We have discovered a soft X-ray shadow covering large parts of the Loop I superbubble in the ROSAT All Sky Survey. The corresponding HI feature is associated with an annular volume of dense neutral gas formed in the collision of the Local Bubble with the Loop I supershell.
Loop I is a Galactic giant radio loop of 58° radius associated with X-ray emission (North Polar Spur) and an HI shell. It has been shown that Loop I can be modelled as a superbubble produced by stellar winds and SNe in the Sco-Cen association at a distance of 170 pc. The Solar system is embedded in the Local Bubble (LB), a cavity of ~100 pc radius, probably filled with hot, tenuous plasma of ~106 K and a density of ~5 x 10-3 cm-3. We have found evidence for a collision between the two interstellar bubbles.
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 NH 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 NH ~ 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.
Roland J. Egger & Bernd Aschenbach, Astronomy & Astrophysics 294, L25-L28 (1995)
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