It's not a normal DOM event, and no, you don't have to (and more unfortunately **can't**, even if you want to) declare custom events in your element's markup. I don't like it, and I started a thread about it on polymer-dev: https://groups.google.com/forum/#!searchin/polymer-dev/we$20have$20attributes$20why$20not$20events/polymer-dev/5Vk1Fnooyeo/HYEE1IpL3V0J. I didn't think the given answers were satisfactory: I still dislike this situation, and I'm going to continue this discussion.

There is also a loosely related new topic of adding a formal and structured way to document custom elements: https://groups.google.com/forum/#!topic/polymer-dev/NFQ4hIgZSo4 . If implemented, that could partially alleviate the pain point with custom event declaration.