751 research outputs found
Observational constraints on annihilation sites in 1E 1740.7-2942 and Nova Muscae
The region of the Galactic center contains several sources which demonstrate
their activity at various wavelengths and particularly above several hundred
keV. Escape of positrons from such a source or several sources into the
interstellar medium, where they slow down and annihilate, can account for the
511 keV narrow line observed from this direction. 1E 1740.7-2942 object has
been proposed as the most likely candidate to be responsible for this variable
source of positrons. Besides, Nova Muscae shows a spectrum which is consistent
with Comptonization by a thermal plasma kT<100 keV in its hard X-ray part,
while a relatively narrow annihilation line observed by SIGMA on January 20-21,
1991 implies that positrons annihilate in a much colder medium.
We estimate the electron number density and the size of the emitting regions
suggesting that annihilation features observed by SIGMA from Nova Muscae and 1E
1740.7-2942 are due to the positron slowing down and annihilation in thermal
plasma. We show that in the case of Nova Muscae the observed radiation is
coming from a pair plasma stream, N(e+)~N(e-), rather than from a gas cloud. We
argue that two models are probably relevant to the 1E source: annihilation in
(hydrogen) plasma N(e+)<~N(e-) at rest, and annihilation in the pair plasma
stream, which involves matter from the source environment.Comment: 5 pages including 2 figures, latex, aipproc.sty, aipproc.cls,
epsfig.sty. To be published in Proc. of 4th Compton Symp., 1997 (27-30 April,
Williamsburg, Virginia
Quantum normal-to-inhomogeneous superconductor phase transition in nearly two-dimensional metals
In multi-band systems, electrons from different orbitals coexist at the Fermi
surface. An attractive interaction among these quasi-particles gives rise to
inter-band or hybrid pairs which eventually condense in a superconducting
state. These quasi-particles have a natural mismatch of their Fermi
wave-vectors, , which depends on the strength of the hybridization
between their orbitals. The existence of this natural scale suggests the
possibility of inhomogeneous superconducting ground states in these systems,
even in the absence of an applied magnetic field. Furthermore, since
hybridization depends on pressure, this provides an external parameter to
control the wave-vectors mismatch at the Fermi surface. In this work, we study
the phase diagram of a two-dimensional, two-band metal with inter-band pairing.
We show that as the mismatch between the Fermi wave-vectors of the two hybrid
bands is reduced, the system presents a normal-to-inhomogeneous superconductor
quantum phase transition at a critical value of the hybridization
. The superconducting ground state for is characterized
by a wave-vector with magnitude . Here
is the superconducting gap in the homogeneous state and
the average Fermi velocity. We discuss the nature of the quantum critical point
(QCP) at and obtain the associated quantum critical exponents.Comment: 6 pages, 4 figure
Spin solitons in magnetized pair plasmas
A set of fluid equations, taking into account the spin properties of the
electrons and positrons in a magnetoplasma, are derived. The
magnetohydrodynamic limit of the pair plasma is investigated. It is shown that
the microscopic spin properties of the electrons and positrons can lead to
interesting macroscopic and collective effects in strongly magnetized plasmas.
In particular, it is found that new Alfvenic solitary structures, governed by a
modified Korteweg-de Vries equation, are allowed in such plasmas. These
solitary structures vanish if the quantum spin effects are neglected. Our
results should be of relevance for astrophysical plasmas, e.g. in pulsar
magnetospheres.Comment: 7 page
Very-High-Energy Gamma-Ray Signal from Nuclear Photodisintegration as a Probe of Extragalactic Sources of Ultrahigh-Energy Nuclei
It is crucial to identify the ultrahigh-energy cosmic-ray (UHECR) sources and
probe their unknown properties. Recent results from the Pierre Auger
Observatory favor a heavy nuclear composition for the UHECRs. Under the
requirement that heavy nuclei survive in these sources, using gamma-ray bursts
as an example, we predict a diagnostic gamma-ray signal, unique to nuclei - the
emission of de-excitation gamma rays following photodisintegration. These gamma
rays, boosted from MeV to TeV-PeV energies, may be detectable by gamma-ray
telescopes such as VERITAS, HESS, and MAGIC, and especially the next-generation
CTA and AGIS. They are a promising messenger to identify and study individual
UHE nuclei accelerators.Comment: 7 pages, 4 figures, accepted for publication in PRD, with extended
descriptions. Conclusions unchange
- …