4,101 research outputs found
Absorption of High Energy Gamma-Rays by Low Energy Intergalactic Photons
Following our previously proposed technique, we have used the recent
gamma-ray observations of Mrk 421 to place theoretically significant
constraints on and possible estimates of the intergalactic infrared radiation
field (IIRF) which are consistent with normal galactic IR production by stars
and dust and rule out exotic mechanisms proposed to produce a larger IIRF.
Using models for the low energy intergalactic photon spectrum from microwave to
UV energies, we calculate the opacity of inter- galactic space to gamma-rays as
a function of energy and redshift. These calculations indicate that the GeV
gamma-ray burst recently observed by the EGRET experiment on CGRO originates at
a redshift less than approximately 1.5.Comment: 12 pg., uuencoded, Z-compressed ps file (includes figures), To be
published in Space Sci. Re
Flavor decomposition of the elastic nucleon electromagnetic form factors
The u- and d-quark contributions to the elastic nucleon electromagnetic form
factors have been determined using experimental data on GEn, GMn, GpE, and GpM.
Such a flavor separation of the form factors became possible up to 3.4 GeV2
with recent data on GEn from Hall A at JLab. At a negative four-momentum
transfer squared Q2 above 1 GeV2, for both the u- and d-quark components, the
ratio of the Pauli form factor to the Dirac form factor, F2/F1, was found to be
almost constant, and for each of F2 and F1 individually, the d-quark portions
of both form factors drop continuously with increasing Q2.Comment: 4 pages, 3 figure
2-Dust : a Dust Radiative Transfer Code for an Axisymmetric System
We have developed a general purpose dust radiative transfer code for an
axisymmetric system, 2-Dust, motivated by the recent increasing availability of
high-resolution images of circumstellar dust shells at various wavelengths.
This code solves the equation of radiative transfer following the principle of
long characteristic in a 2-D polar grid while considering a 3-D radiation field
at each grid point. A solution is sought through an iterative scheme in which
self-consistency of the solution is achieved by requiring a global luminosity
constancy throughout the shell. The dust opacities are calculated through Mie
theory from the given size distribution and optical properties of the dust
grains. The main focus of the code is to obtain insights on (1) the global
energetics of dust grains in the shell (2) the 2-D projected morphologies that
are strongly dependent on the mixed effects of the axisymmetric dust
distribution and inclination angle of the shell. Here, test models are
presented with discussion of the results. The code can be supplied with a
user-defined density distribution function, and thus, is applicable to a
variety of dusty astronomical objects possessing the axisymmetric geometry.Comment: To be published in ApJ, April 2003 issue; 13 pages, 4 tables, 17
figures, 5-page appendix (no figures for the main text included in this
preprint). For the complete preprint and code distribution, contact the
author
Testing the Color Charge and Mass Dependence of Parton Energy Loss with Heavy-to-light Ratios at RHIC and LHC
The ratio of nuclear modification factors of high-pT heavy-flavored mesons to
light-flavored hadrons (``heavy-to-light ratio'') in nucleus-nucleus collisions
tests the partonic mechanism expected to underlie jet quenching. Heavy-to-light
ratios are mainly sensitive to the mass and color-charge dependences of
medium-induced parton energy loss. Here, we assess the potential for
identifying these two effects in D and B meson production at RHIC and at the
LHC. To this end, we supplement the perturbative QCD factorized formalism for
leading hadron production with radiative parton energy loss. For D meson
spectra at high but experimentally accessible transverse momentum (10 < pT < 20
GeV) in Pb-Pb collisions at the LHC, we find that charm quarks behave
essentially like light quarks. However, since light-flavored hadron yields are
dominated by gluon parents, the heavy-to-light ratio of D mesons is a sensitive
probe of the color charge dependence of parton energy loss. In contrast, due to
the larger b quark mass, the medium modification of B mesons in the same
kinematical regime provides a sensitive test of the mass dependence of parton
energy loss. At RHIC energies, the strategies for identifying and disentangling
the color charge and mass dependence of parton energy loss are more involved
because of the smaller kinematical range accessible. We argue that at RHIC, the
kinematical regime best suited for such an analysis of D mesons is 7 < pT < 12
GeV, whereas the study of lower transverse momenta is further complicated due
to the known dominant contribution of additional, particle species dependent,
non-perturbative effects.Comment: 21 pages RevTex, 9 Figure
On the Submillimeter Opacity of Protoplanetary Disks
Solid particles with the composition of interstellar dust and power-law size
distribution dn/da propto a^{-p} for a 3 lambda and 3 <
p < 4 will have submm opacity spectral index beta(lambda) = dln(kappa)/dln(nu)
approx (p-3) beta_{ism}, where beta_{ism} approx 1.7 is the opacity spectral
index of interstellar dust material in the Rayleigh limit. For the power-law
index p approx 3.5 that characterizes interstellar dust, and that appears
likely for particles growing by agglomeration in protoplanetary disks, grain
growth to sizes a > 3 mm will result in beta(1 mm) < ~1. Grain growth can
naturally account for beta approx 1 observed for protoplanetary disks, provided
that a_{max} > ~ 3 lambda.Comment: Submitted to ApJ. 17 pages, 6 figure
Shadowing Effects on the Nuclear Suppression Factor, R_dAu, in d+Au Interactions
We explore how nuclear modifications to the nucleon parton distributions
affect production of high transverse momentum hadrons in deuteron-nucleus
collisions. We calculate the charged hadron spectra to leading order using
standard fragmentation functions and shadowing parameterizations. We obtain the
d+Au to pp ratio both in minimum bias collisions and as a function of
centrality. The minimum bias results agree reasonably well with the BRAHMS data
while the calculated centrality dependence underestimates the data and is a
stronger function of p_T than the data indicate.Comment: 18 pages, 3 figures, final version, Phys. Rev. C in pres
New unidentified H.E.S.S. Galactic sources
H.E.S.S. is one of the most sensitive instruments in the very high energy
(VHE; > 100 GeV) gamma-ray domain and has revealed many new sources along the
Galactic Plane. After the successful first VHE Galactic Plane Survey of 2004,
H.E.S.S. has continued and extended that survey in 2005-2008, discovering a
number of new sources, many of which are unidentified. Some of the unidentified
H.E.S.S. sources have several positional counterparts and hence several
different possible scenarios for the origin of the VHE gamma-ray emission;
their identification remains unclear. Others have so far no counterparts at any
other wavelength. Particularly, the lack of an X-ray counterpart puts serious
constraints on emission models. Several newly discovered and still unidentified
VHE sources are reported here.Comment: ICRC 2009 proceeding
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