late spring 2000 at the Italian Telescopio Nazionale Galileo
(TNG) 3.5 m telescope, La Palma. SARG offers both single object and long slit (up to 26 arcsec) observing modes covering a
spectral range from λ = 0.37 up to1 μm, with resolution ranging from R = 29,000 up to R = 164,000. Cross dispersion is provided by means of a selection of four grisms; interference filters may be used for the
long slit mode (up to 26 arcsec). A dioptric camera images the cross dispersed spectra onto a mosaic of two 2048 × 4096 EEV
CCDs (pixel size: 13.5 μm) allowing complete spectral coverage at all resolving power for λ < 0.8 μm. In order to reach a
high wavelength calibration precision an iodine-absorbing cell is provided. A Distributed Active Temperature Control System
(DATCS) maintains constant the temperature of all spectrograph components at a preset value. Early results show that SARG
works according to original specifications in terms of wavelength coverage, efficiency (measured peak efficiency is about
13%),resolution (maximum resolution R = 164,000 using a 0.3 arcsec slit, R ∼144,000 using an image slicer), and stability (preliminary estimates of radial velocity accuracy is ∼3 m/s using the iodine
cell and ±150 m/s without the iodine cell).
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
12.
K. S. Dwarakanath Siddharth Malu Ruta Kale 《Journal of Astrophysics and Astronomy》2011,32(4):529-532
We have discovered a giant radio halo in the massive merging cluster MACSJ0417.5-1154. This cluster, at a redshift of 0.443,
is one of the most X-ray luminous galaxy cluster in the MAssive Cluster Survey (MACS) with an X-ray luminosity in the 0.1–2.4
keV band of 2.9×1045 erg s − 1. Recent observations from GMRT at 230 and 610 MHz have revealed a radio halo of ∼ 1.2 × 0.3 Mpc2 in extent. This halo is elongated along the North-West, similar to the morphology of the X-ray emission from Chandra. The
1400 MHz radio luminosity (L
r) of the halo is ∼2 × 1025 W Hz − 1, in good agreement with the value expected from the L
x − L
r correlation for cluster halos. 相似文献
13.
D. Moss N. Seymour I. M. McHardy T. Dwelly M. J. Page N. S. Loaring 《Monthly notices of the Royal Astronomical Society》2007,378(3):995-1006
We present the results of a deep 610-MHz survey of the 1 H XMM–Newton / Chandra survey area with the Giant Metre-wave Radio Telescope. The resulting maps have a resolution of ∼7 arcsec and an rms noise limit of 60 μJy. To a 5σ detection limit of 300 μJy, we detect 223 sources within a survey area of 64 arcmin in diameter. We compute the 610-MHz source counts and compare them to those measured at other radio wavelengths. The well-known flattening of the Euclidean-normalized 1.4-GHz source counts below ∼2 mJy, usually explained by a population of starburst galaxies undergoing luminosity evolution, is seen at 610 MHz. The 610-MHz source counts can be modelled by the same populations that explain the 1.4-GHz source counts, assuming a spectral index of −0.7 for the starburst galaxies and the steep spectrum active galactic nucleus (AGN) population. We find a similar dependence of luminosity evolution on redshift for the starburst galaxies at 610 MHz as is found at 1.4 GHz (i.e. ' Q '= 2.45+0.3 −0.4 ). 相似文献
14.
P. N. Best J. N. Arts H. J. A. Röttgering R. Rengelink M. H. Brookes J. Wall 《Monthly notices of the Royal Astronomical Society》2003,346(2):627-683
A new sample of radio sources, with the designated name CENSORS (A Combined EIS–NVSS Survey Of Radio Sources), has been defined by combining the National Radio Astronomy Observatory Very Large Array Sky Survey (NVSS) at 1.4 GHz with the ESO Imaging Survey (EIS) Patch D, a 3° by 2° region of sky centred at RA , Dec. −21°00'00' (J2000). New radio observations of 199 NVSS radio sources with NVSS flux densities S 1.4 GHz > 7.8 mJy are presented, and are compared with the EIS I -band imaging observations which reach a depth of I ∼ 23 ; optical identifications are obtained for over two-thirds of the ∼150 confirmed radio sources within the EIS field. The radio sources have a median linear size of 6 arcsec, consistent with the trend for lower flux density radio sources to be less extended. Other radio source properties, such as the lobe flux density ratios, are consistent with those of brighter radio source samples. From the optical information, 30–40 per cent of the sources are expected to lie at redshifts z ≳ 1.5 .
One of the key goals of this survey is to accurately determine the high-redshift evolution of the radio luminosity function. These radio sources are at the ideal flux density level to achieve this goal; at redshifts z ∼ 2 they have luminosities which are around the break of the luminosity function and so provide a much more accurate census of the radio source population at those redshifts than the existing studies of extreme, high radio power sources. Other survey goals include investigating the dual-population unification schemes for radio sources, studying the radio luminosity dependence of the evolution of radio source environments, and understanding the radio power dependence of the K – z relation for radio galaxies. 相似文献
One of the key goals of this survey is to accurately determine the high-redshift evolution of the radio luminosity function. These radio sources are at the ideal flux density level to achieve this goal; at redshifts z ∼ 2 they have luminosities which are around the break of the luminosity function and so provide a much more accurate census of the radio source population at those redshifts than the existing studies of extreme, high radio power sources. Other survey goals include investigating the dual-population unification schemes for radio sources, studying the radio luminosity dependence of the evolution of radio source environments, and understanding the radio power dependence of the K – z relation for radio galaxies. 相似文献
15.
R. A. Laing J. R. Canvin W. D. Cotton A. H. Bridle 《Monthly notices of the Royal Astronomical Society》2006,368(1):48-64
We present images of the jets in the nearby radio galaxy NGC 315 made with the Very Large Array at five frequencies between 1.365 and 5 GHz with resolutions between 1.5 and 45 arcsec. Within 15 arcsec of the nucleus, the spectral index of the jets is α= 0.61 . Further from the nucleus, the spectrum is flatter, with significant transverse structure. Between 15 and 70 arcsec from the nucleus, the spectral index varies from ≈0.55 on-axis to ≈0.44 at the edge. This spectral structure suggests a change of dominant particle acceleration mechanism with distance from the nucleus and the transverse gradient may be associated with shear in the jet velocity field. Further from the nucleus, the spectral index has a constant value of 0.47. We derive the distribution of Faraday rotation over the inner ±400 arcsec of the radio source and show that it has three components: a constant term, a linear gradient (both probably due to our Galaxy) and residual fluctuations at the level of 1–2 rad m−2 . These residual fluctuations are smaller in the brighter (approaching) jet, consistent with the idea that they are produced by magnetic fields in a halo of hot plasma that surrounds the radio source. We model this halo, deriving a core radius of ≈225 arcsec and constraining its central density and magnetic field strength. We also image the apparent magnetic field structure over the first ±200 arcsec from the nucleus. 相似文献
16.
A radio source with a spectral index of −2.2 between 0.08 and 1.425 GHz has been observed at the Very Large Array (VLA) at 1.425 GHz with resolution ∼ 3 arcsec. The projected linear length of the source is 56 kpc with an average projected distance of 42 kpc from the centroid of the rich southern cluster Abell 4038, assuming the source is in the cluster. The physical parameters of the source include a high minimum-energy field ( B me ) of 38 μG, which is unusual for a source of low surface brightness and relaxed appearance, but is explained by its unusually steep spectrum. Although its radio morphology has some characteristics of a narrow-angle-tail source (NAT), the absence of an identified host galaxy ( m R ≥ 23.0) makes it unlikely that the source is a working radio galaxy. The relic is probably the remains of an FR II radio galaxy that was once energized by a particular bright cluster elliptical now 18 kpc to its east. The density of the intracluster gas has been sufficient to confine the source and preserve its morphology, permitting the source to age and its spectrum to steepen through synchrotron and inverse Compton energy losses. 相似文献
17.
We have observed the region of the Coma cluster at 34.5 MHz with a resolution of 26 arcmin × 40 arcmin. A map of the diffuse
halo (Coma C) is presented. The size of the halo is found to be 54 arcmin × 30 arcmin. The position angle is 50° ± 10° and
the integrated flux is 60 ± 11 Jy.
We have also found an extended source to the south of Coma A. The measured half-power widths of this source are 30 arcmin
× 40 arcmin. The position angle is 135° and the integrated flux is ~ 15 Jy at 34.5 MHz. The spectral index in the frequency
range 408 to 34.5 MHz is -1.0. It is suggested that this source also belongs to the Coma cluster. 相似文献
18.
The Eridanus galaxies follow the well-known radio—FIR correlation. The majority (70%) of these galaxies have their star formation
rates below that of the Milky Way. The galaxies that have a significant excess of radio emission are identified as low luminosity
AGNs based on their radio morphologies obtained from the GMRT observations. There are no powerful AGNs (L
20cm > 1023 W Hz−1) in the group. The two most far-infrared and radio luminous galaxies in the group have optical and HI morphologies suggestive
of recent tidal interactions. The Eridanus group also has two far-infrared luminous but radio-deficient galaxies. It is believed
that these galaxies are observed within a few Myr of the onset of an intense star formation episode after being quiescent
for at least a 100 Myr. The upper end of the radio luminosity distribution of the Eridanus galaxies (L
20cm ∼ 1022 W Hz−1) is consistent with that of the field galaxies, other groups, and late-type galaxies in nearby clusters. 相似文献
19.
N. Udaya Shankar K. Golap S. Sachdev R. Dodson M. Katwaroo Ch.V. Sastry 《Astrophysics and Space Science》2002,282(1):15-28
The Mauritius Radio Telescope (MRT) has been built with the main objective of surveying the southern sky at meter wavelengths.
MRT is a Fourier synthesis, T-shaped non-coplanar array. It consists of a2048 m long East-West arm with 1024 fixed helices
and a 880 m long South arm with 15 trolleys. Each trolley has four helices. A 512 channel, 2-bit 3-level complex correlation
receiver is used to process the data from the EW and S group outputs. At least 60 days of observing are required for obtaining
the Fourier components of the brightness distribution of the sky required to complete the survey. The MRT survey will be one
of the most extensive survey at low frequencies providing a moderately deep radio catalog reaching a source density of about
2 × 104 sr-1 over most of the sky south of δ=-10°with an angular resolution of 4' × 4.6' sec (δ) and a limiting flux density of 200 mJy (3 σ level) at 151 MHz. This paper
will describe the telescope, the observations carried out so far, challenges of imaging with the data acquired over a period
exceeding four years with a non-coplanar array, and summarises the results obtained so far.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
20.
V. K. Kapahi 《Journal of Astrophysics and Astronomy》1995,16(1):1-36
Radio maps at 5 GHz with an angular resolution of 1 to 2 arcsec and a dynamic range ≳ 200:1 are presented for a sample of
45 radio quasars at redshifts between 0.2 and 1.5. The sources were imaged from observations made with the Very Large Array
with the aim of investigating the epoch dependence of misalignments and asymmetries in their extended radio structure. Maps
of some of the larger radio sources are presented also at a frequency of 1.5 GHz with a typical angular resolution of ≈ 4
arcsec. The radio structure of most of the quasars reported here has been delineated in considerably greater detail than available
in the literature. 相似文献