Stellar footprints of a variable G

Theories with varying gravitational constant $G$ have been studied since long time ago. Among them, the most promising candidates as alternatives of the standard General Relativity are known as scalar-tensor theories. They provide consistent descriptions of the observed universe and arise as the low energy limit of several pictures of unified interactions. Therefore, an increasing interest on the astrophysical consequences of such theories has been sparked over the last few years. In this essay we comment on two methodological approaches to study evolution of astrophysical objects within a varying-$G$ theory, and the particular results we have obtained for boson and white dwarf stars.Comment: This essay received Honorable Mention in the 1999 Essay Competition of the Gravity Research Foundatio

Quark matter equation of state and stellar properties

In this paper we study strange matter by investigating the stability window within the QMDD model at zero temperature and check that it can explain the very massive pulsar recently detected. We compare our results with the ones obtained from the MIT bag model and see that the QMDD model can explain larger masses, due to the stiffening of the equation of state

A propeller scenario for the gamma-ray emission of low-mass X-ray binaries: The case of XSS J12270-4859

XSS J12270-4859 is the only low mass X-ray binary (LMXB) with a proposed persistent gamma-ray counterpart in the Fermi-LAT domain, 2FGL 1227.7-4853. Here, we present the results of the analysis of recent INTEGRAL observations, aimed at assessing the long-term variability of the hard X-ray emission, and thus the stability of the accretion state. We confirm that the source behaves as a persistent hard X-ray emitter between 2003 and 2012. We propose that XSS J12270-4859 hosts a neutron star in a propeller state, a state we investigate in detail, developing a theoretical model to reproduce the associated X-ray and gamma-ray properties. This model can be understood as being of a more general nature, representing a viable alternative by which LMXBs can appear as gamma-ray sources. In particular, this may apply to the case of millisecond pulsars performing a transition from a state powered by the rotation of their magnetic field, to a state powered by matter in-fall, such as that recently observed from the transitional pulsar PSR J1023+0038. While the surface magnetic field of a typical NS in a LMXB is lower by more than four orders of magnitude than the much more intense fields of neutron stars accompanying high-mass binaries, the radius at which the matter in-flow is truncated in a NS-LMXB system is much lower. The magnetic field at the magnetospheric interface is then orders of magnitude larger at this interface, and as consequence, so is the power to accelerate electrons. We demonstrate that the cooling of the accelerated electron population takes place mainly through synchrotron interaction with the magnetic field permeating the interface, and through inverse Compton losses due to the interaction between the electrons and the synchrotron photons they emit. We found that self-synchrotron Compton processes can explain the high energy phenomenology of XSS J12270-4859.Comment: 12 pages, 3 figures, accepted for publication in MNRAS. References update

Stability windows for proto-quark stars

We investigate the existence of possible stable strange matter and related stability windows at finite temperature for different models that are generally applied to describe quark stars, namely, the quark-mass density dependent model, the MIT bag model and the Nambu-Jona-Lasinio model. We emphasize that, although the limits for stable strange matter depend on a comparison with the ground state of 56Fe, which is a zero temperature state, the quantity that has to be used in the search for strange matter in proto-quark stars is the free energy and we analyze stability windows up to temperatures of the order of 40 MeV. The effects of strong magnetic fields on stability windows are computed and the resulting mass-radius relations for different stages of the proto-quark star are analyzed.Comment: Published versio

Chromaticity effects in microlensing by wormholes

Chromaticity effects introduced by the finite source size in microlensing events by presumed natural wormholes are studied. It is shown that these effects provide a specific signature that allow to discriminate between ordinary and negative mass lenses through the spectral analysis of the microlensing events. Both galactic and extragalactic situations are discussed.Comment: To appear in Modern Physics Letters A, 200

Comparing supernova remnants around strongly magnetized and canonical pulsars

The origin of the strong magnetic fields measured in magnetars is one of the main uncertainties in the neutron star field. On the other hand, the recent discovery of a large number of such strongly magnetized neutron stars, is calling for more investigation on their formation. The first proposed model for the formation of such strong magnetic fields in magnetars was through alpha-dynamo effects on the rapidly rotating core of a massive star. Other scenarios involve highly magnetic massive progenitors that conserve their strong magnetic moment into the core after the explosion, or a common envelope phase of a massive binary system. In this work, we do a complete re-analysis of the archival X-ray emission of the Supernova Remnants (SNR) surrounding magnetars, and compare our results with all other bright X-ray emitting SNRs, which are associated with Compact Central Objects (CCOs; which are proposed to have magnetar-like B-fields buried in the crust by strong accretion soon after their formation), high-B pulsars and normal pulsars. We find that emission lines in SNRs hosting highly magnetic neutron stars do not differ significantly in elements or ionization state from those observed in other SNRs, neither averaging on the whole remnants, nor studying different parts of their total spatial extent. Furthermore, we find no significant evidence that the total X-ray luminosities of SNRs hosting magnetars, are on average larger than that of typical young X-ray SNRs. Although biased by a small number of objects, we found that for a similar age, there is the same percentage of magnetars showing a detectable SNR than for the normal pulsar population.Comment: 16 pages, 5 figures, Accepted for publication in MNRA

On the possibility of an astronomical detection of chromaticity effects in microlensing by wormhole-like objects

We study the colour changes induced by blending in a wormhole-like microlensing scenario with extended sources. The results are compared with those obtained for limb darkening. We assess the possibility of an actual detection of the colour curve using the difference image analysis method.Comment: Accepted for publication in Modern Physics Letters A. 13 report pages, 7 figure

On the relation of the gravitino mass and the GUT parameters

In this article we consider the local supersymmetry breaking and the broken SU(5) symmetry permisible by dilaton vacuum configuration in supergravity theories. We establish the parameter relation of spontaneuos breaking of supersymmetry and of the GUT theory.Comment: 12 pages, file written in Revtex forma