Found a javascript code in my theme header. Is it some hack?

looks like some kind of code was injected in you page and its wrting a script tag in your document. The script has a scr to some external domain/website. I would suggest scan your site immediately on https://sitecheck.sucuri.net/
Also remove the script from your header and follow all the security measures to secure your site. Thee are lot of posts in this forum whcih tell you how to do it.