Effects of Strong Magnetic Fields in Strange Baryonic Matter

We investigate the effects of very strong magnetic fields upon the equation of state of dense bayonic matter in which hyperons are present. In the presence of a magnetic field, the equation of state above nuclear density is significantly affected both by Landau quantization and magnetic moment interactions, but only for field strengths B > 5 × 10 G. The former tends to soften the EOS and increase proton and lepton abundances, while the latter produces an overall stiffening of the EOS. Each results in a supression of hyperons relative to the field-free case. The structure of a neutron star is, however, primarily determined by the magnetic field stress. We utilize existing general relativistic magneto-hydrostatic calculations to demonstrate that maximum average fields within a stable neutron are limited to values B ≤ 1−3×10 G. This is not large enough to significantly influence particle compositions or the matter pressure, unless fluctuations dominate the average field strengths in the interior or configurations with significantly larger field gradients are considered. PACS numbers: 26.60+c, 97.60.Jd, 98.35.Eg Email addresses: [email protected] (A.E. Broderick), [email protected] (M. Prakash), [email protected] (and J.M. Lattimer). Preprint submitted to Physics Letters B 1 February 2008 Recent discoveries that link soft γ-ray repeaters and perhaps some anomalous X-ray pulsars with neutron stars having ultrastrong magnetic fields the so called magnetars (see Table 1 in [1] for a summary) have spurred theoretical studies of the effects ultrastrong magnetic fields have on the equation of state (EOS) of neutron-star matter and on the structure of neutron stars (cf. [1–3] and references therein). The magnetic field strength B needed to dramatically affect neutron star structure directly can be estimated with a dimensional analysis [4] equating the magnetic field energy EB ∼ (4πR /3)(B/8π) with the gravitational binding energy EB.E. ∼ 3GM /5R, yielding