Soft gamma repeaters outside the Local group

We propose that the best sites to search for SGRs outside the Local group are galaxies with active massive star formation. Different possibilities to observe SGR activity from these sites are discussed. In particular we searched for giant flares from nearby galaxies ($\sim 2$ -- 4 Mpc) M82, M83, NGC 253, and NGC 4945 in the BATSE data. No candidates alike giant SGR flares were found. The absence of such detections implies that the rate of giant flares with energy release in the initial spike above $0.5 \cdot 10^{44}$ erg is less then 1/25 yr$^{-1}$ in our Galaxy. However, hyperflares similar to the one of 27 December 2004 can be observed from larger distances. Nevertheless, we do not see any significant excess of short GRBs from the Virgo galaxy cluster and from galaxies Arp 299 and NGC 3256 with extremely high star formation rate. This implies that the galactic rate of hyperflares with energy release $\sim 10^{46}$ erg is less than $\sim 10^{-3}$ yr$^{-1}$. With this constraint the fraction of possible extragalactic SGR hyperflares among BATSE short GRBs should not exceed few percents. We present a list of short GRBs coincident with galaxies mentioned above, and discuss the possibility that some of them are SGR giant flares. We propose that the best target for observations of extragalactic SGR flares by {\it Swift} is the Virgo cluster.Comment: 14 pages with 3 figures; accepted to MNRAS (final version

Fermion Pair Production From an Electric Field Varying in Two Dimensions

The Hamiltonian describing fermion pair production from an arbitrarily time-varying electric field in two dimensions is studied using a group-theoretic approach. We show that this Hamiltonian can be encompassed by two, commuting SU(2) algebras, and that the two-dimensional problem can therefore be reduced to two one-dimensional problems. We compare the group structure for the two-dimensional problem with that previously derived for the one-dimensional problem, and verify that the Schwinger result is obtained under the appropriate conditions.Comment: Latex, 14 pages of text. Full postscript version available via the worldwide web at http://nucth.physics.wisc.edu/ or by anonymous ftp from ftp://nucth.physics.wisc.edu:/pub/preprints

The Zel'dovich effect and evolution of atomic Rydberg spectra along the Periodic Table

In 1959 Ya. B. Zel'dovich predicted that the bound-state spectrum of the non-relativistic Coulomb problem distorted at small distances by a short-range potential undergoes a peculiar reconstruction whenever this potential alone supports a low-energy scattering resonance. However documented experimental evidence of this effect has been lacking. Previous theoretical studies of this phenomenon were confined to the regime where the range of the short-ranged potential is much smaller than Bohr's radius of the Coulomb field. We go beyond this limitation by restricting ourselves to highly-excited s states. This allows us to demonstrate that along the Periodic Table of elements the Zel'dovich effect manifests itself as systematic periodic variation of the Rydberg spectra with a period proportional to the cubic root of the atomic number. This dependence, which is supported by analysis of experimental and numerical data, has its origin in the binding properties of the ionic core of the atom.Comment: 17 pages, 12 figure

Evolution of isolated neutron stars in globular clusters: number of Accretors

With a simple model from the point of view of population synthesis we try to verify an interesting suggestion made by Pfahl & Rappaport (2001) that dim sources in globular clusters (GCs) can be isolated accreting neutron stars (NSs). Simple estimates show, that we can expect about 0.5-1 accreting isolated NS per typical GC with $M=10^5 M_{\odot}$ in correspondence with observations. Properties of old accreting isolated NSs in GCs are briefly discussed. We suggest that accreting NSs in GCs experienced significant magnetic field decay.Comment: 6 pages, no figures. Submitted to Astronomical and Astrophysical Transactions (style included