The thermal history of the intergalactic medium down to redshift z=1.5:
  a new curvature measurement
  According to the photo-heating model of the intergalactic medium (IGM), He II
reionization is expected to affect its thermal evolution. Evidence for
additional energy injection into the IGM has been found at $3\lesssim
z\lesssim4$, 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 IGM
temperature measurements down to the atmospheric cut-off of the H I
Lyman-$\alpha$ forest at $z\simeq1.5$. Applying the curvature method on a
sample 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 that
the 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 at
the mean density ($T_{0}$) shows a tendency to flatten at $z\lesssim 2.8$. In
the case of $\gamma\sim1.5$, our results are consistent with previous ones
which indicate a falling $T_{0}$ for redshifts $z\lesssim2.8$. Finally, our
$T(\bar{\Delta})$ values show reasonable agreement with moderate blazar heating
models.
