You learned very early in the course that bean becomes an bhean because feminine nouns are lenited after the singular definite article an in the nominative case. But that's not the full story. You can only lenite a consonant. For words that start with a vowel, it is masculine nouns that are marked after the singular definite article, and they are marked with a t- prefix.
aláram is a masculine noun, so you get an t-aláram.