83 research outputs found
Spectrum and Duration of Delayed MeV-GeV Emission of Gamma-Ray Bursts in Cosmic Background Radiation Fields
We generally analyze prompt high-energy emission above a few hundreds of GeV
due to synchrotron self-Compton scattering in internal shocks. However, such
photons cannot be detected because they may collide with cosmic infrared
background photons, leading to electron/positron pair production.
Inverse-Compton scattering of the resulting electron/positron pairs off cosmic
microwave background photons will produce delayed MeV-GeV emission, which may
be much stronger than a typical high-energy afterglow in the external shock
model. We expand on the Cheng & Cheng model by deriving the emission spectrum
and duration in the standard fireball shock model. A typical duration of the
emission is ~ 10^3 seconds, and the time-integrated scattered photon spectrum
is nu^{-(p+6)/4}, where p is the index of the electron energy distribution
behind internal shocks. This is slightly harder than the synchrotron photon
spectrum, nu^{-(p+2)/2}. The lower energy property of the scattered photon
spectrum is dependent on the spectral energy distribution of the cosmic
infrared background radiation. Therefore, future observations on such delayed
MeV-GeV emission and the higher-energy spectral cutoff by the Gamma-Ray Large
Area Space Telescope (GLAST) would provide a probe of the cosmic infrared
background radiation.Comment: 5 pages, accepted for publication in Ap
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Longevity Risk and Capital Markets: The 2015-16 Update
International audienc
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Longevity Risk and Hedging Solutions
Longevity risk—the risk of unanticipated increases in life expectancy—has only recently been recognized as a significant global risk that has materially raised the costs of providing pensions and annuities. We first discuss historical trends in the evolution of life expectancy and then analyze the hedging solutions that have been developed for managing longevity risk. One set of solutions has come directly from the insurance industry: pension buyouts, buy-ins, and bulk annuity transfers. Another complementary set of solutions has come from the capital markets: longevity swaps and q-forwards. This has led to hybrid solutions such as synthetic buy-ins. We then review the evolution of the market for longevity risk transfer, which began in the UK in 2006 and is arguably the most important sector of the broader “life market.” An important theme in the development of the longevity market has been the innovation originating from the combined involvement of insurance, banking, and private equity participants
Evidence for Intergalactic Absorption in the TeV Gamma-Ray Spectrum of Mkn 501
The recent HEGRA observations of the blazar Mkn 501 show strong curvature in
the very high energy gamma-ray spectrum. Applying the gamma-ray opacity derived
from an empirically based model of the intergalactic infrared background
radiation field (IIRF), to these observations, we find that the intrinsic
spectrum of this source is consistent with a power-law: dN/dE~ E^-alpha with
alpha=2.00 +/- 0.03 over the range 500 GeV - 20 TeV. Within current synchrotron
self-Compton scenarios, the fact that the TeV spectral energy distribution of
Mkn 501 does not vary with luminosity, combined with the correlated, spectrally
variable emission in X-rays, as observed by the BeppoSAX and RXTE instruments,
also independently implies that the intrinsic spectrum must be close to
alpha=2. Thus, the observed curvature in the spectrum is most easily understood
as resulting from intergalactic absorption.Comment: 7 pages, 1 figure, accepted in ApJ Letters 1999 April
A kinematic study of Open Clusters: implications for their origin
The Galactic population of open clusters provides an insight into star
formation in the Galaxy. The open cluster catalogue by Dias et al.(2002b) is a
rich source of data, including kinematic information. This large sample made it
possible to carry out a systematic analysis of 481 open cluster orbits, using
parameters based on orbit eccentricity and separation from the Galactic plane.
These two parameters may be indicative of origin, and we find them to be
correlated. We also find them to be correlated with metallicity, another
parameter suggested elsewhere to be a marker for origin in that high values of
any of these two parameters generally indicates a low metallicity ([Fe/H]
Solar0.2 dex). The resulting analysis points to four open clusters in the
catalogue being of extra-Galactic origin by impact of high velocity cloud on
the disk: Berkeley21, 32, 99, and Melotte66, with a possible further four due
to this origin (NGC2158, 2420, 7789, IC1311). A further three may be due to
Galactic globular cluster impact on the disk i.e of internal Galactic origin
(NGC6791, 1817, and 7044).Comment: 14 pages, 816 figures, accepted for publication in MNRAS 14-May-201
Discovery of Very High Energy Gamma-Ray Emission from the BL Lac Object H2356-309 with the H.E.S.S. Cherenkov Telescopes
The extreme synchrotron BL Lac object H2356-309, located at a redshift of z =
0.165, was observed from June to December 2004 with a total exposure of approx.
40 h live-time with the H.E.S.S. (High Energy Stereoscopic System) array of
atmospheric-Cherenkov telescopes (ACTs). Analysis of this data set yields, for
the first time, a strong excess of 453 gamma-rays (10 standard deviations above
background) from H2356-309, corresponding to an observed integral flux above
200 GeV of I(>200GeV) = (4.1+-0.5) 10^12 cm^-2.s^-1 (statistical error only).
The differential energy spectrum of the source between 200 GeV and 1.3 TeV is
well-described by a power law with a normalisation (at 1 TeV) of N_0 = (3.00 +-
0.80_stat +- 0.31_sys) 10^-13 cm^-2.s^-1.TeV^-1 and a photon index of Gamma =
3.09 +- 0.24_stat +- 0.10_sys. H2356-309 is one of the most distant BL Lac
objects detected at very-high-energy gamma-rays so far. Results from
simultaneous observations from ROTSE-III (optical), RXTE (X-rays) and NRT
(radio) are also included and used together with the H.E.S.S. data to constrain
a single-zone homogeneous synchrotron self-Compton (SSC) model. This model
provides an adequate fit to the H.E.S.S. data when using a reasonable set of
model parameters.Comment: 7 pages, 4 figures, accepted for publication in Astronomy and
Astrophysics (05/07/2006
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