Distance and evolutionary state of the supernova remnant G18.1−0.1

We analyse radio and X-ray observations of the supernova remnant (SNR) G18.1−0.1 and the overlapping and surrounding H ii regions. The H i absorption spectra yield a new distance of 5.6 kpc to G18.1−0.1 and confirm previous distance estimates for the surrounding H ii regions. All of the H ii regions are significantly closer (d = 2.9 to 4.1 kpc) than SNR G18.1−0.1, so we conclude that they are not physically associated. Chandra archival X-ray observations of G18.1−0.1 show extended shell-type X-ray emission from the SNR. The X-ray spectrum is used to determine that the SNR is in the Sedov adiabatic expansion phase. G18.1−0.1 is ≃8.9 pc radius, middle-aged (5 kyr), expanding in normal density interstellar medium (≃0.6 cm−3) and has a low explosion energy (≃2.5 × 1050 erg).