In Bushnell and Henniart, The Local Langlands conjecture for GL(2), there is a proposition on p. 184 in which they prove the following:


Let $F$ be a non-archimedean local field, $\mathcal W\_F$ its Weil group, and $\tau$ an irreducible smooth representation of $\mathcal W\_F$. Then, if $\tau(\mathcal W\_F)$ is finite, it can be extended to an irreducible smooth representation of $\Omega\_F = Gal(\overline F/F)$. 


In the proof they fix a Frobenius element $\Phi\in\mathcal W\_F$ and write any element $\omega\in \Omega\_F$ as $\omega = \Phi^a\sigma$ (for some $a\in \hat{\mathbb Z}$ and $\sigma\in I\_F$). My questions is: which element of $\Omega\_F$ is meant by $\Phi^a$? Of course, there is an exact sequence 
$1\to \mathcal I\_F\to \Omega\_F\to Gal(F^{\text{unr}}/F) = \hat{\mathbb Z}\to 1$ but how is $\Phi$ used to cook up an element of $\Omega\_F$ in the fiber of an element $a\in \hat{\mathbb Z}$? 


Thanks in advance!



Weil group of a local field, small notational problem
If $G$ is any profinite group, and $a\in\widehat{\mathbf{Z}}$, then for any sequence $(a\_n)$ of integers converging in $\widehat{\mathbf{Z}}$ to $a$, the sequence $(g^{a\_n})$ converges in $g$ to an element $g^a$ which is independent of the chosen sequence. Thus one can take $\widehat{\mathbf{Z}}$-exponents in a profinite group, and the usual rules apply, e.g., $g^{a+b}=g^ag^b$, $(g^a)^b=g^{ab}$. This is what is meant by the notation $\Phi^a$. It is defined via a limit in the absolute Galois group (of course if $a\in\mathbf{Z}$ it's the usual $a$-th power of $\Phi$). 


