Introduction

Radio emission from Supernova remnants (SNRs) is due to the synchrotron radiation mechanism which has a non-thermal power law dependence on frequency with a negative spectral index $\alpha$ ( $S \propto\nu^\alpha$). This makes the emission progressively stronger at lower frequencies (see Section 1.1). On the other hand, thermal emission from typical HII regions, the other class of objects with extended radio emission in the Galaxy, has a flat spectrum above $\approx 1$ GHz. Below this frequency, the optical depth is much greater than 1 and the spectrum turns over with a spectral index of 2. The low frequency continuum spectra of Galactic objects are therefore frequently used to distinguish between thermal and non-thermal sources of emission (Kassim & Weiler1990; Kassim et al.1989a; Subrahmanyan & Goss1995). The other key observational evidence used to identify SNRs is their morphology. Detection of an extended non-thermal source with no thermal emission has been the criterion used to identify Galactic sources as SNRs.

Kassim (Kassim1989), in a study of non-thermal emission from known Supernova remnants in the Galaxy, demonstrated that the radio emission from most SNRs gets absorbed at frequencies below about 100 MHz. He observed a number of SNRs, widely distributed in the inner Galaxy and showed that about two thirds of all SNRs show this spectral turn-over due to free-free absorption by the intervening ISM; this indicates the presence of an extended low density warm ionized medium (ELDWIM) in the Galaxy. Since then, many more SNRs has been imaged at frequencies between 1 GHz and 327 MHz (and in some cases even below 327 MHz). A collection of the radio spectra of SNRs have been compiled by Trushkin (1998)^8.1. His catalogue of SNR spectra also gives the spectral indices above and below the turn-over frequency and the frequency at which the spectrum turns over. Spectra of about 35% of the SNRs from this catalogue show a low frequency turn-over. The measurement of this turn-over frequency for SNRs distributed throughout the Galaxy can be used to indirectly measure the distribution of this phase of the ISM, which is otherwise inaccessible to direct observations.

Imaging of HII regions at frequencies below $\sim1$ GHz, together with the electron temperature derived from RRL observations have been used to deduce improved estimates of the physical properties of HII regions (Shaver1969; Subrahmanyan1992a; Subrahmanyan1992b; Kassim et al.1989b). Low frequency continuum mapping in the Galactic plane is therefore important from the point of view of identification of SNRs, separating thermal emission from non-thermal emission and for studying the intervening ISM.

Radio emission from SNRs is typically extended, often with a low surface brightness. Telescopes which are sensitive to extended emission as well as to total flux, are required to image such objects. Single dish observations are sensitive to emission at all scales in the field. While interferometric telescopes typically provide much higher resolution, they are insensitive to scales larger than those corresponding to the smallest projected baseline. Single dish telescopes, at high frequencies, provide the sensitivity and moderate resolution required for such work. Till recently many observations were therefore done using single dish instruments. However, single dish observations pose a separate problem. While single-dish observations do not discriminate against extended emission, they are more prone to large scale confusing emission, which is abundantly present in the Galactic plane. Low frequency single dish observations do not provide the required resolution to resolve the extended nature of the SNR emission. For these reasons, most Galactic SNRs have been observed using single dishes at high frequencies ($>$ 1 GHz) which provide reasonable resolution (better than $1{^\prime}$). Higher resolution interferometric observations of some Galactic SNRs have also been done. However, such observations are often restricted to observing relatively small sized SNRs. Imaging at low frequencies using interferometers is also relatively difficult due to problems arising from higher level of RFI, higher phase noise at low frequencies (due to various reasons ranging from cross talk to ionospheric phase corruption), non-co-planarity of array requiring much more complex software and higher computing power. Hence, even interferometric observations have been typically done at frequencies $>1$ GHz.

However, observations at high frequencies have another source of emission, namely the thermal emission from HII regions which, in some cases, is fairly strong. The non-thermal emission from SNRs becomes progressively diminished at higher frequencies, while the thermal emission remain fairly constant as a function of frequency at frequencies higher than about 1 GHz. High frequency observations therefore miss extended low surface brightness emission from SNRs. Such observations alone can also wrongly classify sources which have thermal and non-thermal components in the radiation (for example, due to line-of-sight super-position of thermal and non-thermal emission). Thus, while high frequency telescopes provide the required resolution and sensitivity, they alone are not adequate for the identification of Galactic objects as SNRs.

Recently, synthesis telescopes like the GMRT (Bhatnagar2000) and MOST (Gray1994a) have been used to image parts of the Galactic plane. The MOST survey, at a resolution of $\approx 90\times 43.5 {\mathrm{arcsec^2}}$ and is sensitive to angular scales up to $30'$, identified seventeen candidate SNRs. Most of these object are large ($>5-10'$). Many of the fields however suffer from the grating response due to nearby strong sources and from confusing thermal emission from nearby strong HII regions.

Duncan et al. (1997b) used the Parkes 64-m single dish telescope to image a set of large, high latitude SNRs at 2.4 GHz. These observations identified about a dozen new large SNRs ($>15'$) south of $-25{^\circ}$ declination. Again, besides the lower angular resolution of the single dish, these observations suffered from the problem of separating extended diffused non-thermal emission from thermal emission in and around the Galactic plane using these high frequency.

The GMRT at 327 MHz provides a resolution of up to $\sim 20$ arcsec and is sensitive to spatial scales up to 30 arcmin. The relatively smaller field-of-view (half-power beam width of $\approx 1^\circ.4$) of the GMRT also provides an advantage in terms of attenuating confusing emission from near by strong sources. At this frequency, the thermal emission from typical HII regions is severely diminished, while the emission from SNRs remain relatively strong. The higher resolution and sensitivity at these low frequencies, make the GMRT an ideal instrument for studies of Galactic SNRs. Compared to the resolutions of about $1{^\prime}$ and $1.5{^\prime}$ and RMS noise of $\approx 20$ mJy/beam and $35$ mJy/beam in the survey by Duncan et al. (1997b) and Gray (1994a) respectively, the GMRT at 327 MHz provides a factor of 2 lower RMS noise and, depending upon the largest baselines used, up to 2-5 times better resolution.