From nkassim@shimmer.nrl.navy.mil Thu Oct 3 10:55:54 1996 Date: Thu, 3 Oct 96 10:55:39 EDT From: "Namir E. Kassim" To: gcnews@astro.umd.edu Subject: abstract for gc newsletter Cc: nkassim@shimmer.nrl.navy.mil X-Status: A % mnguide.tex % % v1.3 released 14th September 1995 % v1.2 released 5th September 1994 (M. Reed) % v1.1 released 18th July 1994 % v1.0 released 28th January 1994 \documentstyle{mn} %\documentstyle[doublespacing]{mn} %\documentstyle[referee]{mn} %\documentstyle{mn} % If your system has the AMS fonts version 2.0 installed, MN.sty can be % made to use them by uncommenting the line: %\AMStwofontstrue % % By doing this, you will be able to obtain upright Greek characters. % e.g. \umu, \upi etc. See the section on "Upright Greek characters" in % this guide for further information. % % If you are using AMS 2.0 fonts, bold math letters/symbols are available % at a larger range of sizes for NFSS release 1 and 2 (using \boldmath or % preferably \bmath). \newif\ifAMStwofonts %\AMStwofontstrue %%%%% AUTHORS - PLACE YOUR OWN MACROS HERE %%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \ifoldfss % \newcommand{\rmn}[1] {{\rm #1}} \newcommand{\itl}[1] {{\it #1}} \newcommand{\bld}[1] {{\bf #1}} % \ifCUPmtlplainloaded \else \NewTextAlphabet{textbfit} {cmbxti10} {} \NewTextAlphabet{textbfss} {cmssbx10} {} \NewMathAlphabet{mathbfit} {cmbxti10} {} % for math mode \NewMathAlphabet{mathbfss} {cmssbx10} {} % " " " \fi % \ifAMStwofonts % \ifCUPmtlplainloaded \else \NewSymbolFont{upmath} {eurm10} \NewSymbolFont{AMSa} {msam10} \NewMathSymbol{\upi} {0}{upmath}{19} \NewMathSymbol{\umu} {0}{upmath}{16} \NewMathSymbol{\upartial}{0}{upmath}{40} \NewMathSymbol{\leqslant}{3}{AMSa}{36} \NewMathSymbol{\geqslant}{3}{AMSa}{3E} \let\oldle=\le \let\oldleq=\leq \let\oldge=\ge \let\oldgeq=\geq \let\leq=\leqslant \let\le=\leqslant \let\geq=\geqslant \let\ge=\geqslant \fi % \fi % \fi % End of OFSS \ifnfssone % \newmathalphabet{\mathit} \addtoversion{normal}{\mathit}{cmr}{m}{it} \addtoversion{bold}{\mathit}{cmr}{bx}{it} % \newcommand{\rmn}[1] {\mathrm{#1}} \newcommand{\itl}[1] {\mathit{#1}} \newcommand{\bld}[1] {\mathbf{#1}} % \def\textbfit{\protect\txtbfit} \def\textbfss{\protect\txtbfss} \long\def\txtbfit#1{{\fontfamily{cmr}\fontseries{bx}\fontshape{it}% \selectfont #1}} \long\def\txtbfss#1{{\fontfamily{cmss}\fontseries{bx}\fontshape{n}% \selectfont #1}} % \newmathalphabet{\mathbfit} % math mode version of \textbfit{..} \addtoversion{normal}{\mathbfit}{cmr}{bx}{it} \addtoversion{bold}{\mathbfit}{cmr}{bx}{it} % \newmathalphabet{\mathbfss} % math mode version of \textbfss{..} \addtoversion{normal}{\mathbfss}{cmss}{bx}{n} \addtoversion{bold}{\mathbfss}{cmss}{bx}{n} % \ifAMStwofonts % \ifCUPmtlplainloaded \else % % Make NFSS 1 use the extra sizes available for bold math italic and % bold math symbol. These definitions may already be loaded if your % NFSS format was built with fontdef.max. % \UseAMStwoboldmath % \makeatletter \new@mathgroup\upmath@group \define@mathgroup\mv@normal\upmath@group{eur}{m}{n} \define@mathgroup\mv@bold\upmath@group{eur}{b}{n} \edef\UPM{\hexnumber\upmath@group} % \new@mathgroup\amsa@group \define@mathgroup\mv@normal\amsa@group{msa}{m}{n} \define@mathgroup\mv@bold\amsa@group{msa}{m}{n} \edef\AMSa{\hexnumber\amsa@group} \makeatother % \mathchardef\upi="0\UPM19 \mathchardef\umu="0\UPM16 \mathchardef\upartial="0\UPM40 \mathchardef\leqslant="3\AMSa36 \mathchardef\geqslant="3\AMSa3E % \let\oldle=\le \let\oldleq=\leq \let\oldge=\ge \let\oldgeq=\geq \let\leq=\leqslant \let\le=\leqslant \let\geq=\geqslant \let\ge=\geqslant % \fi \fi % \fi % End of NFSS release 1 \ifnfsstwo % \newcommand{\rmn}[1] {\mathrm{#1}} \newcommand{\itl}[1] {\mathit{#1}} \newcommand{\bld}[1] {\mathbf{#1}} % \def\textbfit{\protect\txtbfit} \def\textbfss{\protect\txtbfss} \long\def\txtbfit#1{{\fontfamily{cmr}\fontseries{bx}\fontshape{it}% \selectfont #1}} \long\def\txtbfss#1{{\fontfamily{cmss}\fontseries{bx}\fontshape{n}% \selectfont #1}} % \DeclareMathAlphabet{\mathbfit}{OT1}{cmr}{bx}{it} \SetMathAlphabet\mathbfit{bold}{OT1}{cmr}{bx}{it} \DeclareMathAlphabet{\mathbfss}{OT1}{cmss}{bx}{n} \SetMathAlphabet\mathbfss{bold}{OT1}{cmss}{bx}{n} % \ifAMStwofonts % \ifCUPmtlplainloaded \else \DeclareSymbolFont{UPM}{U}{eur}{m}{n} \SetSymbolFont{UPM}{bold}{U}{eur}{b}{n} \DeclareSymbolFont{AMSa}{U}{msa}{m}{n} \DeclareMathSymbol{\upi}{0}{UPM}{"19} \DeclareMathSymbol{\umu}{0}{UPM}{"16} \DeclareMathSymbol{\upartial}{0}{UPM}{"40} \DeclareMathSymbol{\leqslant}{3}{AMSa}{"36} \DeclareMathSymbol{\geqslant}{3}{AMSa}{"3E} % \let\oldle=\le \let\oldleq=\leq \let\oldge=\ge \let\oldgeq=\geq \let\leq=\leqslant \let\le=\leqslant \let\geq=\geqslant \let\ge=\geqslant % \fi \fi % \fi % End of NFSS release 2 \ifCUPmtlplainloaded \else \ifAMStwofonts \else % If no AMS fonts \def\upi{\pi} \def\umu{\mu} \def\upartial{\partial} \fi \fi \long\def\boxit#1{\noindent\ignorespaces \framebox[\hsize][l]{\hbox{\vbox{\raggedright #1\par}}}\par \medskip\noindent\ignorespaces } % for guide only \title{A New Supernova Remnant Over The Galactic Centre} \author{N. E.~Kassim,$^1$ and D. A.~Frail$^2$} \institute{$^1$Code 7213, Naval Research Laboratory, Washington, DC, 20375-5351 USA\\ $^2$National Radio Astronomy Observatory, Socorro, New Mexico, 87801 USA} %\date{Accepted 1993 December 11. Received 1993 March 17} \pagerange{\pageref{firstpage}--\pageref{lastpage}} \pubyear{1996} \def\LaTeX{L\kern-.36em\raise.3ex\hbox{a}\kern-.15em T\kern-.1667em\lower.7ex\hbox{E}\kern-.125emX} \newtheorem{theorem}{Theorem}[section] \begin{document} \label{firstpage} \maketitle \begin{abstract} Improved images and a newly determined spectrum from 80 MHz to 15 GHz have clarified the nature of the radio source G\thinspace{0.33}+0.04 at the Galactic Centre. Its non-thermal spectral index and its shell-like morphology favor an interpretation that this is a supernova remnant. Furthermore, the absorption characteristics of the continuum spectrum at the lowest frequencies and its elongation along the plane suggest that, like Sgr A East, it is in physical proximity to the Galactic Centre (see http://rsd-www.nrl.navy.mil/7214/weiler/dragonfarm.html for an image) \end{abstract} \begin{keywords} supernova remnants -- Galaxy: center -- radio continuum: general. \end{keywords} \section{Introduction} There are a host of observations from radio wavelengths to $\gamma$-ray energies that support the hypothesis that the supernova birthrate near the Galactic Centre is, or has recently been, much higher than inferred elsewhere in the Galaxy. The evidence includes such large-scale structures as the Galactic Centre Lobe, the expanding molecular ring, and the G\thinspace{359.1}$-$0.3 superbubble, as well as the large numbers of massive stars, the detection of hot gas (10$^7$-10$^8$ K) in the iron lines and radioactive $^{26}$Al at 1.8 MeV (see reviews by Morris \& Serabyn 1996, Genzel, Hollenbach \& Townes 1994). The most tangible evidence of a heightened supernova birthrate is the recent detection of an excess of supernova remnants (SNRs) within 5$^\circ$ of the nucleus of the Galaxy (Gray 1994a) over that expected from a uniform Galactic distribution. This excess of 14 SNRs is statistically significant at the 3$\sigma$ level but the true total number of SNRs may actually be much higher owing to the difficulties of obtaining an accurate census of extended objects in such a confused region (see Green 1991). Sgr A East (G\thinspace{0.0}+0.0), the closest known SNR to the Galactic Centre, was identified as such by Ekers et al. (1983) on the basis of its non-thermal spectral index at centimetre wavelengths and a shell-like morphology (but see Yusef-Zadeh \& Morris 1987, Mezger et al. 1989, and Khokhlov \& Melia 1996 for different viewpoints). The free-free absorption of Sgr A East at meter wavelengths by the thermal gas in Sgr A West conclusively demonstrates that this SNR is behind the Galactic Centre complex (Pedlar et al. 1989) and yet its interaction with the M$-$0.02$-$0.07 molecular cloud at the dynamical centre of the Galaxy (Zylka, Mezger \& Wink 1990, Yusef-Zadeh et al. 1996) requires that it cannot be too far beyond it. There is another non-thermal radio source located towards the inner 100 pc of the Galaxy called G\thinspace{0.33}+0.04 which LaRosa \& Kassim (1985) first suggested might be a Galactic SNR. The same claim has since been made by others (Dagkesamanskii, Kovalenko \& Udal'tsov 1994, Gray 1994b) but the {\it inferred} steep spectrum ($\alpha \leq -1$, where S$_\nu\propto\nu^\alpha$) seemed to rule out an SNR origin and instead suggested a similarity to the small scale radio lobes exhibited by some Seyfert and spiral galaxies, and thus this source was dubbed the Northern Galactic Lobe (NGL). Anantharamaiah et al. (1991) suggested that the NGL seen at low frequencies was not a discrete source but was only the Galactic continuum background shining through opacity ``windows'' in the optically thick thermal gas, which is widespread in the Galactic Centre. In this paper we present a careful re-examination of the published and archival data on this source to produce a spectrum from 80 MHz to 15 GHz as well as an improved image at 333 MHz using state-of-the-art imaging algorithms. Together these data make a strong case for identifying G\thinspace{0.33}+0.04 as an SNR, making it the second closest SNR to the Centre of our Galaxy after Sgr A East. \section{Archival Data} The low radio frequency observations (i.e. $\nu<$150 MHz) were made by the Clark Lake TPT array (LaRosa \& Kassim 1985, Kassim, LaRosa \& Erickson 1986) and the E-W WBCR-1000 array at the Lebedev Institute of Physics (Dagkesamanskii et al. 1994). At these frequencies, much of the non-thermal emission from the Galactic Centre is absorbed by optically thick gas along the line-of-sight. At frequencies between 327 MHz and 1.5 GHz subarcminute resolution images are available from synthesis arrays such as the Very Large Array (VLA)\footnote{The VLA is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.} and the Molonglo Observatory Synthesis Telescope (MOST). These measurements are ideal for detecting SNRs because they are rarely effected by line of sight absorption, provide high surface brightness sensitivity, and resolve out the background emission. We also obtained VLA archival data at 1.5 GHz from observations made in the DnC hybrid array configuration on 1984 July 20 and processed them following standard practice. A MOST image of G\thinspace{0.33}+0.04 at 843 MHz from Gray (1994b) was kindly provided by A. Gray, while at 333 MHz we reprocessed the calibrated visibility data of Anantharamaiah et al. (1991), kindly provided by A. Pedlar, using wide-field imaging which resulted in an improved image. Fig. 1 is a simple grey-scale plot of this reprocessed 333 MHz data which places the source relative to other well known Galactic Centre features. We also used the 408 MHz Molonglo pencil beam map by Little (1974) and at higher frequencies we used available single dish maps of the Galactic Centre region (Altenhoff et al. 1978, Gordon 1974). \begin{figure} \vspace{8cm} \caption{A grey-scale image of the region around the Galactic Centre at 333 MHz which places G\thinspace{0.33}+0.04 relative to other well known Galactic Centre features. The grey scale flux range varies from -1 to +270 mJy~beam$^{-1}$ as indicated by the scale at the top of the figure. This image is meant as a finder chart and has not been corrected for primary beam attenuation. Residual horizontal and vertical features are artifacts of the wide-field imaging algorithm.} \label{Figure 1} \end{figure} \section{Results} \subsection{New Imaging of G\thinspace{0.33}+0.04 at 333 MHz} At 333 MHz with the background emission mainly resolved out and most HII regions either non-detectable or only weakly optically thick emitters, we can now recognize G\thinspace{0.33}+0.04 for the first time as a well defined shell source similar to many other recognized shell-type Galactic SNRs in appearance. While the 5\arcmin$\times$8\farcm{6} beam (at 80 MHz) of the Clark Lake array was able to marginally resolve G\thinspace{0.33}+0.04 as a $\sim$12\arcmin\ source and determine that it was elongated in a direction parallel to the Galactic plane (LaRosa \& Kassim 1985), our reprocessed 333 MHz image shown in Fig. 2 provides a much better quality image with a resolution of approximately 25\arcsec. Here G\thinspace{0.33}+0.04 is visible above the diffuse background as a shell with dimensions of 14.5\arcmin$\times$7.5\arcmin. Like Sgr A East it is elongated parallel to the Galactic plane, in this case with an aspect ratio of 1.9:1. The shell is brighter on its southeastern edge where there is evidence for a direct connection with the polarized radio filaments associated with the Galactic Centre arc studied by Yusef-Zadeh, Morris \& Chance (1983). We also note that the shell is not completely limb brightened and shows evidence of filamentary structure. In this respect it is similar in appearance to regions of known radiative shocks in other SNR shells, often indicating interaction with molecular clouds (e.g. W44, see Giacani et al. 1996). \begin{figure} \vspace{8cm} \caption{Contour and grey scale image of G\thinspace{0.33}+0.04 at 333 MHz, re-imaged from the original data set of Anantharamaiah et al. (1991). The angular resolution is 42$''\times$23$''$ at PA +6$\degr$. This image was made using wide-field imaging software which accounts and corrects for the non-coplanar characteristics of the VLA. The grey scale flux range varies from 120 to 350 mJy~beam$^{-1}$ as indicated by the scale at the top of the figure. The maximum on the map is 305 mJy~beam$^{-1}$, the rms noise is $\sim$ 10 mJy~beam$^{-1}$ and the contour levels are at [-1,1,2,3,4,5,6,7,8,9,10,11,12] $\times$25 mJy~beam$^{-1}$. The three large crosses are the position of small diameter ($\leq 30''$) sources which are prominent 5 GHz continuum emitters (Downes et al. 1978) and are likely HII regions, and the triangle is the position of the broad recombination line observed with a 6$\arcmin$ beam which probably has contributions from all three (Pauls and Mezger 1975). The 90$\%$ confidence limit for the hard X-ray source 1E 1743.1-2843 is indicated by the circle. The dark line indicates the Galactic plane and the small crosses near the southwest and northeast end of this line are fiducial points at $l=0.25\degr$ and $l=0.45\degr$, respectively. This image has been corrected for primary beam attenuation.} \label{Figure 2} \end{figure} \subsection{New Spectrum for G\thinspace{0.33}+0.04} With its morphology in hand from Fig. 2, we proceeded to re-image the source at the appropriate resolution from the VLA 1.5 GHz archival data. In addition we have re-examined existing images of the Galactic Centre region, including some single dish maps, and been able to follow this source in emission as a discrete source from 57.5 MHz to 15.5 GHz. A subset of these images were then used to derive a set of flux densities and a new and improved spectrum for G\thinspace{0.33}+0.04. In deriving flux densities for G\thinspace{0.33}+0.04 we have taken special care to remove contamination from background emission by fitting planar baselines to the emission plateau in which it is immersed. The flux density of the source at each frequency determined after this baseline fit are tabulated in Table 1 and presented in Fig. 3. The error bars, which can be considerable especially for the single dish data, include the uncertainty in separating the emission of G\thinspace{0.33}+0.04 from its surroundings. Like G\thinspace{0.0}+0.0 (Sgr A East), G\thinspace{0.33}+0.04 (NGL) has an inverted spectrum with the exp($-\tau_{ff}$) behavior expected for thermal absorption (where $\tau_{ff}$ is the free-free optical depth and is proportional to $\nu^{-2.1}$). However, while Sgr A East is seen in emission at 123 MHz but in absorption at lower frequencies (Kassim et al. 1986), G\thinspace{0.33}+0.04 can be followed in emission to 57.5 MHz (LaRosa and Kassim 1985), indicating a relatively lower value of $\tau_{ff}$. A weighted least-squares fit to the data which includes a free-free absorption component and a power-law component is shown as a solid line in Fig. 3. The spectral index is determined to be $-$0.56$\pm{0.10}$ and the free-free optical depth at 100 MHz is 0.77$\pm{0.05}$. \begin{table} \caption{Radio Flux Densities for G\thinspace{0.33+0.04}.} \begin{tabular}{@{}rll@{}} Frequency & Flux Density & Ref. \\ (MHz) & (Jy) & \\[10pt] 80\phantom{0.0} & 40$\pm$12 & LaRosa and Kassim 1985 \\ 83\phantom{0.0} & 25$\pm$5 & Dagkesamanskii et al. 1994 \\ 101\phantom{0.0} & 36$\pm$7 & Dagkesamanskii et al. 1994 \\ 111\phantom{0.0} & 65$\pm$19 & Kassim et al. 1986, 1987 \\ 111\phantom{0.0} & 37$\pm$6 & Dagkesamanskii et al. 1994 \\ 120\phantom{0.0} & 49$\pm$6 & Dagkesamanskii et al. 1994 \\ 333\phantom{0.0} & 37$\pm$4 & Anantharamaiah et al. 1991$^\dagger$\\ 408\phantom{0.0} & 34$\pm$12 & Little 1974 \\ 843\phantom{0.0} & 28$\pm$5 & Gray 1994b$^\ddagger$\\ 1538\phantom{0.0} & 14$\pm$4 & This paper (VLA archival data) \\ 5000\phantom{0.0} & 8$\pm$3 & Altenhoff et al. 1978 \\ 15500\phantom{0.0} & \phantom{0}5$\pm$2 & Gordon 1974 \end{tabular} \medskip $^\dagger${Reprocessed 333 MHz data with wide-field imaging algorithms.} $^\ddagger${Re-analyzed for this paper.} \end{table} The large value of $\tau_{ff}$ suggests that G\thinspace{0.33}+0.04 is unlikely to be a nearby foreground object and may in fact be physically located near the Galactic Centre. Studies of the low frequency turnovers of SNRs (Kassim 1989, Kovalenko, Pynzar \& Udal'tsov 1995) have shown that apart from a few special lines of sight, values of $\tau_{ff}>0.6$ (at 100 MHz) are rare and are concentrated towards the inner degree of the Galactic Centre. Furthermore, if we assume an electron temperature of 8000 K then this $\tau_{ff}$ corresponds to an emission measure of $\sim{10}^4$ pc cm$^{-6}$, a value which agrees with the parameters for the low density HII region which surrounds the inner 130 pc of the Galaxy (Mezger \& Pauls 1979). Above 250 MHz the spectrum in Fig. 3 can be described by a pure power law with a slope of $-$0.56. Since the mean radio spectral index for shell-type SNRs in our Galaxy is $-$0.5 (Kovalenko, Pynzar' \& Udal'tsov 1994), this is entirely consistent with the identification of G\thinspace{0.33}+0.04 as an SNR. The earlier estimates (e.g. LaRosa \& Kassim 1985) which gave a slope $\leq{-1}$ relied on extrapolations between the Clark Lake synthesis images at decametre wavelengths and single-dish surveys (e.g. Reich et al. 1984) at centimetre wavelength. The later surveys contain a strong contribution from the Galactic background, resulting in poor constraints on the spectrum of G\thinspace{0.33}+0.04. \begin{figure} \vspace{8cm} \caption{Continuum spectrum of G\thinspace{0.33}+0.04. The crosses and error bars represent the data listed in Table 1, and the solid line is weighted least squares fit to the data which includes both a power law and a free-free absorption component. The spectrum is typical of Galactic shell-type SNRs (see text).} \label{Figure 3} \end{figure} \subsection{G\thinspace{0.33}+0.04 as a new SNR} Taken together, the shell-like morphology and the non-thermal spectral index argue persuasively for identifying G\thinspace{0.33}+0.04 as a supernova remnant. Indirect evidence favors that it is located at the Galactic Centre and is not merely seen in projection. This includes the large value of $\tau_{ff}$, the orientation of its major axis parallel to the plane, and the bright radio emission nearest the Yusef-Zadeh et al. (1983) filaments. More direct proof will require some sign that G\thinspace{0.33}+0.04 is interacting with the interstellar gas at the Galactic Centre. G\thinspace{0.33}+0.04 appears to lie in a minimum in the integrated $^{13}$CO emission but it is surrounded by molecular complexes around its exterior (Bally et al. 1987, Stark et al. 1989). As in the case of Sgr A East and G\thinspace{359.1}$-$0.5 (Yusef-Zadeh, Uchida \& Roberts 1995, Yusef-Zadeh et al. 1996), the search for shock-excited OH emission at 1720 MHz could provide the needed evidence of an interaction. \subsection{Thermal Sources Near G\thinspace{0.33}+0.04} Three discrete sources likely to be HII regions are located at the eastern periphery of G0.33+0.04 and marked by crosses on Fig. 2. All three are compact ($\leq 30''$) continuum emitters at 5 GHz (Downes et al. 1978) and also are detected on our VLA 1.5 GHz map. All three have inverted spectra between 1.5 GHz and 333 MHz, and only G0.38+0.02 is detected at 333 MHz, indicating that all three are optically thick HII regions at meter wavelengths. The inference that they are thermal is confirmed by the unusually broad ($\sim 70$ km~s$^{-1}$) H$109~\alpha$ radio recombination line (Pauls and Mezger 1975) observed with a 6$\arcmin$ beam which would have been sensitive to emission from all three sources. \section[]{Discussion} If G\thinspace{0.33}+0.04 is an SNR at the Galactic Centre as we have argued, then it is located at a galactocentric radius of 50\thinspace{d}$_{8.5}$ pc, where {d}$_{8.5}$ is its distance in units of 8.5 kpc. Likewise its average diameter D is 26\thinspace{d}$_{8.5}$ pc and its surface brightness at 1 GHz $\Sigma$ is 3$\times{10}^{-20}$ W m$^{-2}$ Hz$^{-1}$ sr$^{-1}$. While G\thinspace{0.33}+0.04 is much larger and fainter compared to Sgr A East with D=9 pc and $\Sigma$=2$\times{10}^{-18}$ W m$^{-2}$ Hz$^{-1}$ sr$^{-1}$ (Goss et al. 1983), its values are well within averages for SNRs in the inner Galaxy (Green 1991). By integrating the spectrum from 10 MHz to 100 GHz we derive a radio luminosity of 3$\times{10}^{34}$ erg s$^{-1}$ and a equipartion minimum energy E$_{min}$ of $\sim{10}^{50}$ ergs in relativistic particles and fields (assuming d$_{8.5}$=1, and a ratio of ions to electrons k=40) (Pacholczyk 1970). The dense gaseous environments into which Sgr A East and G\thinspace{359.1$-$0.5} appear to be expanding have lead Mezger et al. (1989) and Uchida et al. (1992) to conclude that these non-thermal shells have an initial explosion kinetic energy E$_\circ>>$10$^{51}$ ergs and thus they interpret them as being formed by multiple supernovae. In the absence of density constraints we calculate E$_\circ$ by using E$_{min}$ above. If the radio emission from G\thinspace{0.33}+0.04 is driven by the kinetic energy of the shock (via particle acceleration) then it is converting E$_\circ$ to E$_{min}$ with some efficiency $\epsilon$. We conclude that a single supernova event for G\thinspace{0.33}+0.04 with the canonical kinetic energy of a few$\times$10$^{51}$ ergs can be accommodated if $\epsilon$ is of order a few percent. Multiple supernovae are required if $\epsilon<<0.01$, far below the lower limit of $>5$\% found by Duric et al. (1995) for 53 SNRs in M\thinspace{33}. The equipartition field we derive for G\thinspace{0.33}+0.04 of 70 $\mu$G is large but not unreasonable for a compressed ambient field at the Galactic Centre, given the high values that are inferred there (Morris 1993,1994). G\thinspace{0.33}+0.04 is not likely to be a young object given its large diameter. The Caswell and Lerche (1979) $\Sigma$-t relation provides a generic age estimate of t$\sim 5\times 10^3$ yrs for z=0, relevant only if the explosion energy and density around the progenitor star are comparable to average values found for other Galactic SNRs. If the density were much higher as is likely the case for Sgr A East, this generic age increases. An upper limit on its age can be estimated by the timescale for velocity shear to cause a noticeable deviation in the remnant from spherical symmetry. G\thinspace{0.33}+0.04 resembles Sgr A East in that both remnants are elongated parallel to the plane in a direction opposite what one might expect for a shock which is propagating in a density layer with a strong z-dependence. Differential velocities on the order of the shock velocity could create such an effect (Morris 1993). Taking the rotation law given by Sanders (1989) for the inner 300 pc of the Galaxy we estimate a shear velocity of order 15 km s$^{-1}$ from one side of G\thinspace{0.33}+0.04 to another, resulting in an upper limit to the age of t$<$0.5$\times$10$^6$yrs. Yusef-Zadeh \& Morris (1987) have discussed another explanation for the large aspect ratio of Sgr A East which involves the poloidal field geometry at the Galactic Centre. Expansion is impeded in a direction parallel to the field lines when the shock velocity is greater than the Alfven velocity but the deviation from spherical symmetry is a maximum when the two velocities are similar. Perhaps the larger aspect ratio for G\thinspace{0.33}+0.04 over Sgr A East (1.9 versus 1.3) can be explained by a smaller shock velocity for the former. If Sgr A East were a nonstandard explosive event related to activity near Sgr A$^*$ (Yusef-Zadeh and Morris 1987, Mezger et al. 1989, Khokhlov and Melia 1996.) and G\thinspace{0.33}+0.04 represented an earlier epoch of similar activity, our upper limit for its age and its present position imply a peculiar velocity of $>10^2~$km sec$^{-1}$. However the nonstandard hypotheses for Sgr A East are driven by the large explosion energy ($>4\times10^{52}$ ergs) inferred from its unusually dense environment ($\sim10^4~$cm$^{-3}$) (Mezger et al. 1989). With no present evidence for such an environment around G\thinspace{0.33}+0.04 (which, following Mezger et al. 1989, leads to an even more extreme explosion energy of $\sim10^{56}$ ergs for G\thinspace{0.33}+0.04) we find no reason to pursue such a hypothesis at present. Furthermore the new, flatter spectrum for G\thinspace{0.33}+0.04 removes one of the principle arguments for Seyfert-like activity (with the NGL as a steep spectrum lobe), first raised by LaRosa and Kassim (1985). However, as long as Sgr A East continues to be viewed as a unique source related to outburst activity at the Galactic Centre, the possibility that G\thinspace{0.33}+0.04 might represent an earlier episode of similar activity should not be dismissed. It may also be noteworthy that the hard X-ray source 1E1743-2843 (see Fig. 2) is also located on the periphery of G\thinspace{0.33}+0.04 (Skinner et al. 1987). This is where G\thinspace{0.33}+0.04 is brightest and appears to come in contact with the northwestern part of the Galactic Centre arc, though we see no obvious physical connection with the X-ray source. The apparent physical connection between G\thinspace{0.33}+0.04 and the arc is more prominent on our reprocessed 1.5 GHz VLA map than on our 333 MHz image, suggesting a thermal nature to any interaction. Finally, the inferred inverted spectra of the compact sources located at the periphery of G\thinspace{0.33}+0.04 are typical of HII regions (Kassim et al. 1989) and indicate, along with the broad radio recombination line (Pauls and Mezger 1975), that they are thermal. The location of these HII regions near the periphery of a shell-type SNR adds to the list of SNRs with spatially correlated HII regions (e.g. see Kassim and Weiler 1990), further circumstantial evidence for induced star formation by SNR shocks. \section{Summary} %established taht it is a non-thermal source with limb bright and a %spectral insdex of , simplest interpretaion consistent with the data %is that it is an snr but we cannot rul out that it is an multiple snr %or ejection event as advocated by %better values of density will allow us to constrain etot We have reprocessed archival 333 MHz VLA data with wide-field imaging software and uncovered the meter wavelength counterpart to the Northern Galactic Lobe first identified by LaRosa and Kassim at 80 MHz. With its morphology much better defined on the VLA 333 MHz image we have been able to follow the source in emission from 57.5 MHz to 15 GHz and construct a new spectrum with power law index $-$0.56. This is a significantly flatter spectrum than the $\alpha \leq -1$ originally estimated using only the very low frequency maps and poorly constrained upper limit flux densities from single-dish, centimetre wavelength maps. The revised spectrum and far better delineated shell-like morphology now favor re-interpretation of the Northern Galactic Lobe as the supernova remnant G\thinspace{0.33}+0.04. Furthermore the low frequency turnover in the continuum spectrum implies that the source is located physically close to the Galactic Centre, though perhaps closer to us than Sgr A East since it can be followed in emission to lower frequencies. Though it is larger and thus presumably older than Sgr A East, the commonly derived physical properties of G\thinspace{0.33}+0.04, including its continuum spectrum, surface brightness, morphology, and implied physical size, radio luminosity, and equipartition minimum energy are typical of other shell-type, Galactic SNRs. We do note that G\thinspace{0.33}+0.04 is brightest where it nearly overlaps the northwest portion of the Galactic Centre arc, suggesting some type of physical interaction. Finally, while we find no compelling evidence to force us to interpret G\thinspace{0.33}+0.04 as anything else besides a normal SNR, we note that alternative interpretations of Sgr A East do exist which may also apply to this source. In order to test these models estimates of the density of the gas into which G\thinspace{0.33}+0.04 is expanding are needed so we can place better limits on its energetics and perhaps its age. \section*{Acknowledgments} This research has made use of NASA's Astrophysics Data System Abstract Service and the Simbad database, operated at CDS, Strasbourg, France. We thank Andrew Gray and Alan Pedlar for giving us their reduced data. Basic research in radio and infrared astronomy at the Naval Research Laboratory is supported by the Office of Naval Research. We thank N. Duric, M. Goss, and T. 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