A note on some moduli spaces of Ulrich Bundles

Abstract

We prove that the modular component $\mathcal M(r)$, constructed in the Main Theorem of a former paper of us (published in Adv. Math on 2024), paramatrizing (isomorphism classes of) Ulrich vector bundles of rank $r$ and given Chern classes, on suitable $3$-fold scrolls $X_e$ over Hirzebruch surfaces $\mathbb{F}_{e\geq 0}$, which arise as tautological embeddings of projectivization of very-ample vector bundles on $\mathbb{F}_e$, is generically smooth and unirational. A stronger result holds for the suitable associated moduli space $\mathcal M_{\mathbb F_e}(r)$ of vector bundles of rank $r$ and given Chern classes on $\mathbb{F}_e$, Ulrich w.r.t. the very ample polarization $c_1({\mathcal E}_e) = \mathcal O_{\mathbb F_e}(3, b_e),$ which turns out to be generically smooth, irreducible and unirational.