Sinha, Monika

Compact stars having strong magnetic fields (magnetars) have been observationally determined to have surface magnetic fields of order of 10 14-1015 G, the implied internal field strength being several orders larger. We study the equation of state and composition of hypernuclear matter and quark matter - two forms of dense matter in strong magnetic fields. We find that the magnetic field has substantial influence on the properties of hypernuclear matter and quark matter for magnetic field B ≥ 1017 G and B ≥ 1018 G respectively. In particular the matter properties become anisotropic. Moreover, above a critical field B cr, both hypernuclear and quark matter show instability, although the values of Bcr are different for two kinds of matter. © Published under licence by IOP Publishing Ltd.

Compact stars with strong magnetic fields (magnetars) have been observationally determined to have surface magnetic fields of order of 1014-1015 G, the implied internal field strength being several orders larger. We study the equation of state and composition of dense hypernuclear matter in strong magnetic fields in a range expected in the interiors of magnetars. Within the non-linear Boguta-Bodmer-Walecka model we find that the magnetic field has sizable influence on the properties of matter for central magnetic field B ≥ 1017 G, in particular the matter properties become anisotropic. Moreover, for the central fields B ≥ 1018 G, the magnetized hypernuclear matter shows instability, which is signalled by the negative sign of the derivative of the pressure parallel to the field with respect to the density, and leads to vanishing parallel pressure at the critical value Bcr ≃ 1019 G. This limits the range of admissible homogeneously distributed fields in magnetars to fields below the critical value Bcr. © 2012 Elsevier B.V..

We construct equilibrium models of compact stars using a realistic equation of state and obtain the density range occupied by the proton superconductor in strong B-fields. We do so by combining the density profiles of our models with microscopic calculations of proton pairing gaps and the critical unpairing field Hc2 above which the proton type-II superconductivity is destroyed. We find that magnetars with interior homogeneous field within the range 0.1 ≤ B16 ≤ 2, where B16 = B/10¹⁶ G, are partially superconducting, whereas those with B16 > 2 are void of superconductivity. We briefly discuss the neutrino emissivity and superfluid dynamics of magnetars in the light of their (non)-superconductivity.

We describe the microphysics, phenomenology, and astrophysical implication of a B-field induced unpairing effect that may occur in magnetars, if the local B field in the core of a magnetar exceeds a critical value Hc2. Using the Ginzburg-Landau theory of superconductivity, we derive the Hc2 field for proton condensate taking into the correction (≤30%) which arises from its coupling to the background neutron condensate. The density dependence of pairing of proton condensate implies that Hc2 is maximal at the crust-core interface and decreases towards the center of the star. As a consequence, magnetar cores with homogenous constant fields will be partially superconducting for "medium-field" magnetars (1015≤B≤5×1016G) whereas "strong-field" magnetars (B>5×1016G) will be void of superconductivity. The neutrino emissivity of a magnetar's core changes in a twofold manner: (i) the B-field assisted direct Urca process is enhanced by orders of magnitude, because of the unpairing effect in regions where B≥Hc2; (ii) the Cooper-pair breaking processes on protons vanish in these regions and the overall emissivity by the pair-breaking processes is reduced by a factor of only a few.

The magnetars are believed to be highly magnetized neutron stars having surface magnetic field 1014 − 1015 G. It is believed that at the center, the magnetic field may be higher than that at the surface. We study the effect of the magnetic field on the neutron star matter. We model the nuclear matter with the relativistic mean field approach considering the possibility of appearance of hyperons at higher density. We find that the effect of magnetic field on the matter of neutron stars and hence on the mass-radius relation is important, when the central magnetic field is atleast of the order of 1017 G. Very importantly, the effect of strong magnetic field reveals anisotropy to the system. Moreover, if the central field approaches 1019 G, then the matter becomes unstable which limits the maximum magnetic field at the center of magnetars.

MeV-scale magnetic fields in the interiors of magnetars suppress the pairing of neutrons and protons in the S-wave state. In the case of a neutron condensate the suppression is the consequence of the Pauli-paramagnetism of the neutron gas, i.e., the alignment of the neutron spins along the magnetic field. The proton S-wave pairing is suppressed because of the Landau diamagnetic currents of protons induced by the field. The Ginzburg-Landau and BCS theories of the critical magnetic fields for unpairing are reviewed. The macrophysical implications of the suppression (unpairing) of the condensates are discussed for the rotational crust-core coupling in magnetars and the neutrino-dominated cooling era of their thermal evolution.