Monika Sinha

Hypernuclear matter in strong magnetic field

Abstract Compact stars with strong magnetic fields (magnetars) have been observationally determined to have surface magnetic fields of order of 10 14 –10 15 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 ⩾ 10 17 G , in particular the matter properties become anisotropic. Moreover, for the central fields B ⩾ 10 18 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 B cr ≃ 10 19 G . This limits the range of admissible homogeneously distributed fields in magnetars to fields below the critical value B cr .

Super bursts and long bursts as surface phenomena of compact objects

X-ray bursts from compact stars are believed to be a result of Type I thermonuclear processes which are short-lived, typically ∼10 to 100 s. There are some low mass X-ray binaries, such as 4U 1820–30, 4U 1636–53, KS1731–260 and Serpens X-1, known as super bursters (SBs) which emit X-rays close to the Eddington luminosity limit for long periods of several hours. Recently, there have been reports of some long bursters (LBs), which have bursts lasting 6–25 min, whereas 4U 1735–44 has a burst period of 86 min. We suggest that these bursts from SBs and LBs may be a result of breaking and re-formation of diquark pairs, on the surface of realistic strange quark stars. We use the beta equilibrated u, d and s quark model of Dey et al. and Li et al. and allow for spin-dependent hyperfine interaction between quarks. The interaction produces pairing of specific colour-spin diquarks, leading to further lowering of energy by several MeV for each pair, on average. Diquarks are expected to break up because of the explosion and shock of the thermonuclear process. The subsequent production of copious diquark pairing may produce sufficient energy to produce the very long bursts seen in SBs or LBs. We do not claim to be able to model the complicated process fully. However, the estimated total energy liberated, 1042 erg, can be explained in our model with the calculated pair density ∼0.275 fm−3 and a surface thickness of only half a μm, if the entire surface is involved. The depth of the surface involved in the process may be only few μm if the process is restricted to a small part of the surface near the equator, as suggested by Bildsten. If SBs and LBs are surface phenomena, then recurrent super bursts, found near 4U 1636–53 by Wijnands at an interval of 4.7 yr, and the quick cooling of KS 1731–260 could be natural in this model.

CPT and lepton number violation in the neutrino sector: Modified mass matrix and oscillation due to gravity

We study the consequences of CPT and lepton number violation in the neutrino sector. For CPT violation we take gravity with which neutrino and antineutrino couple differently. Gravity mixes neutrino and antineutrino in an unequal ratio to give two mass eigenstates. Lepton number violation interaction together with CPT violation gives rise to neutrino-antineutrino oscillation. Subsequently, we study the neutrino flavor mixing and oscillation under the influence of gravity. It is found that gravity changes flavor oscillation significantly which influences the relative abundance of different flavors in present universe. We show that the neutrinoless double beta decay rate is modified due to the presence of gravity—the origin of CPT violation, as the mass of the flavor state is modified.

HAVE WE OBSERVED THE SKIN VIBRATION OF REALISTIC STRANGE STARS (ReSS)

Skin vibration of ReSS and consequent resonance absorption can account for the absorption lines in the spectrum of X-ray emission from many compact stellar objects and in particular, the stars J1210-5226 and RXJ1856-3754. Observations of the X-ray spectrum of these stars is difficult to explain, if they are neutron stars.

Stability of strange stars (SS) derived from a realistic equation of state.

A realistic EOS (equation of state) leads to strange stars (ReSS) which are compact in the mass–radius (M–R) plot, close to the Schwarzschild limiting line.1 Many of the observed stars fit in with this kind of compactness, irrespective of whether they are X-ray pulsars, bursters or soft γ repeaters or even radio pulsars. We point out that a change in the radius of a star can be small or large, when its mass is increasing and this depends on the position of a particular star on the M–R curve. We carry out a stability analysis against radial oscillations and compare with the EOS of other SS models. We find that the ReSS is stable and an M–R region can be identified to that effect.

Upper critical field and (non)-superconductivity of magnetars

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 H c2 above which the proton type-II superconductivity is destroyed. We find that magnetars with interior homogeneous field within the range 0.1 ≤ B 16 ≤ 2, where B 16 = B/1016 G, are partially superconducting, whereas those with B 16 > 2 are void of superconductivity. We briefly discuss the neutrino emissivity and superfluid dynamics of magnetars in the light of their (non)-superconductivity.

Strange quark matter in strong magnetic fields within a confining model

model. The confinement is modeled by means of the Richardson potential for quark-quark interaction modified suitably to account for a strong magnetic field. We compare our results for the equation of state and magnetization of matter to those derived within the MIT bag model. The differences between these models arise mainly due to the momentum dependence of the strong interaction between quarks in the Richardson model. Specifically, we find that the magnetization of strange quark matter in this model �

Hyperon bulk viscosity in strong magnetic fields

We study the bulk viscosity of neutron star matter including {lambda} hyperons in the presence of quantizing magnetic fields. Relaxation time and bulk viscosity due to both the nonleptonic weak process involving {lambda} hyperons and direct Urca processes are calculated here. In the presence of a strong magnetic field of 10{sup 17} G, the hyperon bulk viscosity coefficient is reduced, whereas bulk viscosity coefficients due to direct Urca processes are enhanced compared with their field free cases when many Landau levels are populated by protons, electrons, and muons.

Strange pulsar hypothesis

It appears that there is a genuine shortage of radio pulsars with surface magnetic fields significantly smaller than ∼10 8 G. We propose that the pulsars with very low magnetic fields are actually strange stars locked in a state of minimum free energy and therefore at a limiting value of the magnetic field which cannot be lowered by the system spontaneously.

Newtonian and general relativistic contribution of gravity to surface tension of strange stars

Surface tension (S ) is due to the inward force experienced by particles at the surface and usually gravitation does not play an important role in this force. But in compact stars the gravitational force on the particles is very large and S is found to depend not only on the interactions in the strange quark matter, but also on the structure of the star, i.e. on its mass and radius. Indeed, it has been claimed recently that 511 keV photons observed by the space probe INTEGRAL from the galactic bulge may be due to e + e − annihilation, and their source may be the positron cloud outside of an antiquark star. Such stars, if they exist, may also go a long way towards explaining away the antibaryon deficit of the universe. For that to happen S must be high enough to allow for survival of quark/antiquark stars born in early stages of the formation of the universe. High value of S may also assist explanation of delayed γ-ray burst after a supernova explosion, as conversion from normal matter to strange matter takes place. The possibility of some implications from formation of surface waves are also discussed.