Candidate Galactic SNRs

The GMRT was finally used at 327-MHz to map a sample of seven fields selected from the surveys by Gray (1994a) and Duncan et al. (1997b) containing candidate SNRs. Most of these SNRs are large in size and their morphology is easily discernible at resolution of about an arcmin. These observations constitute the most sensitive, highest resolution observations for these and other sources which happen to be within the field of view. Partly due to system problems and ionospheric phase corruption, and partly due to the well known problem of deconvolution of extended sources, these extended sources tend to break up at higher resolution. Higher resolution images were therefore made and used only for small angular size objects.

Some of these objects are already listed in the SNR catalogue (Green2000) based only on the morphological evidence from single high frequency observations. These GMRT observations establish the non-thermal nature of the emission and typical SNR morphology for six of these candidate SNRs and confirms them as Galactic SNRs. G001.4$-$0.0 is detected as a clear partial arc of emission, coincident with a faint arc of emission seen in the OH (1720 MHz) emission (Yusef-Zadeh et al.1999). An OH (1720 MHz) spot has also been previously detected towards this direction and coincides with the arc seen in the radio continuum (Yusef-Zadeh et al.1999). Recently it has been argued that the OH (1720 MHz) emission is a good tracer of the interaction between the shock front driven by the SNR blast wave and molecular clouds (Frail et al.1994). The OH (1720 MHz) maser emission is distinguished from the OH maser emission at 1665, 1667 and 1612 MHz by the former being positionally and kinematically associated with SNRs while the later is associated with HII regions. OH (1720 MHz) emission associated with SNRs is believed to be due to the blast wave driving a shock in a denser molecular cloud. OH masers at 1665, 1667 and 1612 MHz cannot be produced under the same physical conditions and the absence of these lines in observations which detect the OH (1720 MHz) line favors this interpretation. The morphology of G001.4$-$0.0 in OH (1720 MHz) emission and radio continuum suggests that the arc of emission corresponds to the shock front. Absence of emission on the eastern side may be explained by the absence of such a cloud on that side. G003.8$+$0.3 is clearly visible as an incomplete arc of emission, embedded within a ring of thermal emission seen in the IRAS 60$\mu$m image. Its morphology is also clearly deciphered in the radio continuum image from the 11 cm 100-m Effelsberg survey (Reich et al.1990). G004.8$+$6.2 is seen as an almost complete shell of emission just east of the shell-type Kepler's SNR. This SNR is also present in the field of view of a VLA observation and is also detected, although at a very low level, in the NVSS image of this region. The morphology and the measured integrated flux from the GMRT and VLA observations are in good agreement. G356.2$+$4.5 is a faint partial shell. This SNR is also visible in the NVSS image of this region, but the images at 327 and 1400 MHz suffer from the problem of missing flux, which did not allow the determination of the spectral index. However, the morphology is strongly in favor of this being an SNR. G356.3$-$1.5 is a barrel shaped SNR. The 834-MHz image of this source was severely affected by the grating response of a nearby strong source. However, the barrel morphology, with significant emission projected between the two rims, is clearly seen in the 327-MHz image. G004.2$-$0.0 is detected as an unresolved source. The spectra measured between 327 and 843 MHz for this source is consistent with it being a flat spectrum source. Presence of significant thermal emission in the IRAS 60$\mu$m image, however, suggests that this may not be an SNR but a flat spectrum thermal source.