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3XMM J185246.6+003317 is a slowly rotating soft-gamma repeater (neutron star)in the vicinity of the supernova remnant Kes\,79. So far, observations haveonly set upper limits to its surface magnetic field and spindown, and there isno estimate for its mass and radius. Using ray-tracing modelling and Bayesianinference for the analysis of several light curves spanning a period of aroundthree weeks, we have found that it may be one of the most massive neutron starsto date. In addition, our analysis suggests a multipolar magnetic fieldstructure with a subcritical field strength and a carbon atmospherecomposition. Due to the time-resolution limitation of the available lightcurves, 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 verifiedby simulations, i.e., data injections with known model parameters, and theirsubsequent recovery. The best-fitting model has three small hot spots, two ofthem in the southern hemisphere. These are, however, just back-of-the-envelopeestimates and conclusions, based on a simple ray-tracing model with anisotropicemission; we also estimate the impact of modelling on the parameteruncertainties and the relevant phenomena on which to focus in more preciseanalyses. We interpret the above best-fitting results as due to accretion ofsupernova layers/interstellar medium onto 3XMM J185246.6+003317 leading toburying and a subsequent re-emergence of the magnetic field, and a carbonatmosphere being formed possibly due to hydrogen/helium diffusive nuclearburning. Finally, we briefly discuss some consequences of our findings forsuperdense matter constraints.

Evidence for 3XMM J185246.6+003317 as a massive magnetar with a low magnetic field