Untwisting the Tornado: X-ray imaging and spectroscopy of G357.7-0.1.
GAENSLER B.M., FOGEL J.K.J., SLANE P.O., MILLER J.M., WIJNANDS R., EIKENBERRY S.S. and LEWIN W.H.G.
Abstract (from CDS):
We report on the detection of X-ray emission from the unusual Galactic radio source G357.7-0.1 (the ``Tornado''). Observations made with the Chandra X-Ray Observatory demonstrate the presence of up to three sources of X-ray emission from the Tornado: a relatively bright region of dimensions 2'x1' coincident with and interior to the brightest radio emission at the ``head'' of the Tornado, plus two fainter extended regions possibly associated with the Tornado's ``tail.'' No X-ray point sources associated with the Tornado are seen down to a 3 σ luminosity (0.5-10 keV) of 1x1033 ergs.s–1, for a distance to the system of 12 kpc. The spectrum of the brightest region of X-rays is consistent with a heavily absorbed (NH~1023 cm–2) thermal plasma of temperature kT∼0.6 keV; an absorbed power law can also fit the data but implies an extremely steep photon index. From these data we tentatively conclude that the Tornado is a supernova remnant (SNR), although we are unable to rule out the possibility that the Tornado is powered either by outflows from an X-ray binary or by the relativistic wind of an unseen pulsar. Within the SNR interpretation, the head of the Tornado is a limb-brightened radio shell that contains centrally filled thermal X-rays and that is interacting with a molecular cloud. We therefore propose that the Tornado is a ``mixed morphology'' supernova remnant. The unusual tail component of the Tornado remains unexplained in this interpretation but might result from expansion of the SNR into an elongated progenitor wind bubble.