Learn Russian: Неужели начальник взял взятку, или это просто слух? - Did the boss really take a bribe, or is it just a rumor?

Неужели expresses strong surprise, shock, or disbelief. The speaker is saying "Is it really possible that...?" and implies they find it hard to believe. If they used разве, the tone would be slightly milder, more like a simple request for clarification.

I notice взял and взятку look similar. Is that a coincidence?

Not a coincidence! Both come from the verb взять (to take). A взятка literally means a "take" or a "taking," which is the Russian word for a bribe. So взял взятку is essentially "took a taking."

Why is the ending of взятка changed to взятку in this sentence?

Because it is the direct object of the verb взял (took). In Russian, feminine nouns ending in -а change to -у in the accusative case to show they are the thing being acted upon.

Does слух always mean "rumor"?

No, слух is also the word for "hearing" (the physical sense). When used as a countable noun like in this sentence, or in the plural (слухи), it translates to "rumor" or "rumors."

Why is there a comma before или? In English we don't always use one before "or".