Continuum flux densities of known SNRs

With a $1{^\circ}.4$ field of view of GMRT at 327 MHz, there were other known SNRs in some of the fields. Many of them have been mapped at 327 MHz using the VLA, but there were some for which these GMRT images constitute the first 327 MHz images at a resolution of $\sim1$ arcmin or less, with an, RMS noise of 3-15 mJy/beam. This section presents the results and discussion for these known SNRs.

G003.7$-$0.2

Figure 5.12: GMRT 327-MHz image of G003.7$-$0.2. The resolution in the image is $\sim 20\times 10 {\mathrm{arcsec^2}}$ and the RMS noise $\sim 5$ mJy/beam. The extended emission ``breaks up'' possibly due to a combination of uncorrected phase errors and the well known problem of deconvolution of extended sources using the CLEAN algorithm.

This is a classic barrel shaped SNR, first reported by Gray (1994a) where it was classified as a SNR based on the morphology alone. Gaensler (1999) mapped this at 1.4 GHz using the VLA in CnD and BnC array configurations and established the non-thermal nature of emission. This higher quality image at 1.4 GHz, and the 843 MHz image by Gray are in good agreement with the GMRT image at 327 MHz. The resolution in the GMRT image, shown in Fig. 5.12, is $20\times10 {\mathrm{arcsec^2}}$, almost the same as that in the 1.4 GHz (Gaensler1999). In all these images, this SNR satisfies the criterion used by Gaensler (1998) to classify it as a barrel shaped SNR. It has a clear ridge where there is practically no emission, even at the lowest frequency, which defines the axis of symmetry. It also has a clear point of maximum emission, almost perpendicular to the axis of symmetry.

The flux density in the GMRT 327-MHz image was found to be $4.5\pm0.3$ Jy. The flux density at 1.4 GHz is $1.7\pm0.1$ Jy and that at 843 MHz is 2.4 Jy, giving a spectral index of $-0.66\pm0.04$. The spectral index listed in Greens catalogue (Green2000)^8.2 is $-0.65$.

G355.9$-$2.5

Figure 5.13: 327-MHz image of G355.9-2.5 using GMRT. The RMS noise in the image is about 10 mJy/beam and the resolution is $1.6{^\prime}\times 0.8{^\prime}$. This SNR lies on the very edge of the GMRT primary beam. The incomplete nature of the shell is apparent. However, there is an indication of weak emission in the east where the shell is incomplete. The vertical ridge passing through the middle is an artifact of combining different facets along the celestial sphere in a polyhedron imaging algorithm.

This SNR, first identified by Clark et al. (1973) using 408 MHz and 5 GHz observations, is listed as a ``distorted shell, brightest towards the south-east'' in Green's catalogue (Green2000) with a spectral index of $-0.5$ and a size of 13 arcmin. The flux density at 408 MHz (resolution of 3 arcmin) is reported to be 12.3 Jy while at 5 GHz (resolution of 4 arcmin) the value is 3.4 Jy. The highest resolution image made by Dubner et al. (1993) using VLA at 1.4 GHz confirms this general morphology. Polarization observations by these authors at 1465 MHz indicate significant linearly polarized intensity with a mean fractional polarization of 6% with the brightest region also most strongly polarized. Weaker emission, towards the east appears to complete the shell.

Gray (1994b) published a MEM deconvolved image at 843 MHz and measured a flux density of 6.5 Jy, a good 24% lower than expected from a source of spectral index of $-0.5$. The flux density of 5.0 Jy at 1465 MHz is also 21% lower than expected. The authors attribute this discrepancy to the missing extended flux in interferometric images.

The GMRT image at 332 MHz is shown in Fig. 5.3.2. In this pointing, this source lies at half power point of the primary beam. The smallest uv-spacing from which the visibility was reliably measured was about $90\lambda$, corresponding to a largest angular scale of $\sim30$ arcmin. The total angular size of this source is about 13 arcmin. Hence this source is not affected by the missing flux problem of interferometric images. A flux density of $14.2\pm0.3$ Jy was measured from the primary beam corrected image for this pointing, in close agreement with the expected flux density of $13.9$ Jy at this frequency, corresponding to a spectral index of $-0.5$. The weak emission seen in the 1.4 GHz image is more clearly seen at 327 MHz and indeed there is a more complete shell than was seen at higher frequencies.

The linear features noted by Gray in the 843-MHz image are also visible in this image. Although it is commented that these features are not seen in the 1465-MHz image (Dubner et al.1993), they are clearly seen in the polarized intensity and although weak, can also be identified in the total intensity at this frequency. The spectral index of these features is probably then not very different from the rest of the source; there may be the usual filaments seen in many other SNRs.

Kepler's SNR (G004.5$+$6.8)

Figure 5.14: 327-MHz image of the Kepler's SNR (G004.5$+$6.8). The RMS noise in the image is about 25 mJy/beam and the resolution is $28 \times 24 {\mathrm{arcsec^2}}$. The edge brightened morphology with the brighter norther rim (away from the Galactic plane) is clearly visible. The ridge running right across the SNR with small protrusions on either size is also clearly visible.

The field of G004.8$+$6.2 contains the well known shell-type Kepler's SNR of angular size 3 arcmin. A higher resolution image of this field was made, for the purpose of imaging this SNR at 327 MHz at the highest possible resolution. This higher resolution, primary beam corrected image is shown in Fig. 5.14. This is the only map known to me at 327 MHz at a resolution comparable to that at higher frequencies. The edge brightened shell towards the north is clearly visible. The morphology in this 327-MHz image is similar to that seen in the 1.4 and 5 GHz VLA images (Matsui et al.1984). The distance to this SNR is estimated to be between 4.8 and 6.4 kpc from HI absorption and emission profiles (Reynoso & Goss1999). The ridge running right across the SNR with small protrusions on both sides is also visible in the image. The integrated flux after primary beam correction is $\sim38.4\pm0.5$ Jy. This SNR is also an X-ray source and Matsui et al. (1984), in a study of correlation between X-ray morphology and de-polarization between 1.4 and 5 GHz, have shown that there is a mixture of thermal and non-thermal gas associated with this SNR. High resolution imaging of other X-ray loud SNRs in radio and X-ray bands will be useful in determining if this is more generally true for other objects of this class.