14,553 research outputs found
Gamma-ray emission from globular clusters
Over the last few years, the data obtained using the Large Area Telescope
(LAT) aboard the Fermi Gamma-ray Space Telescope has provided new insights on
high-energy processes in globular clusters, particularly those involving
compact objects such as Millisecond Pulsars (MSPs). Gamma-ray emission in the
100 MeV to 10 GeV range has been detected from more than a dozen globular
clusters in our galaxy, including 47 Tucanae and Terzan 5. Based on a sample of
known gamma-ray globular clusters, the empirical relations between gamma-ray
luminosity and properties of globular clusters such as their stellar encounter
rate, metallicity, and possible optical and infrared photon energy densities,
have been derived. The measured gamma-ray spectra are generally described by a
power law with a cut-off at a few gigaelectronvolts. Together with the
detection of pulsed gamma-rays from two MSPs in two different globular
clusters, such spectral signature lends support to the hypothesis that
gamma-rays from globular clusters represent collective curvature emission from
magnetospheres of MSPs in the clusters. Alternative models, involving
Inverse-Compton (IC) emission of relativistic electrons that are accelerated
close to MSPs or pulsar wind nebula shocks, have also been suggested.
Observations at >100 GeV by using Fermi/LAT and atmospheric Cherenkov
telescopes such as H.E.S.S.-II, MAGIC-II, VERITAS, and CTA will help to settle
some questions unanswered by current data.Comment: 11 pages, 7 figures, 2 tables, J. Astron. Space Sci., in pres
Is the meson dynamically generated?
We study the problem whether the meson is generated `dynamically'. A
pedagogical analysis on the toy O(N) linear sigma model is performed and we
find that the large limit and the limit does not
commute. The sigma meson may not necessarily be described as a dynamically
generated resonance. On the contrary, the sigma meson may be more appropriately
described by considering it as an explicit degree of freedom in the effective
lagrangian.Comment: Contribution to ``Quark Confinement and Hadron Spectrum VII'', 2--7
Sept. 2006, Ponta Delgada, Acores, Portuga
The Low Column Density Lyman-alpha Forest
We develop an analytical method based on the lognormal approximation to
compute the column density distribution of the Lyman-alpha forest in the low
column density limit. We compute the column density distributions for six
different cosmological models and found that the standard, COBE-normalized CDM
model cannot fit the observations of the Lyman-alpha forest at z=3. The
amplitude of the fluctuations in that model has to be lowered by a factor of
almost 3 to match observations. However, the currently viable cosmological
models like the lightly tilted COBE-normalized CDM+Lambda model, the CHDM model
with 20% neutrinos, and the low-amplitude Standard CDM model are all in
agreement with observations, to within the accuracy of our approximation, for
the value of the cosmological baryon density at or higher than the old Standard
Bing Bang Nucleosynthesis value of 0.0125 for the currently favored value of
the ionizing radiation intensity. With the low value for the baryon density
inferred by Hogan & Rugers (1996), the models can only marginally match
observations.Comment: three postscript figures included, submitted to ApJ
Is the meson a dynamically generated resonance? -- a lesson learned from the O(N) model and beyond
O(N) linear model is solvable in the large limit and hence
provides a useful theoretical laboratory to test various unitarization
approximations. We find that the large limit and the
limit do not commute. In order to get the correct large spectrum one has
to firstly take the large limit. We argue that the meson may
not be described as generated dynamically. On the contrary, it is most
appropriately described at the same level as the pions, i.e, both appear
explicitly in the effective lagrangian. Actually it is very likely the
meson responsible for the spontaneous chiral symmetry breaking in a lagrangian
with linearly realized chiral symmetry.Comment: 15 pages, 3 figurs; references added; discussions slightly modified;
revised version accepted by IJMP
A NuSTAR Observation of the Gamma-ray Emitting Millisecond Pulsar PSR J1723-2837
We report on the first NuSTAR observation of the gamma-ray emitting
millisecond pulsar binary PSR J1723-2837. X-ray radiation up to 79 keV is
clearly detected and the simultaneous NuSTAR and Swift spectrum is well
described by an absorbed power-law with a photon index of ~1.3. We also find
X-ray modulations in the 3-10 keV, 10-20 keV, 20-79 keV, and 3-79 keV bands at
the 14.8-hr binary orbital period. All these are entirely consistent with
previous X-ray observations below 10 keV. This new hard X-ray observation of
PSR J1723-2837 provides strong evidence that the X-rays are from the
intrabinary shock via an interaction between the pulsar wind and the outflow
from the companion star. We discuss how the NuSTAR observation constrains the
physical parameters of the intrabinary shock model.Comment: Accepted for publication in ApJ. 5 pages, 3 figure
Detailed Atmosphere Model Fits to Disk-Dominated ULX Spectra
We have chosen 6 Ultra-Luminous X-ray sources from the {\it XMM-Newton}
archive whose spectra have high signal-to-noise and can be fitted solely with a
disk model without requiring any power-law component. To estimate systematic
errors in the inferred parameters, we fit every spectrum to two different disk
models, one based on local blackbody emission (KERRBB) and one based on
detailed atmosphere modelling (BHSPEC). Both incorporate full general
relativistic treatment of the disk surface brightness profile, photon Doppler
shifts, and photon trajectories. We found in every case that they give almost
identical fits and similar acceptable parameters. The best-fit value of the
most interesting parameter, the mass of the central object, is between 23 and
73 M_\sun in 5 of the 6 examples. In every case, the best-fit inclination
angle and mass are correlated, in the sense that large mass corresponds to high
inclination. Even after allowing for this degeneracy, we find that, with
formal statistical confidence, 3 of the 6 objects have mass
\gtrsim 25 M_\sun; for the other 3, these data are consistent with a wide
range of masses. A mass greater than several hundred M_\sun is unlikely for
the 3 best-constrained objects. These fits also suggest comparatively rapid
black hole spin in the 3 objects whose masses are relatively well-determined,
but our estimate of the spin is subject to significant systematic error having
to do with uncertainty in the underlying surface brightness profile.Comment: 22 pages, 3 figures, accepted by Ap
The Fundamental Plane of Gamma-ray Globular Clusters
We have investigated the properties of a group of -ray emitting
globular clusters (GCs) which have recently been uncovered in our Galaxy. By
correlating the observed -ray luminosities with various
cluster properties, we probe the origin of the high energy photons from these
GCs. We report is positively correlated with the encounter rate
and the metalicity which place an
intimate link between the gamma-ray emission and the millisecond pulsar
population. We also find a tendency that increase with the energy
densities of the soft photon at the cluster location. Furthermore, the
two-dimensional regression analysis suggests that , soft photon
densities, and / possibly span fundamental
planes which potentially provide better predictions for the -ray
properties of GCs.Comment: 17 pages, 4 figures, 3 tables, published in Ap
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