Exact WKB of solutions by Borel summation and open TBA

Abstract: In this paper, we study the exact WKB methods for solutions of the Schr\"{o}dinger equations corresponding to quantum Seiberg-Witten curves in 4d $\mathcal{N}=2$ theories with surface defects. The tools are Borel summation and Gaiotto-Moore-Neitzke (GMN) open TBA, which is a conjecture that has largely stayed unexplored so far. We study the Borel plane of the solution regularized by a BPS soliton charge and find that its poles on the Borel plane have the physical meaning of BPS soliton central charges and 4d BPS state central charges. We discuss in detail the discontinuities associated with the singularities and propose general expressions for these discontinuities which also apply to higher-order ODEs. We apply the open TBA to genuinely nontrivial examples -- the 4d $\mathcal{N}=2$ theory with a single flavor BPS state corresponding to the Weber equation and an example of the pure $SU(2)$ theory corresponding to the modified Mathieu equation -- and show how to match it with Borel summation. We provide numerical evidence for the matching of the two exact WKB methods and also for an exact solution to the modified Mathieu equation via open TBA.