Summary edit

Original description:

An image of the surroundings of the comparatively bright OB star HD77581 and its (optically invisible) companion Vela X-1 was obtained with the 1.54-m Danish telescope at La Silla, through a narrow-band H-alpha filter. It clearly shows the presence of a typical bow shock, thus immediately confirming the runaway status of this system. In fact, this is one of the most `perfect' bow shocks of parabolic form ever observed around an OB-runaway.

Moreover, the orientation of the bow shock indicates that the system is moving towards the north; its origin must therefore lie somewhere south of its present position in the sky. It also turns out that the accordingly deduced path of HD77581 crosses a well-known OB-association with the designation Vel OB1 .

At the measured distance of Vel OB1 of about 6000 light-years, the observed proper motion and radial velocity of HD77581 indicate a space velocity of 90 km/sec. With this velocity, it would have taken HD77581 and its compact companion about 2.5 million years to travel the distance between Vel OB1 and its present position.

This corresponds exactly to the expected time that has passed since the supernova explosion of the progenitor star of Vela~X-1, as deduced from the observed properties of the binary system.

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