Burst Weapon Fusion
(from Starfinder Core Rulebook)
With the burst fusion, a weapon’s energy damage can form a small explosion that spills onto a second target. The weapon gains the arc critical hit effect. If the weapon already has a critical hit effect, when you score a critical hit, you can apply either the weapon’s normal critical hit effect or the arc effect. The amount of damage dealt to the secondary target is equal to one-third the level of the weapon with the burst fusion (minimum 1). Only weapons that deal acid, cold, electricity, fire, or sonic damage can benefit from a burst fusion. If your weapon deals more than one type of energy damage, you select one of those types for the arc critical hit effect to apply to secondary targets when the fusion is added.