32,564 research outputs found
Gamma-ray spectroscopy: The diffuse galactic glow
The goal of this project is the development of a numerical code that provides statistical models of the sky distribution of gamma-ray lines due to the production of radioactive isotopes by ongoing Galactic nucleosynthesis. We are particularly interested in quasi-steady emission from novae, supernovae, and stellar winds, but continuum radiation and transient sources must also be considered. We have made significant progress during the first half period of this project and expect the timely completion of a code that can be applied to Oriented Scintillation Spectrometer Experiment (OSSE) Galactic plane survey data
Properties of GRB Host Galaxies
The transients following GRB970228 and GRB970508 showed that these (and
probably all) GRBs are cosmological. However, the host galaxies expected to be
associated with these and other bursts are largely absent, indicating that
either bursts are further than expected or the host galaxies are underluminous.
This apparent discrepancy does not invalidate the cosmological hypothesis, but
instead host galaxy observations can test more sophisticated models.Comment: 5 pages, AIPPROC LaTeX, to appear in "Gamma-Ray Bursts, 4th
Huntsville Symposium," eds. C. Meegan, R. Preece and T. Koshu
Community Change for Youth Development in Kansas City
Kansas City, Missouri, is one of six sites in Community Change for Youth Development (CCYD), a national demonstration project aiming to increase basic supports and opportunities available to youth aged 12-20. The lead agency is the YMCA of Greater Kansas City; because of its considerable organizational capacity and relationship with funders, the YMCA was successful in operating and expanding CCYD. This report focuses on the benefits of working with the YMCA of Greater Kansas City and the challenges faced by the organization in leading a community-based initiative in three urban neighborhoods
Search for Supergalactic Anisotropies in the 3B Catalog
The angular distribution of GRBs is isotropic, while the brightness
distribution of bursts shows a reduced number of faint events. These
observations favor a cosmological burst origin. If GRBs are indeed at
cosmological distances and if they trace luminous matter, we must eventually
find an anisotropic distribution of bright bursts. If a significant number of
bursts originate at redshifts less than z~1, the concentration of nearby
galaxies towards the supergalactic plane is pronounced enough that we could
discover the corresponding clustering of burst locations. We used the 3B
catalog to search for a pattern visible in supergalactic coordinates. No
compelling evidence for anisotropies was found. The absence of anisotropies in
SG coordinates implies a minimum sampling distance of 200h^-1 Mpc.Comment: 5 pages, uuencoded postscript, to appear in the Proceedings of the
Huntsville Conference on Gamma Ray Burst
A BeppoSAX observation of the supersoft source 1E 0035.4-7230
Results from a 37,000 s BeppoSAX Low-Energy Concentrator Spectrometer (LECS)
observation of the supersoft source SMC 13 (=1E 0035.4-7230) in the Small
Magellanic Cloud are reported. The BeppoSAX spectrum is fitted either with a
blackbody spectrum with an effective temperature kT = 26-58 eV, an LTE white
dwarf atmosphere spectrum with kT = 35-50 eV, or a non-LTE white dwarf
atmosphere spectrum with kT = 25-32 eV. The bolometric luminosity is < 8 10^37
erg s-1 and < 3 10^37 erg s^-1 for the LTE and the non-LTE spectrum. We also
applied a spectral fit to combined spectra obtained with BeppoSAX LECS and with
ROSAT PSPC. The kT derived for the non-LTE spectrum is 27-29 eV, the bolometric
luminosity is 1.1-1.2 10^37 erg s^-1. We can exclude any spectrally hard
component with a luminosity > 2 10^35 erg s^-1 (for a bremmstrahlung with a
temperature of 0.5 keV) at a distance of 60 kpc. The LTE temperature is
therefore in the range 5.5+/-0.2 10^5 K and the non-LTE temperature in the
range 3.25+/-0.16 10^5 K. Assuming the source is on the stability line for
atmospheric nuclear burning, we constrain the white dwarf mass from the LTE and
the non-LTE fit to ~1.1 M-solar and ~0.9 M-solar respectively. However, the
temperature and luminosity derived with the non-LTE model for 1E 0035.4-7230 is
consistent with a lower mass M~0.6-0.7 M-solar white dwarf as predicted by Sion
and Starrfield (1994). At the moment, neither of these two alternatives for the
white dwarf mass can be excluded.Comment: 6 pages, accepted by A&A March 30th 199
Formation Rates of Black Hole Accretion Disk Gamma-Ray Bursts
While many models have been proposed for GRBs, those currently favored are
all based upon the formation of and/or rapid accretion into stellar mass black
holes. We present population synthesis calculations of these models using a
Monte Carlo approach in which the many uncertain parameters intrinsic to such
calculations are varied. We estimate the event rate for each class of model as
well as the propagation distance for those having significant delay between
formation and burst production, i.e., double neutron star (DNS) mergers and
black hole-neutron star (BH/NS) mergers. For reasonable assumptions regarding
the many uncertainties in population synthesis, we calculate a daily event rate
in the universe for i) merging neutron stars: ~100/day; ii) neutron-star black
hole mergers: ~450/day; iii) collapsars: ~10,000/day; iv) helium star black
hole mergers: ~1000/day; and v) white dwarf black hole mergers: ~20/day. The
range of uncertainty in these numbers however, is very large, typically two to
three orders of magnitude. These rates must additionally be multiplied by any
relevant beaming factor and sampling fraction (if the entire universal set of
models is not being observed). Depending upon the mass of the host galaxy, half
of the DNS and BH/NS mergers will happen within 60kpc (for a Milky-Way massed
galaxy) to 5Mpc (for a galaxy with negligible mass) from the galactic center.
Because of the delay time, neutron star and black hole mergers will happen at a
redshift 0.5 to 0.8 times that of the other classes of models. Information is
still lacking regarding the hosts of short hard bursts, but we suggest that
they are due to DNS and BH/NS mergers and thus will ultimately be determined to
lie outside of galaxies and at a closer mean distance than long complex bursts
(which we attribute to collapsars).Comment: 57 pages total, 23 figures, submitted by Ap
Monopoles and dyons in SO(3) gauged Skyrme models
Three dimensional SO(3) gauged Skyrme models characterised by specific
potentials imposing special asymptotic values on the chiral field are
considered. These models are shown to support finite energy solutions with
nonvanishing magnetic and electrix flux, whose energies are bounded from below
by two distinct charges - the magnetic (monopole) charge and a non-integer
version of the Baryon charge. Unit magnetic charge solutions are constructed
numerically and their properties characterised by the chosen asymptotics and
the Skyrme coupling are studied. For a particular value of the chosen
asymptotics, charge-2 axially symmetric solutions are also constructed and the
attractive nature of the like-monopoles of this system are exhibited. As an
indication towards the possible existence of large clumps of monopoles, some
consideration is given to axially symmetric monopoles of charges-2,3,4.Comment: 15 pages, 4 Postscript figure
Symmetries and Triplet Dispersion in a Modified Shastry-Sutherland Model for SrCu_2(BO_3)_2
We investigate the one-triplet dispersion in a modified Shastry-Sutherland
Model for SrCu_2(BO_3)_2 by means of a series expansion about the limit of
strong dimerization. Our perturbative method is based on a continuous unitary
transformation that maps the original Hamiltonian to an effective, energy
quanta conserving block diagonal Hamiltonian H_{eff}. The dispersion splits
into two branches which are nearly degenerated. We analyse the symmetries of
the model and show that space group operations are necessary to explain the
degeneracy of the dispersion at k=0 and at the border of the magnetic Brillouin
zone. Moreover, we investigate the behaviour of the dispersion for small |k|
and compare our results to INS data.Comment: 9 pages, 8 figures accepted by J. Phys.: Condens. Matte
- …