  In this letter we propose a test to detect the linearity of the dark energy
equation of state, and apply it to two different Type Ia Supernova (SN Ia) data
sets, Union2.1 and SNLS3. We find that: a. current SN Ia data are well
described by a dark energy equation of state linear in the cosmic scale factor
a, at least up to a redshift z = 1, independent of the pivot points chosen for
the linear relation; b. there is no significant evidence of any deviation from
linearity. This apparent linearity may reflect the limit of dark energy
information extractable from current SN Ia data.
