In the setting of compactly generated triangulated categories, we show that the heart
of a (co)silting t-structure is a Grothendieck category if and only if the (co)silting
object satisfies a purity assumption. Moreover, in the cosilting case the previous
conditions are related to the coaisle of the t-structure being a definable subcategory.
If we further assume our triangulated category to be algebraic, it follows that the
heart of any nondegenerate compactly generated t-structure is a Grothendieck
category.