68,354 research outputs found
Long-Distance Contributions to D^0-D^0bar Mixing Parameters
Long-distance contributions to the - mixing parameters and
are evaluated using latest data on hadronic decays. In particular, we
take on two-body and decays to evaluate the contributions of
two-body intermediate states because they account for of hadronic
decays. Use of the diagrammatic approach has been made to estimate
yet-observed decay modes. We find that is of order a few
and of order from hadronic and modes. These are in good
agreement with the latest direct measurement of - mixing
parameters using the and decays by
BaBar. We estimate the contribution to from the modes using the
factorization model and comment on the single-particle resonance effects and
contributions from other two-body modes involving even-parity states.Comment: 18 pages and 1 figure; footnotes and references added; to appear in
Phys. Rev.
Discovery of {\gamma}-ray pulsation and X-ray emission from the black widow pulsar PSR J2051-0827
We report the discovery of pulsed {\gamma}-ray emission and X-ray emission
from the black widow millisecond pulsar PSR J2051-0827 by using the data from
the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope and
the Advanced CCD Imaging Spectrometer array (ACIS-S) on the Chandra X-ray
Observatory. Using 3 years of LAT data, PSR J2051-0827 is clearly detected in
{\gamma}-ray with a signicance of \sim 8{\sigma} in the 0.2 - 20 GeV band. The
200 MeV - 20 GeV {\gamma}-ray spectrum of PSR J2051-0827 can be modeled by a
simple power- law with a photon index of 2.46 \pm 0.15. Significant (\sim
5{\sigma}) {\gamma}-ray pulsations at the radio period were detected. PSR
J2051-0827 was also detected in soft (0.3-7 keV) X-ray with Chandra. By
comparing the observed {\gamma}-rays and X-rays with theoretical models, we
suggest that the {\gamma}-ray emission is from the outer gap while the X-rays
can be from intra-binary shock and pulsar magnetospheric synchrotron emissions.Comment: 10 pages, 4 figures, accepted by ApJ on Jan 28, 201
The Origin of Gamma-Rays from Globular Clusters
Fermi has detected gamma-ray emission from eight globular clusters. We
suggest that the gamma-ray emission from globular clusters may result from the
inverse Compton scattering between relativistic electrons/positrons in the
pulsar wind of MSPs in the globular clusters and background soft photons
including cosmic microwave/relic photons, background star lights in the
clusters, the galactic infrared photons and the galactic star lights. We show
that the gamma-ray spectrum from 47 Tuc can be explained equally well by upward
scattering of either the relic photons, the galactic infrared photons or the
galactic star lights whereas the gamma-ray spectra from other seven globular
clusters are best fitted by the upward scattering of either the galactic
infrared photons or the galactic star lights. We also find that the observed
gamma-ray luminosity is correlated better with the combined factor of the
encounter rate and the background soft photon energy density. Therefore the
inverse Compton scattering may also contribute to the observed gamma-ray
emission from globular clusters detected by Fermi in addition to the standard
curvature radiation process. Furthermore, we find that the emission region of
high energy photons from globular cluster produced by inverse Compton
scattering is substantially larger than the core of globular cluster with a
radius >10pc. The diffuse radio and X-rays emitted from globular clusters can
also be produced by synchrotron radiation and inverse Compton scattering
respectively. We suggest that future observations including radio, X-rays, and
gamma-rays with energy higher than 10 GeV and better angular resolution can
provide better constraints for the models.Comment: Accepted by ApJ, Comments may send to Prof. K.S. Cheng:
[email protected]
Discovery of gamma-ray emission from the supernova remnant Kes 17 with Fermi Large Area Telescope
We report the discovery of GeV emission at the position of supernova remnant
Kes 17 by using the data from the Large Area Telescope on board the Fermi
Gamma-ray Space Telescope. Kes 17 can be clearly detected with a significance
of ~12 sigma in the 1 - 20 GeV range. Moreover, a number of gamma-ray sources
were detected in its vicinity. The gamma-ray spectrum of Kes 17 can be well
described by a simple power-law with a photon index of ~ 2.4. Together with the
multi-wavelength evidence for its interactions with the nearby molecular cloud,
the gamma-ray detection suggests that Kes 17 is a candidate acceleration site
for cosmic-rays.Comment: 13 pages, 3 figures, 1 table, accepted for publication in ApJ Lette
NuSTAR observations and broadband spectral energy distribution modeling of the millisecond pulsar binary PSR J1023+0038
We report the first hard X-ray (3-79 keV) observations of the millisecond
pulsar (MSP) binary PSR J1023+0038 using NuSTAR. This system has been shown
transiting between a low-mass X-ray binary (LMXB) state and a rotation-powered
MSP state. The NuSTAR observations were taken in both LMXB state and
rotation-powered state. The source is clearly seen in both states up to ~79
keV. During the LMXB state, the 3-79 keV flux is about a factor of 10 higher
that in the rotation-powered state. The hard X-rays show clear orbital
modulation during the X-ray faint rotation-powered state but the X-ray orbital
period is not detected in the X-ray bright LMXB state. In addition, the X-ray
spectrum changes from a flat power-law spectrum during the rotation-powered
state to a steeper power-law spectrum in the LMXB state. We suggest that the
hard X-rays are due to the intra-binary shock from the interaction between the
pulsar wind and the injected material from the low-mass companion star. During
the rotation-powered MSP state, the X-ray orbital modulation is due to Doppler
boosting of the shocked pulsar wind. At the LMXB state, the evaporating matter
of the accretion disk due to the gamma-ray irradiation from the pulsar stops
almost all the pulsar wind, resulting the disappearance of the X-ray orbital
modulation.Comment: 8 pages, 6 figures; accepted for publication in Ap
Gamma-ray emission from the globular clusters Liller 1, M80, NGC 6139, NGC 6541, NGC 6624, and NGC 6752
Globular clusters (GCs) are emerging as a new class of gamma-ray emitters,
thanks to the data obtained from the Fermi Gamma-ray Space Telescope. By now,
eight GCs are known to emit gamma-rays at energies >100~MeV. Based on the
stellar encounter rate of the GCs, we identify potential gamma-ray emitting GCs
out of all known GCs that have not been studied in details before. In this
paper, we report the discovery of a number of new gamma-ray GCs: Liller 1, NGC
6624, and NGC 6752, and evidence for gamma-ray emission from M80, NGC 6139, and
NGC 6541, in which gamma-rays were found within the GC tidal radius. With one
of the highest metallicity among all GCs in the Milky Way, the gamma-ray
luminosity of Liller 1 is found to be the highest of all known gamma-ray GCs.
In addition, we confirm a previous report of significant gamma-ray emitting
region next to NGC 6441. We briefly discuss the observed offset of gamma-rays
from some GC cores. The increasing number of known gamma-ray GCs at distances
out to ~10 kpc is important for us to understand the gamma-ray emitting
mechanism and provides an alternative probe to the underlying millisecond
pulsar populations of the GCs.Comment: 22 pages, 7 figures, 2 tables; ApJ, in pres
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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