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Publication date
1998
fields
Physics
and research's language is
English
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In a recent paper (Ushov, PRL, 80, 230, 1998), it has been claimed that the bare surface of a strange star can emit electron-positron pairs of luminosity ~10^{51} ergs/s for about 10s. If true, obviously, this mechanism may explain the origin of cosmic Gamma Ray Bursts. However, we point out that such a mechanism is does not work because (i) if pair production really occurs the supposed pre-existing supercritical electric field will be quenched and this discharge process may at best release ~10^{24} ergs of electromagnetic energy, and (ii) there is no way by which the trapped core thermal energy of few 10^{52} ergs can be transmitted electromagnetically on a time scale of ~10s or even on a much larger time scale. The only way the hot core can cool on a time scale of ~10 s or much shorter is by the well known process of emission of nu-antinu pairs.
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A new method for calculations of electron-positron pair-creation probabilities in low-energy heavy-ion collisions is developed. The approach is based on the propagation of all one-electron states via the numerical solving of the time-dependent Dirac equation in the monopole approximation. The electron wave functions are represented as finite sums of basis functions constructed from B-splines using the dual-kinetic-balance technique. The calculations of the created particle numbers and the positron energy spectra are performed for the collisions of bare nuclei at the energies near the Coulomb barrier with the Rutherford trajectory and for different values of the nuclear charge and the impact parameter. To examine the role of the spontaneous pair creation the collisions with a modified velocity and with a time delay are also considered. The obtained results are compared with the previous calculations and the possibility of observation of the spontaneous pair creation is discussed.
The photon emissivity from the bremsstrahlung process ee-> eegamma occuring in the electrosphere at the bare surface of a strange quark star is calculated. For surface temperatures T<10^9K, the photon flux exceeds that of e+e- pairs that are produced via the Schwinger mechanism in the presence of a strong electric field that binds electrons to the surface of the quark star. The average energy of photons emitted from the bremsstrahlung process can be 0.5 MeV or more, which is larger than that in e+e- pair annihilation. The observation of this distinctive photon spectrum would constitute an unmistakable signature of a strange quark star and shed light on color superconductivity at stellar densities.
The probabilities of bound-free electron-positron pair creation are calculated for head-on collisions of bare uranium nuclei beyond the monopole approximation. The calculations are based on the numerical solving of the time-dependent Dirac equation in the target reference frame with multipole expansion of the projectile potential. In addition, the energy dependence of the pair-creation cross section is studied in the monopole approximation.
Observations to date cannot distinguish neutron stars from self-bound bare quark stars on the basis of their gross physical properties such as their masses and radii alone. However, their surface luminosity and spectral characteristics can be significantly different. Unlike a normal neutron star, a bare quark star can emit photons from its surface at super-Eddington luminosities for an extended period of time. We present a calculation of the photon bremsstrahlung rate from the bare quark stars surface, and indicate improvements that are required for a complete characterization of the spectrum. The observation of this distinctive photon spectrum would constitute an unmistakable signature of a strange quark star and shed light on color superconductivity at stellar densities.
We investigate the stability and $e^+e^-$ pair creation of supercritically charged superheavy nuclei, $ud$QM nuggets, strangelets, and strangeon nuggets based on Thomas-Fermi approximation. The model parameters are fixed by reproducing their masses and charge properties reported in earlier publications. It is found that $ud$QM nuggets, strangelets, and strangeon nuggets may be more stable than ${}^{56}$Fe at $Agtrsim 315$, $5times10^4$, and $1.2times10^8$, respectively. For those stable against neutron emission, the most massive superheavy element has a baryon number $sim$965, while $ud$QM nuggets, strangelets, and strangeon nuggets need to have baryon numbers larger than $39$, 433, and $2.7times10^5$. The $e^+e^-$ pair creation will inevitably start for superheavy nuclei with charge numbers $Zgeq177$, $ud$QM nuggets with $Zgeq163$, strangelets with $Zgeq 192$, and strangeon nuggets with $Zgeq 212$. A universal relation $Q/R_e = left(m_e - bar{mu}_eright)/alpha$ is obtained at a given electron chemical potential $bar{mu}_e$, where $Q$ is the total charge and $R_e$ the radius of electron cloud. This predicts the maximum charge number by taking $bar{mu}_e=-m_e$. For supercritically charged objects with $bar{mu}_e<-m_e$, the decay rate for $e^+e^-$ pair production is estimated based on the JWKB approximation. It is found that most positrons are emitted at $tlesssim 10^{-15}$ s, while a long lasting positron emission is observed for large objects with $Rgtrsim 1000$ fm. The emission and annihilation of positrons from supercritically charged objects may be partially responsible for the short $gamma$-ray burst during the merger of binary compact stars, the 511 keV continuum emission, as well as the narrow faint emission lines in X-ray spectra from galaxies and galaxy clusters.
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