Picked up the Imperial Handbook and found this little bit of info about TIE Defenders.
I love how even Wedge is cautious toward this killer TIE fighter
Thrawn: We need a versatile starfighter capable of achieving hyperspace on its own but at the same time providing sufficient firepower should it encounter resistance. The TIE Defender is an elegant solution which will also appear less threatening to local populations than an entire Star Destroyer in orbit.
Krennic: *throws glitter* PROJECT STARDUST! IT BIG and CurVY. it FirEs MAh LazEr. PlAneT gO bOOM! EVERYONE DIE!