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J. E. Horvath

Publications and source records attributed to J. E. Horvath.

At least 109 records · Page 6Linked to original sources
arXiv2002

Color-flavor locked strange matter

We analyze how the CFL states in dense matter work in the direction of enhancing the parameter space for absolutely stable phases (strange matter). We find that the "CFL strange matter" phase can be the true ground state of hadronic matter for a much wider range of the parameters of the model (the gap of the QCD Cooper pairs $Δ$, the strange quark mass $m_s$ and the Bag Constant $B$) than the state without any pairing, and derive a full equation of state and an accurate analytic approximation to the lowest order in $Δ$ and $m_{s}$ which may be directly used for applications. The effects of pairing on the equation of state are found to be small (as previously expected) but not negligible and may be relevant for astrophysics.

hep-ph↗
arXiv2002

The evolution of primordial black hole masses in the radiation- dominated era

We revisit the problem of PBH mass evolution in the radiation-dominated era. We solve the complete differential equation in the semiclassical regime with absorption and evaporation terms and show that PBHs can gain very little mass, if at all, in this era. Relativistic proper motion of PBHs respect to the CMBR, as a possible loophole in the growth argument, is shown to be unlikely. Finally we demonstrate that PBHs can not remain in thermodynamical equilibrium with the ambient radiation, and therefore initially non-evaporating black holes must enter the evaporating regime, supporting several efforts to look for observational signatures.

gr-qc↗
arXiv2002

Quark-diquark equation of state and compact star structure

(abridged) We construct quark-diquark equations of state using the quark mass density-dependent formalism. A region inside a $2-D$ parameter space in which this matter is absolutely stable is identified. Motivated by the reported compactness of some neutron star candidates we present stellar sequences and show that the quark-diquark EOSs allow a very compact structure, even in the cases in which the diquark itself is not bound.

astro-ph↗
arXiv2001

Supershells and the closest GRB remnants

Following recent suggestions of a gamma-ray burst origin of large ISM holes (supershells) detected in the galaxy, we discuss the effects of the recently discovered shell GSH 238+00+09 (and other nearby extremely energetic supershells) on the earth's atmosphere. We argue that the large flux of gammas from these specific events would have provoked major catastrophes to living organisms. The lack of evidence for the latter can then be used to rule out local isotropic nearby GRBs in the last tens of Myr and another origin for large supershells is likely. Alternatively, this argument may be taken as independent evidence for beaming of the gamma rays, although statistics are not yet sufficient to constrain the beaming angle better than $\sim$ tens of degrees. The effects of less-energetic events of the type of SN1998bw are also considered. It is shown that more than a dozen of them should have occurred within $40 (E/10^{48} erg)^{1/2} pc$ of the earth in the last 500 Myr with similar effects. Beaming of the latter would also alleviate the conflict, although another likely possibility (an actual rate much smaller than $10^{-4} yr^{-1}$) may be a solution, at the same time avoiding an excessive number of supershells active in the galaxy.

astro-ph↗
arXiv2001

Bounds on the cosmological abundance of primordial black holes from diffuse sky brightness: single mass spectra

We constrain the mass abundance of unclustered primordial black holes (PBHs), formed with a simple mass distribution and subject to the Hawking evaporation and particle absorption from the environment. Since the radiative flux is proportional to the numerical density, an upper bound is obtained by comparing the calculated and observed diffuse background values, (similarly to the Olbers paradox in which point sources are considered) for finite bandwidths. For a significative range of formation redshifts the bounds are better than several values obtained by other arguments $Ω_{pbh} \leq 10^{-10}$; and they apply to PBHs which are evaporating today.

astro-ph↗
arXiv2001

Nuclear Effects on Bremsstrahlung Neutrino Rates of Astrophysical Interest

We calculate in this work the rates for the neutrino pair production by nucleon-nucleon bremsstrahlung taking into account the full contribution from a nuclear one-pion-exchange potential. It is shown that if the temperatures are low enough ($T \leq 20 MeV$), the integration over the nuclear part can be done for the general case, ranging from the completely degenerate (D) to the non-degenerate (ND) regime. We find that the inclusion of the full nuclear contribution enhances the neutrino pair production by $nn$ and $pp$ bremsstrahlung by a factor of about two in both the D and ND limits when compared with previous calculations. This result may be relevant for the physical conditions of interest in the semitransparent regions near the neutrinosphere in type II supernovae, cooling of neutron stars and other astrophysical situations.

nucl-th↗
arXiv2000

Mu and Tau Neutrino Thermalization and Production in Supernovae: Processes and Timescales

We investigate the rates of production and thermalization of $ν_μ$ and $ν_τ$ neutrinos at temperatures and densities relevant to core-collapse supernovae and protoneutron stars. Included are contributions from electron scattering, electron-positron annihilation, nucleon-nucleon bremsstrahlung, and nucleon scattering. For the scattering processes, in order to incorporate the full scattering kinematics at arbitrary degeneracy, the structure function formalism developed by Reddy et al. (1998) and Burrows and Sawyer (1998) is employed. Furthermore, we derive formulae for the total and differential rates of nucleon-nucleon bremsstrahlung for arbitrary nucleon degeneracy in asymmetric matter. We find that electron scattering dominates nucleon scattering as a thermalization process at low neutrino energies ($ε_ν\lesssim 10$ MeV), but that nucleon scattering is always faster than or comparable to electron scattering above $ε_ν\simeq10$ MeV. In addition, for $ρ\gtrsim 10^{13}$ g cm$^{-3}$, $T\lesssim14$ MeV, and neutrino energies $\lesssim60$ MeV, nucleon-nucleon bremsstrahlung always dominates electron-positron annihilation as a production mechanism for $ν_μ$ and $ν_τ$ neutrinos.

astro-ph↗
arXiv1999

Strange stars in low-mass binary pulsar systems

Based on observational facts and a variety of theoretical arguments we discuss in this work the possibility that pulsars in Low-Mass Binary Pulsar systems could be strange stars rather than neutron stars. It is shown that, although subject to reasonable uncertainties, the consideration of the physics of the SQM core and thin normal crusts leads to the prediction of several observed features of the magnetic field history of these systems whitin this working hypothesis.

astro-ph↗
arXiv1999

Stellar variability in low-extinction regions towards the Galactic Bulge

Intensive monitoring of low-extinction windows towards the galactic bulge has provided in the last years valuable information for studies about the dynamics, kinematics and formation history of this part of the galaxy, mainly by characterizing the bulge stellar populations (Paczyński, 1996). Since 1997, we have been conducting an intensive photometric-astrometric survey of the galactic bulge, with the monitoring of about 120000 stars in 12 windows uniformly distributed in galactic latitude and longitude (Blanco & Terndrup, 1989 e Blanco, 1988) never before submitted to this kind of survey. For this purpose, we have used the IAG/USP CCD Meridian Circle of the Abrahão de Moraes Observatory. The main objective of this work is the identification and classification of variable objects. In this work we present the set up and development of the necessary tools for a project like this and the posterior analysis of our data. We briefly describe the construction of a program to organize and detect variables among the observed stars, including real time alerts (for variations greater than 0.3 magnitudes). The preliminary analysis after the processing of 76 nights of observation yielded 479 variable stars, from which 96.7 % of them are new. We discuss the preliminary classification of this variables, based on: a) the observed amplitude of variation; b) the shape of light curve; c) the expected variable classes among our data and d) the calculated periods, whenever possible. Finally, we discuss the future perspectives for the project and for the applications and analysis of the discovered variable stars.

astro-ph↗
arXiv1999

Extension of Tycho catalog for low-extinction windows in the galactic bulge

We present in this work secondary catalogs up to $m_{Val} \sim 13$ based on the Tycho reference frame (ESA, 1997) for 12 selected low-extinction fields towards the galactic bulge. The observations have been performed with the Askania-Zeiss Meridian Circle equiped with a CCD camera, located at the Abrahão de Moraes Observatory (Valinhos, Brazil) and operated by the Institute of Astronomy and Geophysics, São Paulo University. The presented catalog, though not complete, has been designed to help in intensive search programmes (e.g. microlensing and variable searches) and therefore the selected standards have a high astrometric and photometric ($V$ band, approximately) quality. The mean precisions obtained were $0.0018^{s}$ in $α$, 0.013'' in $δ$, 0.030 for the standard deviation in magnitude and 0.0042 for the magnitude when weighted with the error bars in each night (in the mean, 42 stars for the catalog of each window). Tables B.1 to B.12 are also available in eletronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html.

astro-ph↗
arXiv1998

H-dibaryons and Primordial Nucleosynthesis

The apparent discrepancy between abundances of light nuclides predicted by the standard Big-Bang and observational data is explained, by assuming the presence of metastable H dibaryons at the nucleosynthesis era. These dibaryons could be formed out of a small fraction of strange quarks at the moment of the confinement transition. For a primordial deuterium abundance of the order of 3 10^{-5}, the measured differences in the 4He abundances requires a relative abundance of H dibaryons of the order of n_H/n_B = 0.07, decaying in a timescale of the order of 10^5 s.

astro-ph↗
arXiv1998

Neutrino sparking and the neutron to strange stars conversion

We address the production of strangelets inside neutron stars by means of high-energy neutrino interactions (sparking). Requiring that neutron stars remain as such along their lifetimes, we obtain a bound on the probability of a strangelet in the final state and compare it with existing laboratory limits. It turns out that this mechanism is not likely to drive a neutron $\to$ strange stars conversion for realistic values of the minimum center-mass-energy necessary to produce the quark-gluon plasma, a necessary precondition for the formation of the strangelet.

astro-ph↗
arXiv1998

Strangelet spectra from type II supernovae

We study in this work the fate of strangelets injected as a contamination in the tail of a "strange matter-driven" supernova shock. A simple model for the fragmentation and braking of the strangelets when they pass through the expanding oxygen shell is presented and solved to understand the reprocessing of this component. We find that the escaping spectrum is a scaled-down version of the one injected at the base of the oxygen shell. The supernova source is likely to produce low-energy particles of $A \sim 100-1000$ quite independently of the initial conditions. However, it is difficult that ultrarrelativistic strangelets (such as the hypothetical Centauro primaries) can have an origin in those explosive events.

astro-ph↗
arXiv1998

Evolution of a Primordial Black Hole Population

We reconsider in this work the effects of an energy absorption term in the evolution of primordial black holes (hereafter PBHs) in the several epochs of the Universe. A critical mass is introduced as a boundary between the accreting and evaporating regimes of the PBHs. We show that the growth of PBHs is negligible in the Radiation-dominated Era due to scarcity of energy density supply from the expanding background, in agreement with a previous analysis by Carr and Hawking, but that nevertheless the absorption term is large enough for black holes above the critical mass to preclude their evaporation until the universe has cooled sufficiently. The effects of PBH motion are also discussed: the Doppler effect may give rise to energy accretion in black-holes with large peculiar motions relative to background. We discuss how cosmological constraints are modified by the introduction of the critical mass since that PBHs above it do not disturb the CMBR. We show that there is a large range of admissible masses for PBHs above the critical mass but well below the cosmological horizon. Finally we outline a minimal kinetic formalism, solved in some limiting cases, to deal with more complicated cases of PBH populations

astro-ph↗
arXiv1998

Gravitational-wave bursts from soft gamma-ray repeaters: Can they be detected?

In this letter we suggest a scenario for simultaneous emission of gravitational-wave and $γ$-ray bursts (GRBs) from soft gamma-ray repeaters (SGRs). we argue that both of the radiations can be generated by a super-Eddington accreting neutron stars in X-ray binaries. In this model a supercritical accretion transient takes back onto the remnant star the disk leftover by the hydrodynamic instability phase of a low magnetized, rapidly rotating neutron star in a X-ray binary system. We estimate the rise timescale $Δt_c = 0.21 ms$, minimum mass accretion rate needed to trigger the $γ$-ray emission, $\dot{M}_λ= 4.5 \times 10^{28} g$, and its effective associated temperature $T_{eff} = 740 keV$, and the timescale for repeating a burst of $γ$-rays $Δτ_R = 11.3 yr$. Altogether, we find the associated GW amplitude and frequency to be $h_c = 2.7 \times 10^{-23}/{(Hz)}^{1/2}$ and $f_{gw} = 966 Hz$, for a source distance $\sim 55 kpc$. Detectability of the pulses by t he forthcoming GW anntenas is discussed and found likely.

gr-qc↗
arXiv1997

Implications of a constant observed braking index for young pulsar's spindown

The observed braking index n_{obs} which had been determined for a few young pulsars, had been found to differ from the expected value for a rotating magnetic dipole model. Also, the observational jerk parameter, determined for two of these pulsars, disagree with the theoretical prediction m_{obs} = 15 in both cases. We propose a simple model able to account for these differences, based on a growth of the "torque function" K = -(dot Omega)/(Omega^{n}), under the constraint that n_{obs} is a constant. We show that there is observational evidence supporting the latter hypotesis, and derive initial values for several physical quantities for the four pulsars whose n_{obs} have been measured.

astro-ph↗
arXiv1997

Gravitational Wave Emission from Galactic Radio Pulsars

We consider in this work continuous gravitational wave (GW) emission from non-axisymmetric radio pulsars. We treat in some detail the observational issues related to the known radio pulsar sample with the aim of unveiling the actual number of sources contributing to GW, which are likely to be the main contributors of GWs. It is shown that the operation of spheroidal GW detectors and full-size interferometers could detect this component of the radiation or impose useful limits on the effective oblateness of young radio pulsars.

astro-ph↗