  3XMM J185246.6+003317 is a transient magnetar located in the vicinity of the
supernova remnant Kes\,79. So far, observations have only set upper limits to
its surface magnetic field and spindown, and there is no estimate for its mass
and radius. Using ray-tracing modelling and Bayesian inference for the analysis
of several light curves spanning a period of around three weeks, we have found
that it may be one of the most massive neutron stars to date. In addition, our
analysis suggests a multipolar magnetic field structure with a subcritical
field strength and a carbon atmosphere composition. Due to the time-resolution
limitation of the available light curves, we estimate the surface magnetic
field and the mass to be $\log_{10} (B/{\rm G}) = 11.89^{+0.19}_{-0.93}$ and
$M=2.09^{+0.16}_{-0.09}$~$M_{\odot}$ at $1\sigma$ confidence level, while the
radius is estimated to be $R=12.02^{+1.44}_{-1.42}$ km at $2\sigma$ confidence
level. They were verified by simulations, i.e., data injections with known
model parameters, and their subsequent recovery. The best-fitting model has
three small hot spots, two of them in the southern hemisphere. These are,
however, just first estimates and conclusions, based on a simple ray-tracing
model with anisotropic emission; we also estimate the impact of modelling on
the parameter uncertainties and the relevant phenomena on which to focus in
more precise analyses. We interpret the above best-fitting results as due to
accretion of supernova layers/interstellar medium onto 3XMM J185246.6+003317
leading to burying and a subsequent re-emergence of the magnetic field, and a
carbon atmosphere being formed possibly due to hydrogen/helium diffusive
nuclear burning. Finally, we briefly discuss some consequences of our findings
for superdense matter constraints.
