We revisit in this work the problem of the maximum masses of magnetized White Dwarfs (WD). The impact of a strong magnetic field onto the structure equations is addressed. The pressures become anisotropic due to the presence of the magnetic field and
split into a parallel and perpendicular components. We first construct stable solutions of TOV equations for the parallel pressures, and found that physical solutions vanish for the perpendicular pressure when $B gtrsim 10^{13}$ G. This fact establishes an upper bound for a magnetic field and the stability of the configurations in the (quasi) spherical approximation. Our findings also indicate that it is not possible to obtain stable magnetized WD with super Chandrasekhar masses because the values of the magnetic field needed for them are higher than this bound. To proceed into the anisotropic regime, we derived structure equations appropriated for a cylindrical metric with anisotropic pressures. From the solutions of the structure equations in cylindrical symmetry we have confirmed the same bound for $B sim 10^{13} $ G, since beyond this value no physical solutions are possible. Our tentative conclusion is that massive WD, with masses well beyond the Chandrasekhar limit do not constitute stable solutions and should not exist.
We study the evolution of close binary systems formed by a normal (solar composition), intermediate mass donor star together with a neutron star. We consider models including irradiation feedback and evaporation. These non-standard ingredients deeply
modify the mass transfer stages of these binaries. While models that neglect irradiation feedback undergo continuous, long standing mass transfer episodes, models including these effect suffer a number cycles of mass transfer and detachment. During mass transfer the systems should reveal themselves as low-mass X-ray binaries (LMXBs), whereas when detached they behave as a binary radio pulsars. We show that at these stages irradiated models are in a Roche lobe overflow (RLOF) state or in a quasi-RLOF state. Quasi-RLOF stars have a radius slightly smaller than its Roche lobe. Remarkably, these conditions are attained for orbital period and donor mass values in the range corresponding to a family of binary radio pulsars known as redbacks. Thus, redback companions should be quasi-RLOF stars. We show that the characteristics of the redback system PSR J1723-2837 are accounted for by these models. In each mass transfer cycle these systems should switch from LMXB to binary radio pulsar states with a timescale of sim million years. However, there is recent and fast growing evidence of systems switching on far shorter, human timescales. This should be related to instabilities in the accretion disc surrounding the neutron star and/or radio ejection, still to be included in the model having the quasi-RLOF state as a general condition.
The fact that a magnetic field in a fermion system breaks the spherical symmetry suggest that the intrinsic geometry of this system is axisymmetric rather than spherical. In this work we analyze the impact of anisotropic pressures, due to the presenc
e of a magnetic field, in the structure equations of a magnetized quark star. We assume a cylindrical metric and an anisotropic energy momentum tensor for the source. We found that there is a maximum magnetic field that the star can sustain, closely related to the violation of the virial relations.
The most recent member of the millisecond pulsar with very low-mass companions and short orbital periods class, PSR J1311-3430 (Pletsch et al. 2012) is a remarkable object in various senses. Besides being the first discovered in gamma-rays, its measu
red features include the very low or absent hydrogen content. We show in this Letter that this important piece of information leads to a very restricted range of initial periods for a given donor mass. For that purpose, we calculate in detail the evolution of the binary system self-consistently, including mass transfer and evaporation, finding the features of the new evolutionary path leading to the observed configuration. It is also important to remark that the detailed evolutionary history of the system naturally leads to a high final pulsar mass, as it seems to be demanded by observations.
The accretion of a phantom fluid with non-zero chemical potential by black holes is discussed with basis on the Generalized Second Law of thermodynamics. For phantom fluids with positive temperature and negative chemical potential we demonstrate that
the accretion process is possible, and that the condition guaranteeing the positiveness of the phantom fluid entropy coincides with the one required by Generalized Second Law. In particular, this result provides a complementary confirmation that cosmological phantom fluids do not need to have negative temperatures.
In this work we extend previous work on the evolution of a Primordial Black Hole (PBH) to address the presence of a dark energy component with a super-negative equation of state as a background, investigating the competition between the radiation acc
retion, the Hawking evaporation and the phantom accretion, the latter two causing a decrease on black hole mass. It is found that there is an instant during the matter-dominated era after which the radiation accretion becomes negligible compared to the phantom accretion. The Hawking evaporation may become important again depending on a mass threshold. The evaporation of PBHs is quite modified at late times by these effects, but only if the Generalized Second Law of thermodynamics is violated.
We argue that giant flares in SGRs can be associated to the core conversion of an isolated neutron star having a subcritical magnetic field $sim 10^{12}$ G and a fallback disk around it. We show that, in a timescale of $lesssim 10^5$ yrs, accretion f
rom the fallback disk can increase the mass of the central object up to the critical mass for the conversion of the core of the star into quark matter. A small fraction of the neutrino-antineutrino emission from the just-converted quark-matter hot core annihilates into $e^+e^-$ pairs above the neutron star surface originating the gamma emission of the spike while the further cooling of the heated neutron star envelope originates the tail of the burst. We show that several characteristics of the giant flare of the SGR 1806-20 of 27 December 2004 (spike and tail energies, timescales, and spectra) can be explained by this mechanism.
Strangelets coming from the interstellar medium (ISM) are an interesting target to experiments searching for evidence of this hypothetic state of hadronic matter. We entertain the possibility of a {it trapped} strangelet population, quite analogous t
o ordinary nuclei and electron belts. For a population of strangelets to be trapped by the geomagnetic field, these incoming particles would have to fulfill certain conditions, namely having magnetic rigidities above the geomagnetic cutoff and below a certain threshold for adiabatic motion to hold. We show in this work that, for fully ionized strangelets, there is a narrow window for stable trapping. An estimate of the stationary population is presented and the dominant loss mechanisms discussed. It is shown that the population would be substantially enhanced with respect to the ISM flux (up to two orders of magnitude) due to quasi-stable trapping.
