Figure 1 shows a ROSAT Survey map centered on Loop I in the energy range 0.1 - 2.0 keV. The solid white circle of 58° radius represents radio Loop I. The dashed lines outline an annular X-ray shadow in front of Loop I. A corresponding feature is apparent in the HI map of Fig. 2. An annular volume of dense neutral gas is predicted to form at the interaction zone between two colliding interstellar bubbles. Star counts using the ROSAT Wide field camera confirm the presence of an annular structure in the wall of the Local Bubble in the direction of Loop I. The distance of the interaction feature is estimated by absorption line studies of nearby stars which are projected on the ring. We find that the N_H jumps from < 10²⁰ cm^-2 to > 7 x 10²⁰ cm^-2 near the distance of about 70 pc which places the dense gas between the Local Bubble and Loop I. From the column density we estimate a particle density within the ring of n ~ 15 cm^-3, which agrees well with the theoretical prediction. Optical and UV spectral analysis of stars near the centre of Loop I reveal the presence of a neutral gas wall of N_H ~ 10²⁰ cm^-2 at a distance of 40 ± 25 pc. This is in agreement with our observation that the sightlines towards the center of Loop I, i.e., through the center of the ring feature, are almost opaque in the lowest ROSAT energy band (0.15 keV).

In conclusion the observed annular HI feature is a product of the interaction between the Local Bubble and the Loop I supershell.