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We consider the most energetically favorable symmetry-allowed spin-singletand spin-triplet superconducting pairing symmetries in twisted bilayer grapheneat the magic angle, whose normal state physics is described by a six-bandeffective tight-binding model. We compute the Chern number as a function of thesuperconducting order parameter strength and the chemical potential and we finda topological phase transition only for the chiral $p+ip'$ superconductingstate. Different from the regular graphene systems for which this happens atthe van Hove singularity, for TBG the topological phase transition arises atthe point where the Fermi surface becomes tangent to the boundary of the firstBrillouin zone. For each pairing symmetry we study the formation of subgapimpurity states for both scalar and magnetic impurities. We analyze the numberof subgap states as well as their spin polarized density of states that we findto exhibit peculiar properties that allows one to distinguish betweenspin-singlet and triplet pairing. Thus only triplet-paired states may exhibitopposite-energy impurity states with the same spin, same as for regulargraphene systems\, moreover we find that this spin may flip at thetwist-induced van Hove singularity.

Superconductivity in twisted bilayer graphene: possible pairing symmetries, impurity-induced states and Chern number