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

Abstract

New HI images from the VLA Galactic Plane Survey (VGPS) show clear absorption features associated with the supernova remnant (SNR) G18.8+0.3. High-resolution ${}^{13}$CO images reveal that molecular clouds overlap the radio filaments of G18.8+0.3. The ${}^{13}$CO emission spectrum over the full velocity range in the direction of G18.8+0.3 shows two molecular components with high brightness-temperatureand three molecular components with low brightness-temperature, all with respective HI absorption. This implies that these clouds are in front of G18.8+0.3. In the HI images, the highest velocity absorption feature seen against the continuum image of G18.8+0.3 is at 129 km s${}^{-1}$, which corresponds to the tangent point in this direction. This yields a lower distance limit of 6.9 kpc for G18.8+0.3. Absence of absorption at negative velocities gives an upper distance limit of 15 kpc. The broadened profile at 20$\pm$5 km s${}^{-1}$ in the ${}^{13}$CO emission spectra is a strong indicator of a possible SNR/CO cloud interaction. Thus, G18.8+0.3 is likely to be at the distance of about 12 kpc. The upper mass limit and mean density of the giant CO cloud at 20$\pm$5 km s${}^{-1}$ are \sim 6 \times 10^{5} M_{\odot} and \sim 2 \times 10^{2} cm^{-3}. We find an atomic hydrogen column density in front of G18.8+0.3 of $N_{HI}$ \sim 2 \times 10^{22} cm^{-2}. The ROSAT PSPC observations show a diffuse X-ray enhancement apparently associated with part of the radio shell of G18.8+0.3. Assuming an association, the unabsorbed flux is 2.2 \times10^{-12} erg cm^{-2} s^{-1}, suggesting an intrinsic luminosity of 3.6 \times 10^{34} erg s^{-1} for G18.8+0.3.

8 pages, 6 figures, two tables, accepted by A&A

References in corpus (5)

• The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey
• The VLA Galactic Plane Survey
• The distance and neutral environment of the massive stellar cluster Westerlund 1
• VLA and XMM-Newton observations of the SNR W41/TeV Gamma-ray source HESS J1834-087
• Evidence for Shocked Molecular Gas in the Galactic SNR CTB 109 (G109.1-1.0)

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• The Distances of SNR W41 and overlapping HII regions
• The Distances of SNRs Kes 69 and G21.5-0.9 from HI and 13CO Spectra
• Distance Determination to the Crab-like PWN G54.1+0.3 and Search for its Supernova Remnant Shell
• Progress on multi-waveband observations of supernova remnants