25,528 research outputs found
Evidence for GeV emission from the Galactic Center Fountain
The region near the Galactic center may have experienced recurrent episodes
of injection of energy in excess of 10 ergs due to repeated
starbursts involving more than 10 supernovae. This hypothesis can be
tested by measurements of -ray lines produced by the decay of
radioactive isotopes and positron annihilation, or by searches for pulsars
produced during starbursts. Recent OSSE observations of 511 keV emission
extending above the Galactic center led to the suggestion of a starburst driven
fountain from the Galactic center. We present EGRET observations that might
support this picture.Comment: 5 pages, 1 embedded Postscript figure. To appear in the Proceedings
of the Fourth Compton Symposiu
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
The Production of Ti44 and Co60 in Supernova
The production of the radioactive isotopes Ti and Co in all
types of supernovae is examined and compared to observational constraints
including Galactic --ray surveys, measurements of the diffuse 511 keV
radiation, --ray observations of Cas A, the late time light curve of SN
1987A, and isotopic anomalies found in silicon carbide grains in meteorites.
The (revised) line flux from Ti decay in the Cas A supernova remnant
reported by COMPTEL on the Compton Gamma-Ray Observatory is near the upper
bound expected from our models. The necessary concurrent ejection of Ni
would also imply that Cas A was a brighter supernova than previously thought
unless extinction in the intervening matter was very large. Thus, if confirmed,
the reported amount of Ti in Cas A provides very interesting constraints
on both the supernova environment and its mechanism. The abundances of
Ti and Co ejected by Type II supernovae are such that
gamma-radiation from Ti decay SN 1987A could be detected by a future
generation of gamma-ray telescopes and that the decay of Co might
provide an interesting contribution to the late time light curve of SN 1987A
and other Type II supernovae. To produce the solar Ca abundance and
satisfy all the observational constraints, nature may prefer at least the
occasional explosion of sub-Chandrasekhar mass white dwarfs as Type Ia
supernovae. Depending on the escape fraction of positrons due to Co made
in all kinds of Type Ia supernovae, a significant fraction of the steady state
diffuse 511 keV emission may arise from the annihilation of positrons produced
during the decay of Ti to Ca. The Ca and Ti isotopic anomalies in
pre-solar grains confirm the production of Ti in supernovae and thatComment: 27 pages including 7 figures. uuencoded, compressed, postscript. in
press 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
Probing the evolving massive star population in Orion with kinematic and radioactive tracers
We assemble a census of the most massive stars in Orion, then use stellar
isochrones to estimate their masses and ages, and use these results to
establish the stellar content of Orion's individual OB associations. From this,
our new population synthesis code is utilized to derive the history of the
emission of UV radiation and kinetic energy of the material ejected by the
massive stars, and also follow the ejection of the long-lived radioactive
isotopes 26Al and 60Fe. In order to estimate the precision of our method, we
compare and contrast three distinct representations of the massive stars. We
compare the expected outputs with observations of 26Al gamma-ray signal and the
extent of the Eridanus cavity. We find an integrated kinetic energy emitted by
the massive stars of 1.8(+1.5-0.4)times 10^52 erg. This number is consistent
with the energy thought to be required to create the Eridanus superbubble. We
also find good agreement between our model and the observed 26Al signal,
estimating a mass of 5.8(+2.7-2.5) times 10^-4 Msol of 26Al in the Orion
region. Our population synthesis approach is demonstrated for the Orion region
to reproduce three different kinds of observable outputs from massive stars in
a consistent manner: Kinetic energy as manifested in ISM excavation, ionization
as manifested in free-free emission, and nucleosynthesis ejecta as manifested
in radioactivity gamma-rays. The good match between our model and the
observables does not argue for considerable modifications of mass loss. If
clumping effects turn out to be strong, other processes would need to be
identified to compensate for their impact on massive-star outputs. Our
population synthesis analysis jointly treats kinematic output and the return of
radioactive isotopes, which proves a powerful extension of the methodology that
constrains feedback from massive stars.Comment: Accepted for publication in A&A, 10 page
Photospheric Emission in the Joint GBM and Konus Prompt Spectra of GRB 120323A
GRB 120323A is a very intense short Gamma Ray Burst (GRB) detected
simultaneously during its prompt gamma-ray emission phase with the Gamma-ray
Burst Monitor (GBM) on board the Fermi Gamma-ray Space Telescope and the Konus
experiment on board the Wind satellite. GBM and Konus operate in the keV--MeV
regime, however, the GBM range is broader both toward the low and the high
parts of the gamma-ray spectrum. Analysis of such bright events provide a
unique opportunity to check the consistency of the data analysis as well as
cross-calibrate the two instruments. We performed time-integrated and coarse
time-resolved spectral analysis of GRB 120323A prompt emission. We conclude
that the analyses of GBM and Konus data are only consistent when using a
double-hump spectral shape for both data sets; in contrast, the single-hump of
the empirical Band function, traditionally used to fit GRB prompt emission
spectra, leads to significant discrepancies between GBM and Konus analysis
results. Our two-hump model is a combination of a thermal-like and a
non-thermal component. We interpret the first component as a natural
manifestation of the jet photospheric emission.Comment: 7 pages of article (3 figures and 1 table) + 3 pages of Appendix (3
figures). Submitted to ApJ on 2017 March 2
Gamma ray pulsar analysis from photon probability maps
A new method is presented of analyzing skymap-type gamma ray data. Each photon event is replaced by a probability distribution on the sky corresponding to the observing instrument's point spread function. The skymap produced by this process may be used for source detection or identification. Most important, the use of these photon weights for pulsar analysis promises significant improvement over traditional techniques
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