Improved measurements of the intergalactic medium temperature around
  quasars: possible evidence for the initial stages of He-II reionisation at
  z~6
  We present measurements of the intergalactic medium (IGM) temperature within
~5 proper Mpc of seven luminous quasars at z~6. The constraints are obtained
from the Doppler widths of Lyman-alpha absorption lines in the quasar
near-zones and build upon our previous measurement for the z=6.02 quasar SDSS
J0818+1722. The expanded data set, combined with an improved treatment of
systematic uncertainties, yields an average temperature at the mean density of
log (T_0/K) = 4.21 \pm 0.03 (\pm^0.06_0.07) at 68 (95) per cent confidence for
a flat prior distribution over 3.2 < log (T_0/K) < 4.8. In comparison,
temperatures measured from the general IGM at z~5 are ~0.3 dex cooler, implying
an additional source of heating around these quasars which is not yet present
in the general IGM at slightly lower redshift. This heating is most likely due
to the recent reionisation of He-II in vicinity of these quasars, which have
hard, non-thermal ionising spectra. The elevated temperatures may therefore
represent evidence for the earliest stages of He-II reionisation in the most
biased regions of the high-redshift Universe. The temperature as a function of
distance from the quasars is consistent with being constant, log(T_0/K)~4.2,
with no evidence for a line-of-sight thermal proximity effect. However, the
limited extent of the quasar near-zones prevents the detection of He-III
regions larger than ~5 proper Mpc. Under the assumption the quasars have
reionised the He-II in their vicinity, we infer the data are consistent with an
average optically bright phase of duration in excess of 10^6.5 yr. These
measurements represent the highest-redshift IGM temperature constraints to
date, and thus provide a valuable data set for confronting models of H-I
reionisation.
