We show that vertical contributions to (possibly semistable) Tanaka-Thomas-Vafa-Witten invariants are well defined for surfaces with pg(S) > 0, partially proving conjectures of [TT17b] and [Tho18a]. Moreover, we show that such contributions are computed by the same tautological integrals as in the stable case, which we studied in [Laa18]. Using the work of Kiem and Li, we show that stability of universal families of vertical Joyce-Song pairs is controlled by cosections of the obstruction sheaves of such families.
Contents1. Introduction 1 1.1. Joyce-Song pairs 1 1.2. Vafa-Witten invariants 2 1.3. The fixed locus 3 1.4. Results 3 1.5. Acknowledgement 4 2. A tautological family of Joyce-Song pairs 4 3. The virtual class 6 4. Localising the virtual class 9 5. Stability 10 6. Comparison to the VW moduli space 13 7. The unrefined case 15 8. The integrals 17 9. Refined invariants 20 References 21