The Feynman diagram for Pair production

The Feynman diagram above is for pair production and at the vertex of photon materialization, the arrow representing positron is against the time axis whereas the second arrow at that vertex which I believe is a virtual photon is towards the time axis and later on, after the virtual photon extracts energy from the nucleus, the electron is produced which is towards the time axis.

My question is that is it necessary for the arrow representing virtual photon to point towards the time axis or can it also point against the time axis. An explanation would be greatly appreciated.

• The lines with arrows on are all (anti)fermions. The wavy lines are photons. – dukwon Apr 13 '17 at 10:27