chile.tex to appear in Proceedings of ESO/CTIO Workshop on the Galactic Centre
Cc: agray@cygnus.drao.nrc.ca

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% Paper to appear in the proceedings of the 4th ESO/CTIO workshop: Galactic
% Centre, held in La Serena, Chile, March 1996.

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\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}

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