Spectroscopic Constraints on the Surface Magnetic Field of the Accreting Neutron Star EXO 0748-676

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 $\sim (1–2)\times {10}^9\mathrm{G}$ and for which the corresponding Zeeman components of the iron lines are expected to be blended together. In other systems, a field strength $\gtrsim 5\times {10}^{10}\mathrm{G}$ 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.