What musical symbol is this?

There is one piece that I am playing in band that has a strange musical symbol in it. In one measure, there are two half notes (the first higher than the second, if that is important); however, in between them are three thick diagonal lines, going in the upper-left to lower-right direction. The lines are not connected to the two notes, but are just placed in the middle of them.

So, what is this mysterious symbol, and how would I go about playing it?