Nonlinear Electrodynamics and the Surface Redshift of Pulsars

It is currently argued that the best method of determining neutron star (NS) fundamental properties is by measuring the gravitational redshift z of spectral lines produced in the stellar photosphere. Measurement of z at the stellar surface provides a unique insight on the NS mass-to-radius relation and thus on its equation of state, which reflects the physics of the strong interaction between the particles making up the star. Evidence for such a measurement has been provided quite recently by Cottam, Paerels, & Mendez and also by Sanwal and coworkers. Here we argue that although the quoted observations are undisputed for canonical pulsars, they could be misidentified if the NS is endowed with a superstrong magnetic field (B) as are the so-called magnetars and strange quark magnetars, e.g., the spectral line discovered by Ibrahim and coworkers. The source of this new ``confusion'' redshift is related to nonlinear electrodynamics (NLED) effects.