Untwisting the Tornado: X-Ray Imaging and Spectroscopy of G357.7–0.1

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′ × 1′ 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 1 × 1033 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.