6,414 research outputs found
ON THE MEASUREMENT OF A COSMOLOGICAL DIPOLE IN THE PHOTON NUMBER COUNTS OF GAMMA-RAY BURSTS
If gamma-ray bursts are cosmological or in a halo distribution their
properties are expected to be isotropic (at least to 1st order). However, our
motion with respect to the burst parent population (whose proper frame is
expected to be that of the Cosmic Microwave Background (CMB), or that of a
static halo) will cause a dipole effect in the distribution of bursts and in
their photon number counts (together termed a Compton-Getting effect). We argue
that the photon number count information is necessary to distinguish a genuine
Compton-Getting effect from some other anisotropy and to fully test the proper
frame isotropy of the bursts. Using the 2B burst catalogue and the dipole
determined from the CMB, we find the surprising result that although the number
weighted distribution is consistent with isotropy, the fluence weighted dipole
has a correlation with the CMB dipole that has a probability of occuring only
10% of the time for an isotropic photon distribution. Furthermore, the photon
and number dipoles are inconsistent under the hypothesis of isotropy, at the
2-sigma level. This could be an indication that a non-negligible fraction of
gamma-ray bursts originate in the local, anisotropic universe. (shortened
Abstract)Comment: Accepted by ApJ. Self-unpacking (use csh), uuencoded, compressed
Postscript, 16 pages + 4 Figures (5 files
Electron-positron pair production in the external electromagnetic field of colliding relativistic heavy ions
The results concerning the production in peripheral highly
relativistic heavy-ion collisions presented in a recent paper by Baltz {\em{et
al.}} are rederived in a very straightforward manner. It is shown that the
solution of the Dirac equation directly leads to the multiplicity, i.e. to the
total number of electron-positron pairs produced by the electromagnetic field
of the ions, whereas the calculation of the single pair production probability
is much more involved. A critical observation concerns the unsolved problem of
seemingly absent Coulomb corrections (Bethe-Maximon corrections) in pair
production cross sections. It is shown that neither the inclusion of the
vacuum-vacuum amplitude nor the correct interpretation of the solution of the
Dirac equation concerning the pair multiplicity is able the explain (from a
fundamental point of view) the absence of Coulomb corrections. Therefore the
contradiction has to be accounted to the treatment of the high energy limit.Comment: 6 pages, 4 Postscript figures, uses svjour.cls/svepj.cl
My New Goal: Follow Ben Franklin\u27s Rule to Set Aside 1 Hour a Day to Learn
Benjamin Franklin is famous as one of the Founding Fathers of the United States and helping to draft the Constitution and the Declaration of Independence. Franklin was a Renaissance man. Besides his career as a diplomat, public servant and accomplished printer, he is credited with discoveries about electricity and inventing bifocals, among other things
With a Little Help From My Friends...
Do I have enough friends
Do We Need to Use (and Discard) So Many Plastic Bags Each Year?
Much of the time I hate plastic bags
A turbojet-boosted two-stage-to-orbit space transportation system design study
The concept to use twin turbo-powered boosters for acceleration to supersonic staging speed followed by an all rocket powered orbiter stage was proposed. A follow-on design study was then made of the concept with the performance objective of placing a 29,483 Kg payload into a .2.6 X 195.3 km orbit. The study was performed in terms of analysis and trade studies, conceptual design, utility and economic analysis, and technology assessment. Design features of the final configuration included: strakes and area rule for improved take off and low transonic drag, variable area inlets, exits and turbine, and low profile fixed landing gear for turbojet booster stage. The payload required an estimated GLOW of 1,270,000 kg for injection in orbit. Each twin booster required afterburning turbojet engines each with a static sea level thrust rating of 444,800 N. Life cycle costs for this concept were comparable to a SSTO/SLED concept except for increased development cost due to the turbojet engine propulsion system
A possible dearth of hot gas in galaxy groups at intermediate redshift
We examine the X-ray luminosity of galaxy groups in the CNOC2 survey, at
redshifts 0.1 < z < 0.6. Previous work examining the gravitational lensing
signal of the CNOC2 groups has shown that they are likely to be genuine,
gravitationally bound objects. Of the 21 groups in the field of view of the
EPIC-PN camera on XMM-Newton, not one was visible in over 100 ksec of
observation, even though three of the them have velocity dispersions high
enough that they would easily be visible if their luminosities scaled with
their velocity dispersions in the same way as nearby groups' luminosities
scale. We consider the possibility that this is due to the reported velocity
dispersions being erroneously high, and conclude that this is unlikely. We
therefore find tentative evidence that groups at intermediate redshift are
underluminous relative to their local cousins.Comment: 16 pages, 5 figures, reference added in section 1, typos corrected,
published in Ap
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