  The Grothendieck construction establishes an equivalence between fibrations,
a.k.a. fibred categories, and indexed categories, and is one of the fundamental
results of category theory. Cockett and Cruttwell introduced the notion of
fibrations in the context of tangent categories and proved that the fibres of a
tangent fibration inherit a tangent structure from the total tangent category.
The main goal of this paper is to provide a Grothendieck construction for
tangent fibrations. Our first attempt will focus on providing a correspondence
between tangent fibrations and indexed tangent categories, which are
collections of tangent categories and tangent morphisms indexed by the objects
and morphisms of a base tangent category. We will show that this construction
inverts Cockett and Cruttwell's result but it does not provide a full
equivalence between these two concepts. In order to understand how to define a
genuine Grothendieck equivalence in the context of tangent categories, we
introduce a new concept: the formal notion of tangent objects. Tangent objects
play a similar role in tangent category theory as formal monads for monad
theory. We show that tangent fibrations arise as tangent objects of a suitable
$2$-category and we employ this characterization to lift the Grothendieck
construction between fibrations and indexed categories to a genuine
Grothendieck equivalence between tangent fibrations and tangent indexed
categories.
