Spectroscopic Constraints on the Surface Magnetic Field of the Accreting Neutron Star EXO 0748-676
Abraham Loeb
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 ~(1-2)x109 gauss, and for which the corresponding Zeeman components of the iron lines are expected to be blended together. In other systems, a field strength >5x1010 gauss 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.
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