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According to the photo-heating model of the intergalactic medium (IGM), He IIreionization is expected to affect its thermal evolution. Evidence foradditional energy injection into the IGM has been found at $3\lesssimz\lesssim4$, though the evidence for the subsequent fall-off below $z\sim2.8$is weaker and depends on the slope of the temperature--density relation,$\gamma$. Here we present, for the first time, an extension of the IGMtemperature measurements down to the atmospheric cut-off of the H ILyman-$\alpha$ forest at $z\simeq1.5$. Applying the curvature method on asample of 60 UVES spectra we investigated the thermal history of the IGM at$z<3$ with precision comparable to the higher redshift results. We find thatthe temperature of the cosmic gas traced by the Ly-$\alpha$ forest[$T(\bar{\Delta})]$ increases for increasing overdensity from$T(\bar{\Delta})\sim 22670$ K to 33740 K in the redshift range $z\sim2.8-1.6$.Under the assumption of two reasonable values for $\gamma$, the temperature atthe mean density ($T_{0}$) shows a tendency to flatten at $z\lesssim 2.8$. Inthe case of $\gamma\sim1.5$, our results are consistent with previous oneswhich indicate a falling $T_{0}$ for redshifts $z\lesssim2.8$. Finally, our$T(\bar{\Delta})$ values show reasonable agreement with moderate blazar heatingmodels.

The thermal history of the intergalactic medium down to redshift z=1.5: a new curvature measurement