chile.tex to appear in Proceedings of ESO/CTIO Workshop on the Galactic Centre Cc: agray@cygnus.drao.nrc.ca % % Paper to appear in the proceedings of the 4th ESO/CTIO workshop: Galactic % Centre, held in La Serena, Chile, March 1996. \documentstyle[11pt,paspconf,epsf]{article} \begin{document} \title{G357.1$-$00.2: a peculiar nonthermal radio source \\ near the Galactic Centre} \author{Andrew D.\,Gray} \affil{Dominion Radio Astrophysical Observatory, Herzberg Institute of Astrophysics, National Research Council Canada, P.O. Box 248, Penticton, BC, V2A 6K3, Canada} \begin{abstract} The Galactic Centre is home to a number of unusual non-thermal radio emitting objects, such the filaments and threads near Sgr~A, the Snake, and G357.7$-$0.1. This paper presents recent high resolution radio observations of an object first catalogued at low resolution nearly 20 years ago, which reveal it to have unsuspected properties. The possible classifications of this object are discussed. \end{abstract} \keywords{Galactic Centre -- radio sources: nonthermal -- polarisation} \section{Introduction} G357.1$-$00.2 was first catalogued (as G357.2$-$0.2) in the $4'$ resolution Parkes 5\,GHz radio continuum survey of the southern Galactic Plane (Haynes, Caswell \& Simons 1978, 1979), where it appeared as an unremarkable, amorphous feature. It was later listed as a supernova remnant (SNR) candidate based on a low infrared-to-radio flux density ratio (Broadbent, Haslam \& Osborne 1989). An image obtained at $1'$ resolution in the MOST 843\,MHz radio continuum survey of the Galactic Centre (Gray 1994a) revealed an unusual ``S''-shaped core structure surrounded by a weak halo. The radio pulsar PSR~B1736$-$31 (listed in Taylor, Manchester \& Lyne 1993) was noted to lie about $1'$ away (Gray 1994b), but no conclusive classification of the object or its relationship, if any, with the pulsar could be made. This paper presents the preliminary results of a new high resolution investigation aimed at clarifying these issues. \section{Observations and Reduction} Radio continuum data were obtained using the VLA at 4860\,MHz ($\lambda=6.17$\,cm) in the DnC configuration on 1995 January 23, and at 1490\,MHz (20.1\,cm) in the CnB configuration on 1996 January 28. To image the whole object, four overlapping fields were observed at 6\,cm, while at 20\,cm only a single field was necessary. 3C286 was used for primary and polarisation angle calibration, while J1751$-$253 was used for secondary calibration. The data were reduced in NRAO's $\cal AIPS$ package using standard methodologies, including maximum entropy deconvolution, with ``mosaicing'' of the four fields at 6\,cm. The final images had spatial resolutions of approximately $13''$, with rms noise of $\sim\!\!25\,\mu$Jy/beam. \section{Results} Figure~1 shows the 6\,cm total intensity image (the 20\,cm image is similar). % \begin{figure} \plotfiddle{G357_I.PS}{8cm}{0}{100}{100}{-312}{-280} \caption{Total intensity at 6\,cm. The cross marks PSR~B1736$-$31.} \end{figure} % The object is revealed to be composed of several compact components embedded amongst curved and kinked filaments, with no obvious centre of activity. PSR~B1736$-$31, not visible in Figure~1 owing to its $\alpha = -2.47$ ($S_\nu \propto \nu^\alpha$) spectrum, is marked with a cross. The halo detected at 843\,MHz is also absent owing to the lack of interferometer spacings short enough to image that component. At 6\,cm most of the object is highly polarised (see Figure~2), with the polarised fraction peaking at $\sim\!\!60$\% on the N-S filament near the centre of the object. % \begin{figure} \plotfiddle{G357_P.PS}{8cm}{0}{100}{100}{-312}{-280} \caption{Polarised intensity at 6\,cm (see text for details).} \end{figure} % However, the compact components show no polarisation, and there is only weak polarisation from the bright southern filaments. At 20\,cm very little polarisation is seen, suggesting the presence of strong depolarisation (a rotation measure of $\sim\!\!2000$\,rad\,m$^{-2}$ is inferred). The observed polarisation angles at 6\,cm are thus probably not intrinsic to the emission. Nonetheless, it is noted that they tend to be aligned along the filaments in the north-western part of the object, and perpendicular to them in the eastern region, with the most strongly polarised emission having angles at $\sim\!\!45^\circ$ to the filament. An examination of spectral index shows that most of the object has $\alpha \simeq -0.5$, although the three ``spurs'' at the northern edge are noticeably steeper at $\alpha \simeq -0.6$, and there is a region of flattening to $\alpha \simeq -0.3$ over the eastern region of the source, where fractional polarisation is also highest. Despite their lack of polarisation, the embedded compact components have nonthermal spectra. Other compact sources in the field also have nonthermal spectra and are probably background sources. \section{Discussion} The morphology of this object is unusual for an SNR, at least in the sense of a shock-wave expanding in the interstellar medium. However, despite its $\alpha=-0.5$ spectrum, it is possible that it is a Crab-like object, as the flat spectrum commonly associated with such objects only holds below a spectral break. Landecker et~al. (in preparation) show that in Crab-like SNR G76.9+1.0 the spectral break is $\sim\!\!1$\,GHz, and further argue that the break frequency in such objects is correlated with surface brightness. For G357.1$-$00.2 the predicted break is also near 1\,GHz. Although PSR~B1736$-$31 lies close to G357.1$-$00.2, its characteristic age of $\sim\!\!0.5$\,Myr and location well outside the nebula probably rule it out as the powering object\footnote{It is interesting to note, however, that the distance to PSR~B1736$-$31 is estimated at $\sim\!\!7.7$\,kpc, putting it close to the Galactic Centre, which is home to several other unusual radio sources.}. However, only two Crab-like objects have had their central pulsar identified (the Crab and Vela~X), so this does not rule out this possible classification. A second possibility is that G357.1$-$00.2 is a radio-galaxy, or perhaps several superimposed galaxies. However, it does not appear to be ``normal'' for this scenario either, although many unusual morphologies are known among that class of objects, so such a subjective argument is hardly conclusive. Nonetheless, none of the compact components in G357.1$-$00.2 are flat spectrum, which is a property common to core sources in radio galaxies, and there is no obvious core-jet-lobe structure readily identifiable in this object. It is also not clear how the diffuse halo component seen at 843\,MHz would be interpreted in this model. A third possibility is that G357.1$-$00.2 is another member of a loosely defined class of peculiar nonthermal phenomena which inhabit the Galactic Centre region, including the ``threads'' and ``filaments'' near Sgr~A (e.g.\ Yusef-Zadeh, Morris \& Chance 1984; Morris \& Yusef-Zadeh 1985), ``the Snake'' (G359.1$-$00.2; Gray et~al.\ 1995), and ``the Tornado'' (G357.7$-$0.1; Becker \& Helfand 1985; Stewart et~al.\ 1994). Most of these objects are still poorly understood, and in the case of the Tornado, as for G357.1$-$00.2, it is still unclear whether the object is an SNR, or even if it is Galactic! \section{Conclusion} G357.1$-$00.2 is an intriguing nonthermal object in the Galactic Centre region, showing very strong polarisation from narrow, tangled filamentary structures, with a normal nonthermal spectrum. No compelling evidence has been found to associate the object with the adjacent radio-pulsar PSR~B1736$-$31; indeed, with the data available from the short observational history of this feature it is still not possible to make a conclusive classification of the object itself. A more thorough analysis of the preliminary results shown here is planned, and data with higher spatial resolution (probably also at a higher frequency to defeat depolarisation) as well as H{\sc i} absorption data may be acquired to help resolve this nature of this object. \acknowledgments The MOST data were collected and analysed while supported by an Australian Postgraduate Research Award. The Very Large Array is operated by Associated Universities Incorporated under a co-operative agreement with the U.S. National Science Foundation. \begin{references} \reference Becker, R.\,H., Helfand, D.\,J. 1985, Nature, 313, 115 \reference Broadbent, A., Haslam, C.\,G.\,T., Osborne, J.\,L. 1989, \mnras, 237, 381 \reference Gray, A.\,D. 1994a, \mnras, 270, 822 \reference Gray, A.\,D. 1994b, \mnras, 270, 835 \reference Gray, A.\,D., Nicholls, J., Ekers, R.\,D., Cram, L.\,E. 1995, \apj, 448, 164 \reference Haynes, R.\,F., Caswell, J.\,L., Simons, L.\,W.\,J. 1978, Aust.\ J. Phys.\ Astrophys.\ Suppl., 45, 1 \reference Haynes, R.\,F., Caswell, J.\,L., Simons, L.\,W.\,J. 1979, Aust.\ J. Phys.\ Astrophys.\ Suppl., 48, 1 \reference Landecker, T.\,L., Yi-jia, Z., Xi-zhen, Z., Higgs, L.\,A., in preparation \reference Morris, M., Yusef-Zadeh, F. 1985, \aj, 90, 2511 \reference Stewart, R.\,T., Haynes, R.\,F., Gray, A.\,D., Reich, W. 1994, \apjl, 432, L39 \reference Taylor, J.\,H., Manchester, R.\,N., Lyne, A.\,G. 1993, \apjs, 88, 529 \reference Yusef-Zadeh, F., Morris, M., Chance, D. 1984, Nature, 310, 557 \end{references} \end{document}