an isolated massive neutron star with a low magnetic field and a carbon atmosphere

[Submitted on 13 Oct 2022]

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Abstract: 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 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. The robustness of
these estimates was verified by simulations, i.e., data injections with known
model parameters, and their subsequent recovery. The best-fit model has three
small hot spots, two of them in the southern hemisphere. We interpret the above
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.