R. K. Shevgaonkar
Articles written in Journal of Astrophysics and Astronomy
Volume 2 Issue 3 September 1981 pp 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.
Volume 3 Issue 2 June 1982 pp 207-216
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.
Volume 4 Issue 1 March 1983 pp 47-51
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.
Volume 7 Issue 4 December 1986 pp 275-292
Minimum-relative-entropy method (MREM) has been presented as a solution to the missing short-baseline problem in the synthesis observations. It is shown that a measure of distance between the prior model and the image in the plane of pixel brightness is an adequate measure of relative entropy. The method has been further extended for polarization observations and the potential of the method against the conventional MEM has been demonstrated by simulated examples.
Volume 43, 2022
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