Snapshot of the axisymmetric toroidal magnetic field (top) and a slice of the radial magnetic field B_R (bottom) in the core of the remnant neutron star at t − t_merger = 0.060 s and y = 0 km. The gray contours correspond to the regions stable to the Tayler instability according to the geometric criterion (Eq. (42) with m = 1). Density contours for ρ ∈ [10¹², 10¹³, 10¹⁴] g cm⁻³ are respectively the dotted, dashed-dotted, solid black lines. The dashed green line corresponds to the peak of the rotation rate in the HMNS (see Figure 1).