The thermal history of the intergalactic medium at $3.9\le z\le 4.3$

A new determination of the temperature of the intergalactic medium over $3.9\le z\le 4.3$ is presented. We applied the curvature method on a sample of 10 high resolution quasar spectra from the Ultraviolet and Visual Echelle Spectrograph on the VLT/ESO. We measured the temperature at mean density by determining the temperature at the characteristic overdensity, which is tight function of the absolute curvature irrespective of $\gamma$. Under the assumption of fiducial value of $\gamma =1.4$, we determined the values of temperatures at mean density $T_0=7893_{-1226}^{+1417}$ K and $T_0=8153_{-993}^{+1224}$ K for redshift range of $3.9\le z\le 4.1$ and $4.1\le z\le 4.3$, respectively. Even though the results show no strong temperature evolution over the studied redshift range, our measurements are consistent with an intergalactic medium thermal history that includes a contribution from He II reionization.