how can I make single variables inside a formula bold?

Question

I got this formula:

$$ \hat{p} = h_{\theta}(x) = \sigma(\theta^T x) $$ 

which gives me this:

But I would like to have some of these variables fat, basically like this:

$$ hat{p} = h_{**\theta**}(x) = \sigma(**\theta**^T**x**) $$ 

I have tried to nest it with \fat{} but that did not work.

Is it possible to fatten single variables in a formula?

Answer 1

\bm{\theta} where \bm is defined by \usepackage{bm}