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

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.     

A synthesis map of the sky at 34.5 MHz

This paper describes a wide-field survey made at 34.5 MHz using GEETEE,1 the low frequency telescope at Gauribidanur (latitude 13°36′12′′N). This telescope was used in the transit mode and by per forming 1-D synthesis along the north-south direction the entire observable sky was mapped in a single day. This minimized the problems that hinder wide-field low-frequency mapping. This survey covers the declination range of-50° to + 70° (- 33° to +61° without aliasing) and the complete 24 hours of right ascension. The synthesized beam has a resolution of 26′ x 42′ sec (δ- 14°. 1). The sensitivity of the survey is 5 Jy/beam (1σ). Special care has been taken to ensure that the antenna responds to all angular scale structures and is suitable for studies of both point sources and extended objects This telescope is jointly operated by the Indian Institute of Astrophysics, Bangalore and the Roman Research Institute, Bangalore.     

The brightness of the galactic radio loops at 1420 MHz: Some indications for the existence of Loops V and VI

In this article we use 1420 MHz data to demonstrate the likely reality of Galactic radio Loops V and VI. We further estimate distances and spectral indices for both these and the four main radio loops. In the cases of Loops I–IV, radio spectral indices are calculated from the mean brightnesses at 1420 and 820/404 MHz. The spectral indices of Loops V and VI are obtained from T –T plots between 1420 and 408 MHz. Using the supernova remnant (SNR) hypothesis for the origin of radio loops, distances are calculated from the surface brightnesses and the angular diameters at 1420 MHz. We also study how results for brightnesses and distances of radio loops agree with current theories of SNR evolution. For this purpose, the ambient density and initial explosion energy of the loops are discussed. We also discuss applications of different Σ–D relations. The results obtained confirm a non‐thermal origin and nearby locations for the Galactic radio loops. Therefore, we have indications that they are very old SNRs that evolve in low ambient densities, with high initial explosion energies. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Monoceros radio loop: Temperature,brightness, spectral index,and distance

In this paper we estimated the temperatures and brightnesses of the Monoceros radio loop at 1420, 820 and 408 MHz. The linear spectrum is estimated for mean temperatures versus frequency between 1420, 820 and 408 MHz. The spectral index of Monoceros loop is also obtained. The brightness temperatures and surface brightnesses of the loop are computed using data taken from radio‐continuum surveys at the three frequencies. The spectral index of the loop is also obtained from T‐T plots between 1420–820, 1420–408, and 820–408 MHz. The obtained results confirm non‐thermal origin of the Monoceros radio loop (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Σ–D relation for supernova remnants and its dependence on the density of the interstellar medium

During the last couple of decades of work on the  Σ– D   (radio surface brightness to diameter) relation for supernova remnants (SNRs), it has been generally accepted that no single  Σ– D   relation can be constructed for all SNRs. However, it may still be possible to construct the relations for some classes of SNRs. In our previous paper we analysed  Σ– D   relation(s) for remnants in the dense environments of molecular clouds. The aim of this paper is to examine, in the same context, a class of oxygen-rich SNRs, and to extend the analysis to remnants evolving in lower-density interstellar media, namely Balmer-dominated SNRs. We have obtained good relations with certain similarities to our previous findings – similarities that emphasize, again, the role of ambient density in the evolution of SNRs.     

The Rhodes/HartRAO 2326-MHz radio continuum survey

We have made scanning observations with the HartRAO 26-m radio telescope to obtain a pencil-beam map of 67 per cent of the sky at 2326 MHz. This is the highest resolution and highest frequency radio continuum map of this type made of such a large area of sky.   In this paper we describe the observations and data reduction procedures used to produce the survey. The resulting map has an angular resolution (HPBW) of 20 arcmin, and the rms pointing accuracy is 1.2 arcmin. The rms noise fluctuations are less than 30-mK T _FB over the whole map. We estimate that the uncertainty in the temperature scale is less than 5 per cent, and that the error in the absolute zero level is better than 80-mK T _FB in any direction.   High-contrast half-tone images of the data with a model of the diffuse galactic background subtracted are presented. These images show many complex emission structures up to and beyond 50° latitude, and illustrate the quality of the data. Extracts from the survey data are available via FTP by arrangement with the authors.     

G353.9−2.0, a new Galactic supernova remnant identified from the NRAO Very Large Array Sky Survey

G353.9−2.0 is a likely Galactic supernova remnant (SNR) identified from a visual inspection of the National Radio Astronomy Observations (NRAO) Very Large Array (VLA) Sky Survey (NVSS) observations in the Galactic plane. It shows a shell structure, about 13 arcmin in diameter, with a central extended source. VLA observations of G353.9−2.0 at 1.4 GHz, with a resolution of ≈40 arcsec, and a significantly better sensitivity than the NVSS observations, are presented here, together with observations at 327 MHz and observations of the central source at 8.4 GHz. These new observations and existing results from the literature are discussed. G353.9−2.0 is confirmed as a SNR, and the central source as a likely extragalactic double source.     

VLA observations of three M 33 supernova remnants at 20 cm

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.     

Re-identification of G35.6−0.4 as a supernova remnant

G35.6−0.4 is an extended radio source in the Galactic plane which has previously been identified as either a supernova remnant or an H  ii region. Observations from the Very Large Array Galactic Plane Survey at 1.4 GHz with a resolution of 1 arcmin allow the extent of G35.6−0.4 to be defined for the first time. Comparison with other radio survey observations show that this source has a non-thermal spectral index, with   S ∝ν^{−0.47±0.07}  . G35.6−0.4 does not have obvious associated infrared emission, so it is identified as a Galactic supernova remnant, not an H  ii region. It is  ≈15 × 11 arcmin²  in extent, showing partial limb brightening.     

VLA observations of ‘Compact’ sources in CTB 80

High resolution (3 arcsec-4 arcsec) maps of three compact sources in CTB 80, observed with the Very Large Array at 6 and 20 cm wavelengths are presented. The central core consists of a limb-brightened shell (of diameter 30 arcsec) superimposed on a diffuse emission of size 75 arcsec x 45 arcsec. The compact sources 1949 + 324 and 1952 + 332 located near the tips of the southwest and northeast ridges are rather remarkable and possibilities for these to be fragments ejected by the SN are examined. 1949 + 324 is fully resolved by the 4 arcsec beam and shows structure with multiple components oriented perpendicular to the ridge. 1950 + 326 is a background radio source.     

VLBI observations of supernova remnants in Messier 82

We have used the European VLBI Network (EVN) at 18 cm to study five of the more compact radio sources in the starburst galaxy M82. The angular resolution of the observations is 15 mas, corresponding to 0.2 pc at the distance of M82. The observations reveal shells ranging in diameter from 40 to 90 mas (0.6 to 1.4 pc), although the strongest source (41.95+575) is only marginally resolved by these measurements (∼20×10 mas²).
We have found clear evidence for expansion in one of the shell sources (43.31+592) by re-analysing, in wide-field mode, EVN data taken in 1986. Between 1986 and 1997 this source has increased its diameter by 13.6±2 mas, corresponding to an average expansion velocity of 9850±1500 km s⁻¹. If we assume that the remnant is in free expansion, this is consistent with a supernova event in the early 1960s. Hence this remnant is almost certainly younger than the strongest, most compact source (41.95+575) which was known to be present in the 1960s. 41.95+575 shows no clear evidence for expansion (<4000 km s⁻¹), consistent with a greater age; this is further evidence of its anomalous status. Comparison of the EVN images with earlier MERLIN data is also consistent with expansion in at least two more of the sources. We discuss the flux density variability of the compact sources in M82 and conclude that, with the exception of 41.95+575 and two transient sources, there is little evidence for significant changes in flux density of most of the remnants since the early 1980s.