• Ch. V. Sastry

      Articles written in Journal of Astrophysics and Astronomy

    • Pulsating radio emission at decametre wavelengths from the sun

      Ch. V. Sastry V. Krishan K. R. Subramanian

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      Observations on the pulsation pattern in the time profile of short duration solar radio bursts at decametre wavelengths are presented. The pulsations are found to be present predominantly in the saturation phase of the burst. A tentative physical model based on the non-linear development of the waves interacting in a turbulent medium is invoked to explain the origin of the pulsations.

    • The structure of the Cygnus loop at 34.5 MHz

      Ch. V. Sastry K. S. Dwarakanath R. K. Shevgaonkar

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      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.

    • Time structure of solar decametre type III radio bursts

      G. Thejappa Ch. V. Sastry

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      The time structure of solar radio decametre Type III bursts occurring during the periods of enhanced emission is investigated. It is found that the time profiles can take a variety of forms of which three distinct types are the following: (1) profiles where the intensity rises to a small but steady value before the onset of the main burst, (2) the intensity of the main burst reduces to a finite level and remains steady before it decays to the base level, (3) the steady state is present during the rise as well as the decay phase of the main burst.

      It is shown that these profiles are not due to random superposition of bursts with varying amplitudes. They are also probably not manifestations of fundamental-harmonic pairs. Some of the observed time profiles can be due to superposition ot bursts caused by ordered electron beams ejected with a constant time delay at the base of the corona.

    • Observations of the supernova remnants HB 9 and 1C 443 at 34.5MHz

      K. S. Dwarakanath R. K. Shevgaonkar Ch. V. Sastry

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      We have observed the extended supernova remnants HB 9 (G 160.5 + 2.8) and IC 443 (G 189.1 + 2.9) at 34.5 MHz with a resolution of 26 arcmin × 40 arcmin. A map of HB 9 is presented. The integrated flux density of HB 9 at 34.5 MHz is 750 ± 150 Jy. The spectral index in the frequency range from 34.5 MHz to 2700 MHz is found to be constant (- 0.58 ± 0.06) without any spectral break such as was reported earlier by Willis (1973). There is no significant variation of the spectral index across the remnant. The integrated flux density of IC 443 at 34.5 MHz is 440 ± 88 Jy. The spectral index in the frequency range from 20 MHz to 10700 MHz is - 0.36 ± 0.04. The reduction in flux at very low frequencies (10 MHz) is attributable to free-free absorption in the interstellar medium and/or in the H II region S 249.

    • Diffuse radio emission from the Coma cluster of galaxies at decametre wavelengths

      Ch. V. Sastry R. K. Shevgaonkar

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      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.

    • Observations and interpretation of solar decametric absorption bursts

      N. Gopalswamy G. Thejappa Ch. V. Sastry

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      The observations of intensity reductions or absorption bursts in the solar decametric radio-continuum are reported. The reductions are interpreted as the absorption of continuum radiation by a shock-generated ion-sound turbulence present in the layer above the continuum level. The duration of the absorption is attributed to the lifetime of the ion-sound turbulence while the depth of absorption is determined by the level of Langmuir waves generated as a result of absorption.

    • The low-frequency radio spectrum of the continuum emission from the undisturbed sun

      K. R. Subramanian Ch. V. Sastry

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      The low-frequency radio spectrum of the continuum emission from the undisturbed Sun is determined for 24 days during the period 1985 May-September. It is found that the spectral index varied from + 1.6 to +3.6 during this period. It is suggested that the large positive spectral indices are due to the existence of temperature gradients in the outer corona.

    • A low frequency radio telescope at mauritius for a southern sky survey

      K. Golap N. Udaya Shankar S. Sachdev R. Dodson Ch. V. Sastry

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      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.

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      Posted on January 27, 2016

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