Radio and X-ray images of the SNR G18.8+0.3 interacting with molecular clouds

New HI images from the VLA Galactic Plane Survey (VGPS) show clear absorption features associated with supernova remnant (SNR) G18.8+0.3. High-resolution CO images reveal that molecular clouds morphologically overlap the radio filaments of G18.8+0.3. The CO emission spectrum over the full velocity range in the direction of G18.8+0.3 shows two high column density molecular clouds and three low column density molecular clouds, all with respective HI absorption, which means these clouds are in front of G18.8+0.3. In the HI images the largest velocity absorption feature seen against the continuum image of G18.8+0.3 is at 129 km s which is at the tangent point in this direction. This yields a lower distance limit of 7.6 kpc for G18.8+0.3. Absence of absorption at negative velocities gives an upper distance limit of 15.2 kpc. The broadened profile at 20±5 km s in the CO emission spectra is a strong indicator of a possible SNR/CO cloud interaction. Then, G18.8+0.3 is at the distance of 11.6 kpc. The upper mass limit and mean density of the giant CO cloud at 20±5 km s are ∼6 ×105M⊙ and ∼ 2× 10 2 cm. We find an atomic hydrogen column density to G18.8+0.3 of NHI ∼ 2×10 22 cm. The ROSAT PSPC observations show a diffuse X-ray enhancement apparently associated with part of the radio shell of G18.8+0.3. We estimate the unabsorbed flux (2.2×10 erg cms) and intrinsic luminosity (3.3×10 erg s) of G18.8+0.3. 2 Tian, Leahy & Wang: Radio and X-ray images of SNR G18.8+0.3