196 research outputs found
The proton low-mass microquasar: high-energy emission
A population of unidentified gamma-ray sources is forming a structure
resembling a halo around the Galactic center. These sources are highly
variable, and hence they should be associated with compact objects.
Microquasars are objects undergoing accretion with relativistic jets; if such
an object has a low-mass, evolved, donor star, it might be found in the
Galactic halo. If these low-mass microquasars can generate detectable gamma-ray
emission, then they are natural candidates to account for the halo high-energy
sources. We aim to construct models for high-energy emission of low-mass
microquasars, which could produce a significant luminosity in the gamma-ray
domain. We consider that a significant fraction of the relativistic particles
in the jets of low-mass microquasars are protons and then we study the
production of high-energy emission through proton synchrotron radiation and
photopion production. Photopair production and leptonic processes are
considered as well. We compute a number of specific models with different
parameters to explore the possibilities of this scenario.} We find that
important luminosities, in the range of erg s, can be
achieved by proton synchrotron radiation in the Gamma-Ray Large Area Space
Telescope (GLAST) energy range, and lower, but still significant luminosities
at higher energies for some models. We conclude that the "proton microquasar"
model offers a very interesting alternative to account for the halo gamma-ray
sources and presents a variety of predictions that might be tested in the near
future by instruments like GLAST, the High-Energy Stereoscopic System II (HESS
II), the Major Atmospheric Gamma-ray Imaging Cherenkov telescope II (MAGIC II),
and neutrino telescopes like IceCube.Comment: 11 pages, 7 figures, final version, accepted for publication in A&
Neutral Beams from Blazar Jets
We treat the production of neutrons, photons, and neutrinos through
photomeson interactions of relativistic protons with ambient photons in the
compact inner jets of blazars. We show that the presence of the external UV
radiation field makes possible strong energy losses already for protons with
energies > 1 PeV, while without this component effective energy losses of
protons begin only at E > 10^{18} eV. We develop a model describing the
production and escape of neutrons from a comoving spherical blob, which
continue to interact with the ambient external radiation field on the
parsec-scale broad line region (BLR). Neutrons may carry ~10% of the overall
energy of the protons accelerated beyond E ~ 1 PeV outside the BLR. Ultra-high
energy (UHE) gamma rays produced in photomeson interactions of neutrons outside
the blob can also escape the BLR. The escaping neutrons, gamma rays and
neutrinos form a collimated neutral beam with a characteristic opening angle ~
1/Gamma, where Gamma is the bulk Lorentz factor of the inner jet. The energy
and the momentum of such beam is then mostly deposited in the extended jet due
to neutron decay at distances ~ (E_n/10^{17} eV}) kpc, and through
pair-production attenuation of gamma rays with energies E_g ~ 10^{15}-10^{18}
eV which can propagate to distances beyond (10-100) kpc. In this scenario,
neutral beams of UHE gamma rays and neutrons can be the reason for straight
extended jets such as in Pictor A. Fluxes of neutrinos detectable with km-scale
neutrino telescopes are predicted from flat spectrum radio quasars such as 3C
279.Comment: to appear in ApJ 586, No.1, March 20 issu
Precision Study of Positronium: Testing Bound State QED Theory
As an unstable light pure leptonic system, positronium is a very specific
probe atom to test bound state QED. In contrast to ordinary QED for free
leptons, the bound state QED theory is not so well understood and bound state
approaches deserve highly accurate tests. We present a brief overview of
precision studies of positronium paying special attention to uncertainties of
theory as well as comparison of theory and experiment. We also consider in
detail advantages and disadvantages of positronium tests compared to other QED
experiments.Comment: A talk presented at Workshop on Positronium Physics (ETH Zurich, May
30-31, 2003
Properties of continuous Fourier extension of the discrete cosine transform and its multidimensional generalization
A versatile method is described for the practical computation of the discrete
Fourier transforms (DFT) of a continuous function given by its values
at the points of a uniform grid generated by conjugacy classes
of elements of finite adjoint order in the fundamental region of
compact semisimple Lie groups. The present implementation of the method is for
the groups SU(2), when is reduced to a one-dimensional segment, and for
in multidimensional cases. This simplest case
turns out to result in a transform known as discrete cosine transform (DCT),
which is often considered to be simply a specific type of the standard DFT.
Here we show that the DCT is very different from the standard DFT when the
properties of the continuous extensions of these two discrete transforms from
the discrete grid points to all points are
considered. (A) Unlike the continuous extension of the DFT, the continuous
extension of (the inverse) DCT, called CEDCT, closely approximates
between the grid points . (B) For increasing , the derivative of CEDCT
converges to the derivative of . And (C), for CEDCT the principle of
locality is valid. Finally, we use the continuous extension of 2-dimensional
DCT to illustrate its potential for interpolation, as well as for the data
compression of 2D images.Comment: submitted to JMP on April 3, 2003; still waiting for the referee's
Repor
Gamma-ray binaries
Recent observations have shown that some compact stellar binaries radiate the
highest energy light in the universe. The challenge has been to determine the
nature of the compact object and whether the very high energy gamma-rays are
ultimately powered by pulsar winds or relativistic jets. Multiwavelength
observations have shown that one of the three gamma-ray binaries known so far,
PSR B1259-63, is a neutron star binary and that the very energetic gamma-rays
from this source and from another gamma-ray binary, LS I +61 303, may be
produced by the interaction of pulsar winds with the wind from the companion
star. At this time it is an open question whether the third gamma-ray binary,
LS 5039, is also powered by a pulsar wind or a microquasar jet, where
relativistic particles in collimated jets would boost the energy of the wind
from the stellar companion to TeV energies.Comment: 4 pages, 3 figures. Invited talk to appear in Proceedings of the
conference "The Multi-Messenger Approach to High-Energy Gamma-ray Sources",
Barcelona, 4-7 July 200
A new measurement of the properties of the rare decay K -> pi+ e+ e-
A large low-background sample of events (10300) has been collected for the
rare decay of kaons in flight K+ -> pi+ e+ e- by experiment E865 at the
Brookhaven AGS. The decay products were accepted by a broad band
high-resolution charged particle spectrometer with particle identification. The
branching ratio (2.94 +- 0.05(stat.) +- 0.13(syst.) +- 0.05(model))*10**{-7}
was determined normalizing to events from the decay chain K+ -> pi+ pi0; pi0 ->
e+ e- gamma. From the analysis of the decay distributions the vector nature of
this decay is firmly established now, and limits on scalar and tensor
contributions are deduced. From the (e+ e-) invariant mass distribution the
decay form factor f(z)=f0(1+ delta*z) (z=M(ee)**2/m(K)**2) is determined with
delta=2.14 +- 0.13 +- 0.15. Chiral QCD perturbation theory predictions for the
form factor are also tested, and terms beyond leading order O(p**4) are found
to be important.Comment: 4 pages, 5 figure
Implications of a Nonthermal Origin of the Excess EUV Emission from the Coma Cluster of Galaxies
The inverse Compton (IC) interpretation of the excess EUV emission, that was
recently reported from several clusters of galaxies, suggests that the amount
of relativistic electrons in the intracluster medium is highly significant,
W_e>10^{61} erg. Considering Coma as the prototype galaxy cluster of nonthermal
radiation, we discuss implications of the inverse Compton origin of the excess
EUV fluxes in the case of low intracluster magnetic fields of order 0.1 muG, as
required for the IC interpretation of the observed excess hard X-ray flux, and
in the case of high fields of order 1 muG as suggested by Faraday rotation
measurements. Although for such high intracluster fields the excess hard X-rays
will require an explanation other than by the IC effect, we show that the
excess EUV flux can be explained by the IC emission of a `relic' population of
electrons driven into the incipient intracluster medium at the epoch of
starburst activity by galactic winds, and later on reenergized by adiabatic
compression and/or large-scale shocks transmitted through the cluster as the
consequence of more recent merger events. For high magnetic fields B > 1 muG
the interpretation of the radio fluxes of Coma requires a second population of
electrons injected recently. They can be explained as secondaries produced by a
population of relativistic protons. We calculate the fluxes of gamma-rays to be
expected in both the low and high magnetic field scenarios, and discuss
possibilities to distinguish between these two principal options by future
gamma-ray observations.Comment: LaTeX, 6 figures; accepted for publication in Ap
Pulsar Jets: Implications for Neutron Star Kicks and Initial Spins
We study implications for the apparent alignment of the spin axes,
proper-motions, and polarization vectors of the Crab and Vela pulsars. The spin
axes are deduced from recent Chandra X-ray Observatory images that reveal jets
and nebular structure having definite symmetry axes. The alignments indicate
these pulsars were born either in isolation or with negligible velocity
contributions from binary motions. We examine the effects of rotation and the
conditions under which spin-kick alignment is produced for various models of
neutron star kicks. If the kick is generated when the neutron star first forms
by asymmetric mass ejection or/and neutrino emission, then the alignment
requires that the protoneutron star possesses an original spin with period
much less than the kick timescale, thus spin-averaging the kick forces.
The kick timescale ranges from 100 ms to 10 s depending on whether the kick is
hydrodynamically driven or neutrino-magnetic field driven. For hydrodynamical
models, spin-kick alignment further requires the rotation period of an
asymmetry pattern at the radius near shock breakout (>100 km) to be much less
than ~100 ms; this is difficult to satisfy unless rotation plays a dynamically
important role in the core collapse and explosion (P_s\lo 1 ms). Aligned kick
and spin vectors are inherent to the slow process of asymmetric electromagnetic
radiation from an off-centered magnetic dipole. We reassess the viability of
this effect, correcting a factor of 4 error in Harrison and Tademaru's
calculation that increases the size of the effect. To produce a kick velocity
of order a few hundred km/s requires that the neutron star be born with an
initial spin close to 1 ms and that spindown due to r-mode driven gravitational
radiation be inefficient compared to standard magnetic braking.Comment: Small changes/additions; final version to be published in ApJ,
Vol.549 (March 10, 2001
A new measurement of K+(e4) decay and the s-wave pi-pi-scattering length a00
A sample of 400000 events from the decay K+->pi+pi-e+nu(e)(K(e4)) has been
collected in experiment E865 at the Brookhaven AGS. The analysis of these data
yields new measurements of the K(e4) branching ratio
(4.11+-0.01+-0.11)*10**(-5)), the s-wave pi-pi scattering length
a00=0.228+-0.012+-0.003, and the form factors F, G, and H of the hadronic
current and their dependence on the invariant pi-pi mass
First observation of the decay K+ -> e+ nu mu+ mu-
Experiment 865 at the Brookhaven AGS has observed the decay K^+ -> e^+ nu
mu^+ mu^-. The branching ratio extracted is (1.72 +/- 0.37(stat) +/- 0.17(syst)
+/- 0.19(model)) x 10^{-8} where the third term in the error results from the
use of a model to extrapolate into a kinematic region dominated by background.Comment: 4 pages, 6 figures, Revtex4. Correction to figure and minor text
change
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