The Borel subgroup and branes on the Higgs moduli space

We consider two families of branes supported on the singular locus of the moduli space of Higgs bundles over a smooth projective curve $X$. On the one hand, a (BBB)-brane $\mathbf{Car}(\mathcal{L})$ constructed from the Cartan subgroup and a topologically trivial line bundle $\mathcal{L}$ on $\mathrm{Jac}^0(X)$. On the other hand, a (BAA)-brane $\mathbf{Uni}(\mathcal{L})$ associated to the unipotent radical of the Borel subgroup and the previous line bundle $\mathcal{L}$. We give evidence of both branes being dual under mirror symmetry, in the sense that an ad-hoc Fourier--Mukai integral functor relates the restriction of the hyperholomorphic bundle of the (BBB)-brane to a generic Hitchin fibre, with the support of the (BAA)-brane. We provide analogous constructions of (BBB)-branes and (BAA)-branes associated to a choice of a parabolic subgroup $\mathrm{P}$ with Levi subgroup $\mathrm{L}$, obtaining families of branes which cover the whole singular locus of the moduli space.