12,087 research outputs found
Unidentified EGRET sources in the Galaxy
Identifying gamma-ray sources in the Galaxy is hampered by their poor
localization, source confusion, and the large variety of potential emitters.
Neutron stars and their environment offer various ways to power gamma-ray
sources: pulsed emission from the open magnetosphere and unpulsed gamma rays
from the wind nebula and from the cosmic rays accelerated in the supernova
remnant. While the latter still awaits confirmation, new candidate associations
bring forward the importance of 10-kyr old pulsars as GeV sources, with a
diversity that will help constrain the acceleration mechanisms near the pulsar
and in the wind. Theoretical interest in the gamma-ray activity of X-ray
binaries and micro-quasars has also been revived by the emergence of a subset
of variable sources in the inner Galaxy and another one in the halo.Comment: 8 pages, Proc. of the Texas Symposium, 2002, Florence, Ital
Off-Beam Gamma-Ray Pulsars and Unidentified EGRET Sources in the Gould Belt
We investigate whether gamma-ray pulsars viewed at a large angle to the
neutron star magnetic pole could contribute to the new population of galactic
unidentified EGRET sources associated with the Gould Belt. The faint, soft
nature of these sources is distinctly different from both the properties of
unidentified EGRET sources along the galactic plane and of the known gamma-ray
pulsars. We explore the possibility, within the polar cap model, that some of
these sources are emission from pulsars seen at lines of sight that miss both
the bright gamma-ray cone beams and the radio beam. The off-beam gamma-rays
come from high-altitude curvature emission of primary particles, are radiated
over a large solid angle and have a much softer spectrum than that of the main
beams. We estimate that the detectability of such off-beam emission is about a
factor of 4-5 higher than that of the on-beam emission. At least some of the
radio-quiet Gould Belt sources detected by EGRET could therefore be such
off-beam gamma-ray pulsars. GLAST should be able to detect pulsations in most
of these sources.Comment: 5 pages, uses emulateapj.sty, accepted for publication in ApJ Letter
Young and middle age pulsar light-curve morphology: Comparison of Fermi observations with gamma-ray and radio emission geometries
Thanks to the huge amount of gamma-ray pulsar photons collected by the Fermi
Large Area Telescope since June 2008, it is now possible to constrain gamma-ray
geometrical models by comparing simulated and observed light-curve
morphological characteristics. We assumed vacuum-retarded dipole pulsar
magnetic field and tested simulated and observed morphological light-curve
characteristics in the framework of two pole emission geometries, Polar Cap
(PC), radio, and Slot Gap (SG), and Outer Gap (OG)/One Pole Caustic (OPC)
emission geometries. We compared simulated and observed/estimated light-curve
morphological parameters as a function of observable and non-observable pulsar
parameters. The PC model gives the poorest description of the LAT pulsar
light-curve morphology. The OPC best explains both the observed gamma-ray peak
multiplicity and shape classes. The OPC and SG models describe the observed
gamma-ray peak-separation distribution for low- and high-peak separations,
respectively. This suggests that the OPC geometry best explains the single-peak
structure but does not manage to describe the widely separated peaks predicted
in the framework of the SG model as the emission from the two magnetic
hemispheres. The OPC radio-lag distribution shows higher agreement with
observations suggesting that assuming polar radio emission, the gamma-ray
emission regions are likely to be located in the outer magnetosphere. The
larger agreement between simulated and LAT estimations in the framework of the
OPC suggests that the OPC model best predicts the observed variety of profile
shapes. The larger agreement between observations and the OPC model jointly
with the need to explain the abundant 0.5 separated peaks with two-pole
emission geometries, calls for thin OPC gaps to explain the single-peak
geometry but highlights the need of two-pole caustic emission geometry to
explain widely separated peaks.Comment: 28 pages, 20 figures, 8 tables; accepted for publication in Astronomy
and Astrophysic
Raman scattering through surfaces having biaxial symmetry
Magnetic Raman scattering in two-leg spin ladder materials and the
relationship between the anisotropic exchange integrals are analyzed by P. J.
Freitas and R. R. P. Singh in Phys. Rev. B, {\bf 62}, 14113 (2000). The angular
dependence of the two-magnon scattering is shown to provide information for the
magnetic anisotropy in the Sr_14Cu_24O_41 and La_6Ca_8Cu_24O_41 compounds. We
point out that the experimental results of polarized Raman measurements at
arbitrary angles with respect to the crystal axes have to be corrected for the
light ellipticity induced inside the optically anisotropic crystals. We refer
quantitatively to the case of Sr_14Cu_24O_41 and discuss potential implications
for spectroscopic studies in other materials with strong anisotropy.Comment: To be published as a Comment in Phys. Rev.
- …