Discussion

Observations of new candidate SNRs at 327 MHz, presented here, confirm these objects as SNRs on the basis of their morphology seen clearly at resolutions of $1{^\prime}$ or less, as well as based on the non-thermal nature of emission and absence of thermal emission in the $60\mu$m IRAS images. Only one of the objects (G004.2$-$0.0) is not detected at 327 MHz at the resolution required to decipher its morphology. There is also significant emission in the IRAS image at its location, implying the presence of thermal emission. At lower resolution, a 100-mJy, unresolved sources is detected, at the location of this object. The estimated flux density at 843 MHz is between 100 and 200 mJy. The existing radio flux densities and emission at IR band are therefore consistent with this source being a flat spectrum thermal source.

The morphology of some SNRs has long been suspected to be shaped by their interaction with nearby molecular clouds. Recently it has been argued that the OH (1720 MHz) emission is a good tracer of this interaction (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 latter are associated with HII regions. Both theoretical and observational evidence (Reach & Rho1998; Reach & Rho1999; Frail & Mitchell1998) suggest that the OH (1720 MHz) masers are associated with C-type shocks and are collisionally pumped in molecular clouds at temperatures and densities of $50-125$ K and $10^5-10^6$ cm$^{-3}$ respectively (Lockett et al.1999, and references therein). OH masers at 1665, 1667 and 1612 MHz cannot be produced under these physical conditions and the absence of these lines in observations which detect the OH (1720 MHz) line favors this interpretation. Measurements of the post shock density and temperature for IC443 (van Dishoeck et al.1993), W28, W44 and 3C291 (Frail & Mitchell1998) are in excellent agreement with these theoretical predictions. A solution to the problem of producing OH, which is not directly formed by shocks, is proposed by Wardle et al. (1999). They suggest that the molecular cloud is irradiated by X-rays produced by hot gas in the interior of the SNR. This leads to photo-dissociation of the H$_2$O molecules, which are produced by the shock wave in copious amounts, behind the C-type shock resulting into the required enhancement of OH just behind the C-type shock. The observed association of OH (1720 MHz) with X-ray-composite SNRs (i.e, SNRs with a shell-type radio morphology and a filled centre morphology in X-ray) and strong correlation between the morphology of the molecular gas and synchrotron emitting relativistic gas (Frail & Mitchell1998) offers observational evidence for the hypothesis that the OH (1720 MHz) maser originates in post-shock gas, heated by the SNR shock passing through dense molecular cloud.

If the extended OH (1720 MHz) emission seen towards G001.4$-$0.1 (Yusef-Zadeh et al.1999) is due to the SNR driving a C-type shock in a molecular cloud, one would expect the OH (1720 MHz) emission to be morphologically and kinematically associated with the general morphology of the radio continuum emission. The detection of a clear arc in the radio continuum image and the remarkable correlation with the OH (1720 MHz) emission towards this source is suggestive of such a model. If this association is true, the arc of emission is the boundary of the C-type shock driven in the molecular cloud by the SNR. The absence of any such cloud on the north eastern side of this SNR can explain the incomplete arc seen in the radio continuum as well as the OH (1720 MHz) emission. Also, the similarity of the radio morphology of this SNR, with G000.9$-$0.1, which is known to be a X-ray-composite SNR, further suggests that this SNR may also be of the X-ray-composite class. Interestingly, an X-ray source designated as 1RXS J175017.6-274646, is detected in the ROSAT All-Sky Survey Faint Sources catalogue about 10 arcmin away. An alternative explanation for the correlation between the OH (1720 MHz) and radio continuum emission is that the maser emission is excited in the intervening material by the background emission of the SNR, having a similar origin as in the widespread OH (1720 MHz) emission in the Galactic plane (Haynes & Caswell1977; Turner1982). However, as pointed out by Yusef-Zadeh et al. (1999), the absence of any reported OH (1720 MHz) absorption line, the correlation of OH (1720 MHz) and radio continuum emission and the coincidence of the compact and extended OH (1720 MHz) emission argues against this alternative. Determination of the radio spectra of the shell and the central source and X-ray observations of this SNR would be future useful observations to determine the nature of this SNR.

G003.6$-$0.1, located about 8 arcmin west of G003.7$-$0.2 appears in the field of view of the GMRT 327-MHz observation as well as at 1.4 GHz VLA observations by Gaensler (1999). This source is detected in the $60\mu$m IRAS image as well as at 2.7, 4.9 and 14.8 GHz and is discussed in greater detail in Chapter 6.

Barring G356.3-1.5, which appears to be barrel shaped, all others are shell-type SNRs. All these shells (and the rims for G356.3-1.5) are well resolved and show significant variations in the brightness along the shell. Caswell & Lerche (1979) proposed a correction to the $\Sigma-D$ relation (Clark & Caswell1976; Case & Bhattacharya1998, and references therein), by correcting for the variation in the ISM density away from the Galactic plane. They used the variation of the brightness across SNRs with respect to the Galactic plane to estimate the scale height of the ISM and derived the $\Sigma-D-z$ relation. In their sample, $\sim25\%$ of SNRs show enhanced emission towards the Galactic plane which they attribute to the variation of the ISM density as a function of the distance from the Galactic plane. However, 5 of the 13 SNRs which did show asymmetry, were brighter away from the plane leaving the statistical significance of this result in serious doubt (Green1984). Except for G355.9$-$2.5, G356.2$+$4.5 and G358.0$+$3.8, all the objects presented here, also show an enhancement in the brightness away from the Galactic plane ( $\approx 30\%$) hinting that brightness variations across SNRs might be dominated more by local effects (interaction with the ISM, variations in the local ISM density, interaction with nearby molecular clouds, etc.) rather than a more global effect. It is also curious that in the current data set, all sources in the fourth quadrant of the Galaxy show an enhancement in the emission towards the plane. This new data plus higher resolution data for known SNRs which has now become available can be used to re-evaluate the $\Sigma-D-z$ relation. The current sample of SNRs for which higher resolution images better resolve the structure better is significantly larger than that available to Caswell & Lerche (1979).

With the exception of Cas-A and a handful of other remnants, few SNRs have reliably determined spatially resolved spectral index maps. A reliable continuum spectral index is an indirect measure of the relativistic electron energy spectrum. Simple Fermi shock acceleration theory predicts a spectral index of $-0.5$, consistent with many shell-type SNRs. However, this inconsistent with substantially higher values of the spectral index, reliably measured for only a few SNRs. Reynolds & Ellison (1992) successfully reproduced the observed radio spectra for Tycho's and Kepler's SNRs by invoking self-consistent, nonlinear shock model of first-order Fermi acceleration. Such models predict a slightly concave spectrum and can also estimate the mean magnetic field, if the precise shape of the lower frequency spectrum is known.

The GMRT covers the low frequency range between 100 MHz and 1 GHz in 4 bands at 150, 233, 327 and 610 MHz and can also be used to observe at 1060 MHz. With a resolution of $\sim 20$ arcsec at the lowest frequency to $\sim2$ arcsec at the highest frequency, the GMRT can be used most effectively to determine the low frequency integrated spectra for most of the SNRs in the Galaxy and potentially remove the '?' (indicating that the quoted number is not reliable) from the quoted spectral indices in the SNR catalogue (Green2000).

The GMRT frequency coverage is also well suited to the separation of sources of thermal and non-thermal emission. Emission from a typical Galactic thermal sources have a flat spectra above $\sim1$ GHz. Below 1 GHz, the emission progressively decreases, with a typical spectral index of 2.0. The spectrum of a typical Galactic SNR has a negative spectral index (typically $-0.5$) above about 100 MHz and turns over below this frequency due to free-free absorption (see Fig. 5.15). The frequency range from 100 MHz to about 1 GHz is therefore very well suited to distinguish between thermal and non-thermal emission in the Galaxy. At the range of frequencies available at the GMRT, the distinction between the typical thermal and non-thermal spectra is most pronounced. With the high angular resolution provided by the GMRT, multi-frequency GMRT observations in the Galactic plane will easily distinguish between thermal and non-thermal sources as well as separate superimposed thermal and non-thermal components and help in identifying compact SNRs which may have been classified as thermal sources due to lack of resolution.

Figure 5.15: Comparison of typical spectra of Galactic SNRs and HII regions (thermal and non-thermal respectively). The first vertical line corresponds to 74 MHz (now available for observing with the VLA). Other vertical lines mark the GMRT frequency bands. GMRT covers the region of the spectra best suited for separating thermal and non-thermal sources in the Galaxy.

Measurement of a low frequency turnover below 100 MHz in the integrated spectrum of a sample of SNRs, due to free-free absorption, is a powerful probe of the distribution of low density ionized gas in the ISM, providing continuum optical depths towards various lines of sight in the Galaxy. It has been suggested that this same low density gas gives rise to low frequency RRLs which have been detected in almost every direction in the inner Galaxy (Kassim1989). The detection of low frequency RRLs (Roshi & Anantharamaiah2000) suggests (Anantharamaiah1985b; Anantharamaiah1986) that this same gas may be the low density extended HII envelope (EHEs) surrounding higher density HII regions. However, past low frequency observations have had insufficient resolution to study individual cases in sufficient detail to confirm this physical picture. The detection of spatially resolved continuum absorption against SNRs using the VLA at 74 MHz and using the GMRT and/or the VLA at 327 MHz for RRL observations will make significant advances in this area and resolve a long standing and important question.

A large number of known SNRs will be well resolved by the GMRT at all GMRT frequency bands. The spectral index of the radio emission changes across the SNR, giving information about the radiation and electron acceleration mechanism as well as about the interaction with the ISM. Apart from morphology, the difference in the spectral index between the Plerionic emission (filling the central portion of the SNR) and the emission from the shell is crucial in identifying filled-center or composite SNRs. Both the low frequency integrated spectral index as well as high resolution images of these SNRs, will be very useful for statistical studies of Galactic SNRs, as well as for the studies of the interaction of individual SNRs with the local ISM. For many of these SNRs, such observations will also be the first observations below 1 GHz and often the first observations below 2.4 GHz. The GMRT primary beam at 1420 MHz has a FWHM of 25 arcmin. Hence, SNRs of size $\le15$ arcmin can be easily observed for HI absorption to estimate the kinematic distance using the Galactic rotation model.

Strictly speaking, spectral index maps can be reliably made only when the spatial frequency coverage at different frequencies is identical (or close to it). However, one can determine the average spectral index at different positions across the SNR via the ``T-T plots'' successfully used by Anderson & Rudnick (1993) to generate spectral index maps for G039.2$-$0.3 and G041.1$-$0.3. Essentially, the local gradient of the emission at two frequencies is used to measure the local (average) spectral index eliminating the error due to differing constant offset between images due to differing spatial frequency coverages. Such spectral index maps, although at a lower resolution, provide information needed to relate the observed spectral properties to the local dynamical situations in individual SNRs. Recently, Katz-Stone & Rudnick (1995) and others have used the technique of spectral tomography to study the spectral variations within the Tycho's SNR (Katz-Stone et al.2000b).

All these observations can be done with the GMRT in the current state. This dissertation has extensively used the 327-MHz band and a RMS noise of about $\le10$ mJy/beam has been regularly achieved. RMS noise of few mJy/beam has also been achieved at 1420-MHz band and 610 MHz bands. The imaging performance at 233 and 150 MHz is yet to be established with some careful data analysis. Therefore, as a future extension of this work, the above mentioned observations should be done with the GMRT.