18,162 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
Black Hole Entropy and Viscosity Bound in Horndeski Gravity
Horndeski gravities are theories of gravity coupled to a scalar field, in
which the action contains an additional non-minimal quadratic coupling of the
scalar, through its first derivative, to the Einstein tensor or the analogous
higher-derivative tensors coming from the variation of Gauss-Bonnet or Lovelock
terms. In this paper we study the thermodynamics of the static black hole
solutions in dimensions, in the simplest case of a Horndeski coupling to
the Einstein tensor. We apply the Wald formalism to calculate the entropy of
the black holes, and show that there is an additional contribution over and
above those that come from the standard Wald entropy formula. The extra
contribution can be attributed to unusual features in the behaviour of the
scalar field. We also show that a conventional regularisation to calculate the
Euclidean action leads to an expression for the entropy that disagrees with the
Wald results. This seems likely to be due to ambiguities in the subtraction
procedure. We also calculate the viscosity in the dual CFT, and show that the
viscosity/entropy ratio can violate the bound for
appropriate choices of the parameters.Comment: 30 pages, no figure, minor revision
Experimental Quantum Cloning with Prior Partial Information
When prior partial information about a state to be cloned is available, it
can be cloned with a fidelity higher than that of universal quantum cloning. We
experimentally verify this intriguing relationship between the cloning fidelity
and the prior information by reporting the first experimental optimal quantum
state-dependent cloning, using nuclear magnetic resonance techniques. Our
experiments may further have important implications into many quantum
information processing protocols.Comment: 4 pages, 2 figure
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
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
The Linear Polarization of Sagittarius A* I. VLA Spectro-polarimetry at 4.8 and 8.4 GHz
Synchrotron radiation from active galactic nuclei (AGN) is often highly
polarized. We present a search for linear polarization with the Very Large
Array (VLA) at 4.8 GHz and 8.4 GHz from the nearest AGN, Sagittarius A*. As a
part of this study we used spectro-polarimetric data that were sensitive to a
rotation measure (RM) as large as 3.5 x 10^6 rad m^-2 at 4.8 GHz and 1.5 x 10^7
rad m^-2 at 8.4 GHz. The upper limit to the linear polarization of Sgr A* over
a broad range of RM is 0.2% at both frequencies. We also present continuum
observations with the VLA at 4.8 GHz which give an upper limit of 0.1% for RMs
less than 10^4 rad m^-2. We conclude that depolarization is unlikely to occur
in the Galacter Center scattering medium. However, it is possible for
depolarization to occur in the accretion region of Sgr A* if the outer scale of
turbulence is small enough. We also consider the implications of a very low
intrinsic polarization for Sgr A*.Comment: 16 pages, 3 figures, accepted for publication in the Astrophysical
Journal, August 20, 1999, Vol 521 #
- …