Synchrotron-Dominated X-ray Emission from the Galactic SNR G1.9+0.3

Stephen Reynolds (North Carolina State University) , K.J. Borkowski (NC State U), U. Hwang (NASA/GSFC), I. Harrus (NASA/GSFC), R. Petre (NASA/GSFC)

The shell supernova remnant G1.9+0.3 has the smallest angular diameter of any Galactic SNR, at about 1.2 arcmin at radio wavelengths (Green 2004). Our 50 ks Chandra observation of G1.9+0.3 shows a complete shell structure with strong bilateral symmetry. The mean diameter is about 100", though fainter extensions on opposite sides extend about 10" further on each side. The azimuthal brightness variations around the shell are quite different from the single bright maximum in radio. The spectrum is featureless and well-described by the exponentially cut off synchrotron model srcut, with a very high absorbing column of (6.0 +/- 0.3) x 10^22 cm^-2. With the radio flux at 1 GHz fixed at 0.9 Jy, we find a spectral index of 0.65 +/- 0.02 and a rolloff frequency of 1.1 (0.5, 2.5) x 10^18 Hz, one of the highest values known. (All errors are 90onfidence intervals.) There are no indications of spectral lines. The high column implies that G1.9+0.3 is at least as far as the Galactic center, and perhaps far across the Galaxy. At 8.5 kpc, the diameter is 4.4 pc, comparable to Kepler's SNR; even at 20 kpc, it would be only half the size of SN 1006. Simple age estimates give values between 200 and 1000 yr. The high rolloff frequency requires shock velocities of several thousand km/s. G1.9+0.3 thus becomes the fourth clear member of the class of Galactic synchrotron-dominated shell supernova remnants.