What to learn from dilepton transverse momentum spectra in heavy-ion collisions?

Recently the NA60 collaboration has presented high precision measurements of dimuon spectra double differential in invariant mass M and transverse pair momentum p_T in In-In collisions at 158 A GeV. While the M-dependence is important for an understanding of in-medium changes of light vector mesons and is p_T integrated insensitive to collective expansion, the p_T-dependence arises from an interplay between emission temperature and collective transverse flow. This fact can be exploited to derive constraints on the evolution model and in particular on the contributions of different phases of the evolution to dimuon radiation into a given M window. We present arguments that a thermalized evolution phase with T > 170 MeV leaves its imprint on the spectra.