Candidate SNRs

This section presents the results from the 327-MHz GMRT observations of fields containing candidates SNRs. The list of fields observed and the parameters of SNRs found in these fields are list in Table 5.2.

Table 5.1: Observed and derived physical parameters of the candidate SNRs in the fields. Type code 'S' implies shell-type, 'B' implies barrel-type while 'N' means that the object is most likely not an SNR.

Name	${\mathrm{RA}_{J2000}}$	${\mathrm{Dec}_{J2000}}$	$S_{327MHz}$	Size	Type	$\alpha$
	($h m s$)	( ${^\circ}  {^\prime} {^{\prime\prime}}$)	(Jy)	(arcmin)		$(S\propto\nu^\alpha$)
G001.4$-$0.1	17 49 39	-27 45	$4.2\pm0.5$	8.0	S	$-0.8\pm0.3?$
G003.7$-$0.2	17 55 29	-25 50	$4.5\pm0.3$	-	B	$-0.7\pm0.1$
G003.8$+$0.3	17 53 02	-25 24	$8.7\pm0.3$	18	S	$-0.9$?
G004.2$-$0.0	17 55 22	-25 15	$0.1\pm0.2$	-	N	-
G004.8$+$6.2	17 33 24	-24 34	$5.5\pm1.2$	$17$	S	$-0.6\pm0.1$
G356.2$+$4.5	17 18 58	-29 40	$8.1\pm1.7$	25	S	$-0.7\pm0.2$
G356.3$-$1.5	17 42 40	-32 52	$5.7\pm0.2$	15	S/B	$>-0.7\pm0.1$
G358.0$+$3.8	17 26 03	-28 36	$2.5\pm1.3$	$37\times39$	S	-

Figure 5.1: 327-MHz image of G001.4$-$0.0 using GMRT. A partial shell with compact source located almost at the centre is clearly detected.

G001.4$-$0.1

The GMRT 327-MHz image of G001.4$-$0.0 is shown in Fig. 5.1. A partial arc of $\sim 8$ arcmin diameter is clearly visible in this image. A compact source, almost at the geometric centre of the arc is also clearly visible. The morphology is very similar to that of the nearby composite SNR G000.9$+$0.1 which has a flat spectrum, X-ray emitting compact central source (Mereghetti et al.1998). This SNR lies just north-east of the Sgr D HII region and at the very edge of the VLA 327-MHz wide-field image of the Galactic centre region (LaRosa et al.2000). A weak fuzz of emission is seen in this wide-field image corresponding to this SNR. However, the image quality for this region is too poor (due to primary beam attenuation) to be able measure the flux density or decipher the morphology.

The 327-MHz flux density measured from our observation was found to be $4.2\pm0.5$ Jy. The 843-MHz image (Gray1994a) suffers from artifacts due to the grating response of Sgr A and the flux density of $\sim2$ Jy is hence only a ``tentative'' figure. From their 1616 MHz VLA observations of this region, Liszt (1992) reported ``an arc of incomplete shell'' of diameter $\sim 7$ arcmin at this location. The incomplete arc seen in the GMRT image agrees well with the incomplete arc seen in the 1616-MHz image. The MOST image of this object shows a relatively featureless source of emission and the ``complete shell'' reported by Gray (1994a) is difficult to decipher.

This source was also the target of observations for the detection of OH (1720 MHz) maser emission by Yusef-Zadeh et al. (1999). In the VLA A-array observation, they detect a maser spot, coincident with the western edge of the arc. Their VLA D-array observations detect an extended arc of emission, almost coincident with the arc seen in the radio continuum images.

The spectral index between 327 and 843 MHz is $-0.8\pm0.3$ for the shell. However, given that the 843-MHz flux density is quite uncertain, this value of the spectral index remains tentative and is used only to show the non-thermal nature of emission. Based on the morphology, non-thermal nature of emission and association of OH (1720 MHz) emission, we propose that this is a shell-type SNR in the Galactic plane.

Figure 5.2: GMRT image at 325 MHz of G003.8$+$0.3. The shell morphology of G003.8$+$0.3 is clearly visible.
Figure 5.3: 327-MHz contours overlayed on the $60\mu$m IRAS gray scale image of G003.8$+$0.3. A shell of IR emission is visible and the radio emission from the SNR is completely within the IR shell and almost fills the IR shell in the north. There is, however, no significant IR emission at the location of the SNR itself. The small extension in the eastern edge of the radio shell is coincident with the strong IR compact source.

Figure 5.4: Image of G003.8$+$0.3 at 11 cm from the 100-m Effelsberg single dish survey. An arc of emission similar to that seen at lower frequencies is clearly visible.

G003.8$+$0.3

Gray (1994a) describes G003.8$+$0.3 as a ``fairly weak, incomplete ring structure most perfectly centered on a slightly extended source''. The morphology of this object in the GMRT 327-MHz image, shown in Fig. 5.2, matches well with the 843-MHz MOST image. This SNR is also in the field of view for the pointings for G003.7$-$0.2 and G004.2$-$0.0. The partial shell is clearly visible in both the images. The IRAS 60$\mu$m image, shown in Fig. 5.3, does not show any significant emission at this position. However, this SNR lies within a ring of IR emission seen clearly in this image. The northern rim of the radio ring is significantly brighter and more extended, almost filling the IR ring, making it difficult to define the center of the radio ring structure. The ``central source'' in the radio image is fairly close to the center defined by the inner edge of the ring structure, but not close to the center defined by the outer edges. The bridge of emission connecting the central source and the ring is not so clearly seen at 327 MHz.

The integrated flux density of this region at 327 MHz is $8.7\pm0.3$ Jy. The diameter of the ring structure (including the northern extension) is $\approx 18{^\prime}$ while its center is at ${\mathrm{RA}_{J2000}}=17^h53^m02^s$, ${\mathrm{Dec}_{J2000}}=-25^\circ24{^\prime}$. The position of the emission peak for the 'central source' is ${\mathrm{RA}_{J2000}}= 17^h52^m 54^s$, ${\mathrm{Dec}_{J2000}}=-25^\circ28{^\prime}$. The flux density of the VLA 21-cm calibrator $J1751-253$, which is just west of this region is measured to be $1.3\pm0.2$ Jy. Flux density of this source from Texas survey (Douglas et al.1996) is $1.41\pm.09$ Jy.

The flux density reported by Gray (1994a) at 843 MHz is $\approx 3.5$ Jy giving a spectral index of $-0.9$. Based on the morphological evidence and evidence for non-thermal nature of emission, we propose that this source is a weak, Galactic SNR.

The 'central source' is detected as a point source at 1400 MHz in the NRAO-VLA All Sky Survey (NVSS) with a flux density of $15.1$ mJy. Although it is a weak source in the 327-MHz image with flux density barely at the $2\sigma$ level, it is nonetheless stronger than $15$ mJy, proving that it is a non-thermal source. With a large error in the measurement of the 327-MHz flux density, it is difficult to determine an accurate spectral index.

Image of this SNR from the radio continuum survey of the Galactic plane at 11 cm using the 100-m Effelsberg telescope (Reich et al.1990) is shown in Fig. 5.4. Here too, a partial shell of emission is visible, which matches well with the morphology of this SNR seen at 843 and 327 MHz. Significant amount of linear polarization is also detected in the 11 cm polarized images.

Figure 5.5: GMRT 327-MHz image of the region containing G004.2$+$0.0. The strong extended source in this field is a classified Ultra Compact H II region G004.417$+$0.126. The left panel shows the high resolution image ( $\sim15\times 11 {\mathrm{arcsec^2}}$) of this region where the core-halo morphology of G004.417$+$0.126 is clearly visible. It's south-western tail is visible in Grey's 843-MHz image of this region. The RMS noise in the map is about 5 mJy/beam. The right panel shows the low resolution image where an unresolved source is detected at the location of the candidate SNR just south of G004.417$+$0.126. The RMS noise in this image is $\sim 8$ mJy/beam.

Figure 5.6: The IRAS $60\mu$m image of the region containing G004.2$+$0.0. The strong source seen just north of G004.2$+$0.0 in the GMRT 327-MHz image is the strongest source in the IRAS image. A faint source, indicated by a cross at the position of G004.2$+$0.0 is also detected in this IRAS image.

G004.2$-$0.0

This source is the smallest diameter candidate SNR (size $\sim3.5$ arcmin) reported by Gray (1994a). He reported the location of this object as ${\mathrm{RA}_{J2000}}=17^h55^m17^s, {\mathrm{Dec}_{J2000}}=-25{^\circ}14{^\prime}51{^{\prime\prime}}$. The total 843-MHz flux density was reported to be $200$ mJy. However this object sits in a negative bowl and the measured value after tentative correction for this bowl is in the range of $100-300$ mJy (Gaensler, private communication). A high resolution image of this field was made to look for the shell-type structure at 327 MHz. There is a hint of a compact source in this image at ${\mathrm{RA}_{J2000}}=17^h55^m 22^s, {\mathrm{Dec}_{J2000}}= -25^\circ15{^\prime} 01{^{\prime\prime}}$, but barely at the $2\sigma$ level. No shell-type structure was detected at the level of $\sim 5$ mJy/beam with a resolution of $\sim 15$ arcsec. The low resolution image, shown in Fig. 5.5 has a $\sim 100$ mJy object at the location of this source. There is probably a compact source in the NVSS image at this location, but again at the $1-1.5\sigma$ level. The $60\mu m$ IRAS image of this region (Fig. 5.6) also shows significant extended emission at the location of this source (indicated by a cross in the figure), which appears to be associated with the HII region in the north, indicating that this may be a thermal source. This source, based on the available radio flux densities is therefore consistent with it being a flat spectrum thermal source and may not be an SNR.

The dominant extended source in the image shown in Fig. 5.5 is a known HII region, G004.4$+$0.1 located at ${\mathrm{RA}_{J2000}}=17^h55^m26^s, {\mathrm{Dec}_{J2000}}=-25{^\circ}05{^\prime}08{^{\prime\prime}}$ (Kuchar & Clark1997). A compact core surrounded by a halo of lower surface brightness is clearly visible in this image and this core-halo morphology is suggestive of this being a compact HII region (Wood & Churchwell1989b). In combination with high resolution images at other frequencies, these data can provide information about the physical conditions in this HII region.

Figure 5.7: GMRT image of G004.8$+$6.2 at 327 MHz and NVSS image at 1400 MHz. The resolution in the GMRT image is $2.2{^\prime}\times1.3{^\prime}$ along PA $-07^\circ$ and the RMS noise is 23 mJy/beam. The NVSS image has been smoothed to the resolution of the GMRT image and has a RMS noise of 0.5 mJy/beam.

Figure 5.8: VLA image of G004.8$+$6.2 at 325 MHz. The resolution in this image is $6{^\prime}\times4{^\prime}$ along PA $-0.8^\circ$ and the RMS noise is 24 mJy/beam.

G004.8$+$6.2

Fig. 5.7 shows the GMRT 327-MHz and NVSS 1400-MHz images of G004.8$+$6.2 (formerly designated as G004.5$+$6.2). This object of size $17{^\prime}\times 18{^\prime}$ is located $40{^\prime}$ east of Kepler's SNR (Fig. 5.14). The total flux density at 327 MHz is $5.5\pm1.2$ Jy and the co-ordinates of the center of the ring are $\mathrm{RA}_{J2000}=17^{\mathrm{h}}33^{\mathrm{m}}24^{\mathrm{s}}$, $\mathrm{Dec}_{J2000}=-21^\circ34{^\prime}$. The apparent flux density of Kepler's SNR in the GMRT image before primary beam correction was approximately 20 percent lower than the value of 38 Jy after primary beam correction. The integrated flux density of G004.8$+$6.2 at 2.4 GHz was reported to be $1.3\pm0.2$ Jy and the value at 4.85 GHz from PMN image of this region was found to be $1.12 \pm 0.07$ Jy, which gives a spectral index of $-0.57\pm0.13$. The NVSS image shows a well resolved ring coincident with the emission at 327 MHz. Since NVSS misses most of the extended emission, we did not attempt to use the NVSS flux density to determine the spectral index of the source. We attribute the lower signal-to-noise ratio in this map to the presence of side lobes of the synthesized beam from Kepler's SNR contaminating the entire map. It could also be partly due to the non-isoplanacity of the ionosphere at these scales as well as pointing errors in the antennas.

This SNR was also in the field of view of a VLA D-array multiple snap-shot observation at 325 MHz in March 1999. This observation was part of another project not included in this dissertation to make continuum images of Galactic SNRs at 325 and 74 MHz to get reliable low frequency spectra and, wherever possible, spectral index maps. The image from these observations is shown in Fig. 5.8. The resolution is $6{^\prime}\times4{^\prime}$, but the image is in good agreement with the higher resolution GMRT image. The integrated flux density from the VLA 327-MHz image is $4.9\pm1.3$ Jy, which is consistent with the flux density from the GMRT image within the errors bars.

Figure 5.9: 327-MHz image of G356.3-1.5 using the GMRT. The box shaped morphology of the object is apparent. The emission filling the center of the object is not detected in the 843-MHz MOST image of Gray. The RMS noise in the image is 7 mJy/beam and the total flux density of the source is $5.7\pm0.2$ Jy

G356.3-1.5

This SNR is classified as a 'classic barrel' SNR by Gray (1994a) from the 843-MHz image. The GMRT 327-MHz image (Fig. 5.9) shows the basic structure seen in the 843-MHz image where the two edges are relatively brightened compared to the center of the remnant. However, at 327 MHz, the center is also filled with significant emission, not seen in the 843-MHz image. Although it does show the brightened rims (probably of the shell), which were seen as the dominant sources of emission in the 843-MHz image, there is no well defined minimum of emission in a direction perpendicular to these rims. Gaensler (1998) laid down the following criterion to classify a SNR as 'barrel' shaped:

1. It must be of the shell- or composite-class
2. The highest resolution image available must have a minimum of 10 beams across its diameter
3. It must have clear minima in emission separated by position angles of $180{^\circ}\pm 30{^\circ}$ relative to the assumed center of the SNR (this defines the axis of symmetry for the 'barrel').
4. It must have well-defined maxima and at approximately perpendicular position angles to the minima
5. A clear bilateral axis should be identifiable, passing through the two minima and through the center of the SNR.

The clear minima along the circumference of an otherwise shell-type SNR makes its classification as a 'barrel' shaped secure. The 843-MHz image, where questionable data processing was done (namely, the subtraction of a smooth component to remove the effect of the grating response from a nearby strong source), shows a well defined minimum of emission between the two rims of emission which is filled by emission in the 327-MHz image. Although morphologically this central emission appears to be emission from the edges of the 'barrel' seen in projection, it is important to measure the spectral index of this emission to evaluate the possibility of this being a filled-center SNR. With a size of $\approx 15{^\prime}$, this can be reliably mapped at 610 and 233 MHz with the GMRT.

The integrated flux density measured at 332 MHz from the GMRT image is $5.7\pm0.2$ Jy. The RMS noise in the image in the vicinity of this object is about 4 mJy/beam. The integrated flux density in the modified image at 843 MHz is reported to be 2.8 Jy. This implies a spectral index of $-0.7\pm0.1$ between 843 and 332 MHz. However the 843-MHz image is marred by a grating response due to G357.7-0.1 and a smooth model of this artifact has been removed, though not entirely successfully (as reported by Gray (1994a)). The 843-MHz flux density is therefore likely to be under estimated and the resulting spectral index an upper limit.

Figure 5.10: The GMRT image of G356.2+4.5 at 327 MHz is shown in the right panel and NVSS image at 1400 MHz in the left panel. The resolution in the GMRT image is $3{^\prime}\times1.5{^\prime}$ along PA $-34^\circ$ and the RMS noise in the map is 10 mJy/beam. The NVSS image has been smoothed to the resolution of the GMRT image and has a RMS noise of 0.5 mJy/beam.

G356.2$+$4.5

Fig. 5.10 shows the GMRT 327-MHz and NVSS 1400-MHz images of G356.2+4.5 where a well defined circular shell of emission of size $25{^\prime}$ is evident. The co-ordinates of the center of the shell are $\mathrm{RA}_{J2000}=17^{\mathrm{h}}18^{\mathrm{m}}58^{\mathrm{s}}$, $\mathrm{Dec}_{J2000}=-29^\circ40{^\prime}$. The RMS noise in the GMRT map is 10 mJy/beam. Low-level emission is seen projected against the central region. The NVSS image is shown in the right panel of Fig. 5.10. The SNR appears to be of shell-type morphology in both maps. The larger scale emission could be missing in the NVSS map due to poorer short spacing uv-coverage, plus the poorer overall uv-coverage of snap-shot observations.

There is significant variation in the brightness along the shell seen in both the 327- and 1400-MHz maps. Although there is a broad correlation between these variations at both frequencies, there are significant differences too. The gap in the emission seen to the south-east of the ring in the NVSS image is also seen in the PMN survey image (Duncan et al.1997b). However it is filled with prominent emission in the 327-MHz GMRT image. These variations in the morphology could imply variations in the spectral index around the ring. The integrated flux density for this SNR at 327 MHz is $8.1\pm1.7$ Jy. The integrated flux density at 2.4 GHz was reported to be $3.0\pm0.3$ Jy and the value at 4.85 GHz from the PMN image was found to be $1.48 \pm 0.13$ Jy. This gives a spectral index of $-0.66 \pm 0.17$ ( $S \propto\nu^\alpha$). We did not use the total flux density from NVSS to determine the spectral index because of the missing emission in this image.

The presence of pulsar PSR B1717-29 in the field was noted by Duncan et al. (1997b). The Taylor et al. (1993) pulsar catalogue provides a characteristic age for this pulsar of $7.12\times 10^6$ yr and a dispersion measure of $42.6\pm 0.4$ pc cm$^{-3}$. Using the Taylor & Cordes (1993) model for the electron density distribution in the Galaxy, the derived distance to the pulsar is $1.4$ kpc, placing it just in front of the Sagittarius arm.

Figure 5.11: GMRT image of G358.0+3.8 at 327 MHz and NVSS image at 1400 MHz. The resolution in the GMRT image is $2.6{^\prime}\times1.8{^\prime}$ along PA $-48^\circ$ and the RMS noise is 15 mJy/beam. The NVSS image has been smoothed to the resolution of the GMRT image and has a RMS noise of 0.5 mJy/beam.

G358.0$+$3.8

Fig. 5.11 shows a $37{^\prime}\times 39{^\prime}$ region of emission, roughly circular in morphology with marginally brightened ring, whose emission closer to the plane, namely the south-eastern part is significantly brighter than on the opposite side. The morphology appears to be that of a shell-type SNR. Again there is broad correlation between the emission at 1400 and 327 MHz along the ring, though with significant detailed differences. Although the signal-to-noise ratio in both maps is low, there is good morphological correlation between the 327-MHz image and 4.85-GHz image from the PMN survey presented by Duncan et al. at a comparable resolution. Brightening of the eastern arc is seen in both the images, while the rest of the ring is fainter. The integrated flux density at 2.4 GHz for this SNR was reported to be $2.4 \pm 0.4$ Jy, with a peak flux density of 400 mJy/beam. The value at 327 MHz is $2.5\pm1.3$ Jy and $0.79\pm 0.15$ Jy at 4.85 GHz from the PMN image. Given the large angular size of this SNR, it is possible that some flux density may be missing in the GMRT image at 327 MHz. Duncan et al. (1997b) also noted that the 2.4-GHz emission might include a thermal component and therefore the flux densities at 2.4 and 4.85 GHz might be over estimated. Because of all these uncertainties, we did not compute the spectral index for this SNR. The co-ordinates of the center of the shell are ( $\mathrm{RA}_{J2000}=17^{\mathrm{h}}26^{\mathrm{m}}03^{\mathrm{s}}$, $\mathrm{Dec}_{J2000}=-28^\circ36{^\prime}$).

Note that the south-east protrusion seen in the GMRT image coincides with faint emission in the form of a smaller arc at the same location in the NVSS map, suggesting either the presence of another faint SNR in the field or a bi-annular morphology. Higher resolution, more sensitive mapping of this region is required to evaluate this possibility.