  We present a recent Chandra observation of the quiescent low-mass X-ray
binary containing a neutron star, located in the globular cluster M30. We fit
the thermal emission from the neutron star to extract its mass and radius. We
find no evidence of flux variability between the two observations taken in 2001
and 2017, nor between individual 2017 observations, so we analyse them together
to increase the signal to noise. We perform simultaneous spectral fits using
standard light-element composition atmosphere models (hydrogen or helium),
including absorption by the interstellar medium, correction for pile-up of
X-ray photons on the detector, and a power-law for count excesses at high
photon energy. Using a Markov-chain Monte Carlo approach, we extract mass and
radius credible intervals for both chemical compositions of the atmosphere:
$R_{\textrm{NS}}=7.94^{+0.76}_{-1.21}$ km and $M_{\textrm{NS}}<1.19$
M$_{\odot}$ assuming pure hydrogen, and $R_{\textrm{NS}}=10.50^{+2.88}_{-2.03}$
km and $M_{\textrm{NS}}<1.78$ M$_{\odot}$ for helium, where the uncertainties
represent the 90% credible regions. For H, the small radius is difficult to
reconcile with most current nuclear physics models (especially for nucleonic
equations of state) and with other measurements of neutron star radii, with
recent preferred values generally in the 11-14 km range. Whereas for He, the
measured radius is consistent with this range. We discuss possible sources of
systematic uncertainty that may result in an underestimation of the radius,
identifying the presence of surface temperature inhomogeneities as the most
relevant bias. According to this, we conclude that either the atmosphere is
composed of He, or it is a H atmosphere with a significant contribution of hot
spots to the observed radiation.
