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

Abstract

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-sigma luminosity (0.5-10 keV) of 1e33 ergs/s, for a distance to the system of 12 kpc. The spectrum of the brightest region of X-rays is consistent with a heavily absorbed (N_H ~ 1e23 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 containing centrally-filled thermal X-rays and which 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.