Today I came across this symbol: (on top of the first four notes)

What is this symbol and how am I supposed to play it?
A friend of mine told me it's Marcato, but Wiki gave me this:

as Marcato, which isn't the same.
I have to play this on double bass with a bow, so maybe some more specific answers would be most welcome
Edit: I know what staccato is, I'm only asking for the little 'a'.