Instantonic string solitons on M5 branes

The (I)nstantonic strings are described as extended states occupying a flat (isometry) direction on the multiple M5-brane world-volume. These are constituted by 4-dimensional gauge instantons and 2-dimensional chiral axion. In this light we revisit the covariant action for M5-branes by adding explicit axion terms. This leads to a new gauge symmetry and simple modification of some field equations and the constraints in the theory. The equations of motion include a conserved toplological current of I-strings. When 6D theory is compactified on a circle the light-like I-strings manifest as heavy monopole states in 5D super-Yang-Mills. While extremely light I-strings tend to become degenerate with the M5 vacuum but only at strong SYM coupling. This indicates that 6D vacuum is infinitely degenerate.