Title: Spectroscopic Constraints on the Surface Magnetic Field of the Accreting Neutron Star EXO 0748-676
Abstract: Gravitationally redshifted absorption lines of Fe XXVI, Fe XXV, and O VIII were inferred recently in the x-ray spectrum of the bursting neutron star EXO 0748-676. We place an upper limit on the stellar magnetic field based on the iron lines. The oxygen absorption feature shows a multiple component profile that is consistent with Zeeman splitting in a magnetic field of $\ensuremath{\sim}(1--2)\ifmmode\times\else\texttimes\fi{}{10}^{9}\text{ }\mathrm{G}$ and for which the corresponding Zeeman components of the iron lines are expected to be blended together. In other systems, a field strength $\ensuremath{\gtrsim}5\ifmmode\times\else\texttimes\fi{}{10}^{10}\text{ }\mathrm{G}$ could induce a blueshift of the line centroids that would counteract gravitational redshift and complicate the derivation of constraints on the equation of state of the neutron star.