Answer
${\bf 1.6}\;\rm mT$
Work Step by Step
We know that the cyclotron frequency of a charged particle in a magnetic field is given by
$$f=\dfrac{qB}{2\pi m}$$
We need to find the strength of the magnetic filed, so
$$B=\dfrac{2\pi m f}{q}$$
and since the astronomers suspect that this radiation is emitted by electrons spiraling in a magnetic field, $m$ is the mass of an electron and $q$ is its charge.
Plug the known;
$$B=\dfrac{2\pi (9.11\times 10^{-31})(45\times 10^6) }{1.6\times 10^{-19}}$$
$$B=\color{red}{\bf 1.6}\;\rm mT$$
