In the addition of hbr to 1-butyne the electrophile in the first step of the mechanism is

            In the addition of hbr to 1-butyne the electrophile in the first step of the mechanism is Hydrogen atom of HBr.

Explanation:
                   In this reaction first of all HBr approaches the triple bond. A Pi Complex (weak inter-molecular interactions) is formed between the two molecules. And the triple bond attacks the partial positive hydrogen atom creating a negative charge on Bromine along with positive charge on itself (Sigma Complex). In second step the negative Bromide attacks the positive carbon of Butyne.