We just covered Feynman diagrams for the first time today, so please bear with me as a student who is not yet educated well in this subject. One question we did asked: "Draw the Feynman diagram for the interaction between an electron and a positron, producing two photons". The correct answer allegedly is this:
I understand that $\beta^-$, $\beta^+$ and $\gamma$ refer to the electron, positron and photons respectively, but we learnt Feynman diagrams as representative of boson interactions, or exchange particle interactions (is there a difference?), and in this diagram there is an unlabelled line running from the electron to the positron - all the diagrams we covered in class had labels with the relevant boson. I understand that this is the annihilation process of matter-antimatter, but what is the boson that triggers it? I've read around the subject (erratically and informally) and have heard of bosons for the EM force and strong/weak forces, but... an annihilation boson? I've no idea what this unlabelled line is supposed to represent, and I didn't have a chance to ask my teacher today.
If someone could please suggest what the unlabelled line is representing, and whether or not it even necessarily represents anything at all, I would be grateful. Also, are bosons synonymous with exchange particles in this context?
P.S. I am familiar with StackExchange guidelines (but never really use the physics site) and if this question doesn't make the cut as is stands I'll happily edit it.