Probing the thermal state of the intergalactic medium at $z>5$ with the
  transmission spikes in high-resolution Ly$\alpha$ forest spectra
  We compare a sample of five high-resolution, high S/N Ly$\alpha$ forest
spectra of bright $6<z \lesssim 6.5$ QSOs aimed at spectrally resolving the
last remaining transmission spikes at $z>5$ with those obtained from mock
absorption spectra from the Sherwood and Sherwood-Relics suites of
hydrodynamical simulations of the intergalactic medium (IGM). We use a profile
fitting procedure for the inverted transmitted flux, $1-F$, similar to the
widely used Voigt profile fitting of the transmitted flux $F$ at lower
redshifts, to characterise the transmission spikes that probe predominately
underdense regions of the IGM. We are able to reproduce the width and height
distributions of the transmission spikes, both with optically thin simulations
of the post-reionization Universe using a homogeneous UV background and full
radiative transfer simulations of a late reionization model. We find that the
width of the fitted components of the simulated transmission spikes is very
sensitive to the instantaneous temperature of the reionized IGM. The internal
structures of the spikes are more prominant in low temeperature models of the
IGM. The width distribution of the observed transmission spikes, which require
high spectral resolution ($\leq $ 8 km/s) to be resolved, is reproduced for
optically thin simulations with a temperature at mean density of $T_0= (11000
\pm 1600,10500\pm 2100,12000 \pm 2200)$ K at $z= (5.4,5.6,5.8)$. This is weakly
dependent on the slope of the temperature-density relation, which is favoured
to be moderately steeper than isothermal. In the inhomogeneous, late
reionization, full radiative transfer simulations where islands of neutral
hydrogen persist to $z\sim5.3$, the width distribution of the observed
transmission spikes is consistent with the range of $T_0$ caused by spatial
fluctuations in the temperature-density relation.
