  We investigate whether the intrinsic spin of neutron stars leaves an
observable imprint on the spectral properties of X-ray binaries. To evaluate
this we consider a sample of nine NSs for which the spins have been measured
that are not accreting pulsars (for which the accretion geometry will be
different). For each source, we perform spectroscopy on a majority of RXTE hard
state observations. Our sample of sources and observations spans the range of
the Eddington ratios Lx/Ledd ~0.005-0.100. We find a clear trend between key
Comptonization properties and the NS spin for a given accretion rate.
Specifically, at a given Lx/Ledd, for more rapidly rotating NSs we find lower
seed photon temperatures and a general increase in Comptonization strength, as
parametrised by the Comptonization y parameter and amplification factor A. This
is in good agreement with the theoretical scenario whereby less energy is
liberated in a boundary layer for more rapidly spinning NSs, resulting in a
lower seed photon luminosity and, consequently, less Compton cooling in the
corona. This effect in extremis results in the hard states of the most rapidly
spinning sources encroaching upon the regime of Comptonization properties
occupied by BHs.
