3,988 research outputs found
Gamma-ray probe of cosmic-ray pressure in galaxy clusters and cosmological implications
Cosmic rays produced in cluster accretion and merger shocks provide pressure
to the intracluster medium (ICM) and affect the mass estimates of galaxy
clusters. Although direct evidence for cosmic-ray ions in the ICM is still
lacking, they produce gamma-ray emission through the decay of neutral pions
produced in their collisions with ICM nucleons. We investigate the capability
of the Gamma-ray Large Area Space Telescope (GLAST) and imaging atmospheric
Cerenkov telescopes (IACTs) for constraining the cosmic-ray pressure
contribution to the ICM. We show that GLAST can be used to place stringent
upper limits, a few per cent for individual nearby rich clusters, on the ratio
of pressures of the cosmic rays and thermal gas. We further show that it is
possible to place tight (<~10%) constraints for distant (z <~ 0.25) clusters in
the case of hard spectrum, by stacking signals from samples of known clusters.
The GLAST limits could be made more precise with the constraint on the
cosmic-ray spectrum potentially provided by IACTs. Future gamma-ray
observations of clusters can constrain the evolution of cosmic-ray energy
density, which would have important implications for cosmological tests with
upcoming X-ray and Sunyaev-Zel'dovich effect cluster surveys.Comment: 12 pages, 5 figures; extended discussions; accepted by MNRA
Magnetic Reconnection with Radiative Cooling. I. Optically-Thin Regime
Magnetic reconnection, a fundamental plasma process associated with a rapid
dissipation of magnetic energy, is believed to power many disruptive phenomena
in laboratory plasma devices, the Earth magnetosphere, and the solar corona.
Traditional reconnection research, geared towards these rather tenuous
environments, has justifiably ignored the effects of radiation on the
reconnection process. However, in many reconnecting systems in high-energy
astrophysics (e.g., accretion-disk coronae, relativistic jets, magnetar flares)
and, potentially, in powerful laser plasma and z-pinch experiments, the energy
density is so high that radiation, in particular radiative cooling, may start
to play an important role. This observation motivates the development of a
theory of high-energy-density radiative magnetic reconnection. As a first step
towards this goal, we present in this paper a simple Sweet--Parker-like theory
of non-relativistic resistive-MHD reconnection with strong radiative cooling.
First, we show how, in the absence of a guide magnetic field, intense cooling
leads to a strong compression of the plasma in the reconnection layer,
resulting in a higher reconnection rate. The compression ratio and the layer
temperature are determined by the balance between ohmic heating and radiative
cooling. The lower temperature in the radiatively-cooled layer leads to a
higher Spitzer resistivity and hence to an extra enhancement of the
reconnection rate. We then apply our general theory to several specific
astrophysically important radiative processes (bremsstrahlung, cyclotron, and
inverse-Compton) in the optically thin regime, for both the zero- and
strong-guide-field cases. We derive specific expressions for key reconnection
parameters, including the reconnection rate. We also discuss the limitations
and conditions for applicability of our theory.Comment: 31 pages, 1 figur
On particle acceleration and trapping by Poynting flux dominated flows
Using particle-in-cell (PIC) simulations, we study the evolution of a
strongly magnetized plasma slab propagating into a finite density ambient
medium. Like previous work, we find that the slab breaks into discrete magnetic
pulses. The subsequent evolution is consistent with diamagnetic relativistic
pulse acceleration of \cite{liangetal2003}. Unlike previous work, we use the
actual electron to proton mass ratio and focus on understanding trapping vs.
transmission of the ambient plasma by the pulses and on the particle
acceleration spectra. We find that the accelerated electron distribution
internal to the slab develops a double-power law. We predict that emission from
reflected/trapped external electrons will peak after that of the internal
electrons. We also find that the thin discrete pulses trap ambient electrons
but allow protons to pass through, resulting in less drag on the pulse than in
the case of trapping of both species. Poynting flux dominated scenarios have
been proposed as the driver of relativistic outflows and particle acceleration
in the most powerful astrophysical jets.Comment: 25 pages, Accepted by Plasma Physics and Controlled Fusio
The dissipative effect of thermal radiation loss in high-temperature dense plasmas
A dynamical model based on the two-fluid dynamical equations with energy
generation and loss is obtained and used to investigate the self-generated
magnetic fields in high-temperature dense plasmas such as the solar core. The
self-generation of magnetic fields might be looked at as a
self-organization-type behavior of stochastic thermal radiation fields, as
expected for an open dissipative system according to Prigogine's theory of
dissipative structures.Comment: 4 pages, 1 postscript figure included; RevTeX3.0, epsf.tex neede
Thermal instability of an expanding dusty plasma with equilibrium cooling
We present an analysis of radiation induced instabilities in an expanding
plasma with considerable presence of dust particles and equilibrium cooling. We
have shown that the equilibrium expansion and cooling destabilize the radiation
condensation modes and the presence of dust particles enhances this effect. We
have examined our results in the context of ionized, dusty-plasma environments
such as those found in planetary nebulae (PNe). We show that due to the
non-static equilibrium and finite equilibrium cooling, small-scale localized
structures formed out of thermal instability, become transient, which agrees
with the observational results. The dust-charge fluctuation is found to heavily
suppress these instabilities, though in view of non-availability of convincing
experimental data, a definitive conclusion could not be made.Comment: 23 pages, 14 figure
Ohm's Law for a Relativistic Pair Plasma
We derive the fully relativistic Ohm's law for an electron-positron plasma.
The absence of non-resistive terms in Ohm's law and the natural substitution of
the 4-velocity for the velocity flux in the relativistic bulk plasma equations
do not require the field gradient length scale to be much larger than the
lepton inertial lengths, or the existence of a frame in which the distribution
functions are isotropic.Comment: 12 pages, plain TeX, Phys. Rev. Lett. 71 3481 (1993
Performance, defect behavior and carrier enhancement in low energy, proton irradiated p+nn+ InP solar cells
The highest AMO efficiency (19.1 percent) InP solar cell consisted of an n+pp+ structure epitaxially grown on a p+ InP substrate. However, the high cost and relative fragility of InP served as motivation for research efforts directed at heteroepitaxial growth of InP on more viable substrates. The highest AMO efficiency (13.7 percent) for this type of cell was achieved using a GaAs substrate. Considering only cost and fracture toughness, Si would be the preferred substrate. The fact that Si is a donor in InP introduces complexities which are necessary in order to avoid the formation of an efficiency limiting counterdiode. One method used to overcome this problem lies in employing an n+p+ tunnel junction in contact with the cell's p region. A simpler method consists of using an n+ substrate and processing the cell in the p+ nn+ configuration. This eliminates the need for a tunnel junction. Unfortunately, the p/n configuration has received relatively little attention the best cell with this geometry having achieved an efficiency of 17 percent. Irradiation of these homoepitaxial cells, with 1 Mev electrons, showed that they were slightly more radiation resistant than diffused junction n/p cells. Additional p/n InP cells have been processed by some activity aimed at diffusion. Currently, there has been some activity aimed at producing heteroepitaxial p+nn+ InP cells using n+ Ge substrates. Since, like Si, Ge is an n-dopant in InP, use of this configuration obviates the need for a tunnel junction. Obviously, before attempting to process heteroepitaxial cells, one must produce a reasonably good homoepitaxial cell. In the present case we focus our attention on homoepitaxially on an n+ Ge substrate
Cosmic Microwave Background Polarization
Cosmic microwave background (CMB) anisotropy is our richest source of
cosmological information; the standard cosmological model was largely
established thanks to study of the temperature anisotropies. By the end of the
decade, the Planck satellite will close this important chapter and move us
deeper into the new frontier of polarization measurements. Numerous
ground--based and balloon--borne experiments are already forging into this new
territory. Besides providing new and independent information on the primordial
density perturbations and cosmological parameters, polarization measurements
offer the potential to detect primordial gravity waves, constrain dark energy
and measure the neutrino mass scale. A vigorous experimental program is
underway worldwide and heading towards a new satellite mission dedicated to CMB
polarization.Comment: Review given at TAUP 2005; References added; Additional reference
A magnetic field diagnostic for sonoluminescence
This study is motivated by the extraordinary process of single bubble
sonoluminescence (SBSL), where an acoustically driven spherical shock is
thought to power the emitted radiation. We propose new experiments using an
external magnetic field which can induce anisotropies in both the shock
propagation and radiation pattern. The effects will depend on the temperature,
conductivity, and size of the radiating region. Our predictions suggest that
such a laboratory experiment could serve as an important diagnostic in placing
bounds on these parameters and understanding the physics of sonoluminescence.Comment: Latex File, Two .eps files, 5 pages, submitted to PR
Radio and gamma-ray constraints on dark matter annihilation in the Galactic center
We determine upper limits on the dark matter (DM) self-annihilation cross
section for scenarios in which annihilation leads to the production of
electron--positron pairs. In the Galactic centre (GC), relativistic electrons
and positrons produce a radio flux via synchroton emission, and a gamma ray
flux via bremsstrahlung and inverse Compton scattering. On the basis of
archival, interferometric and single-dish radio data, we have determined the
radio spectrum of an elliptical region around the Galactic centre of extent 3
degrees semi-major axis (along the Galactic plane) and 1 degree semi-minor axis
and a second, rectangular region, also centered on the GC, of extent 1.6
degrees x 0.6 degrees. The radio spectra of both regions are non-thermal over
the range of frequencies for which we have data: 74 MHz -- 10 GHz. We also
consider gamma-ray data covering the same region from the EGRET instrument
(about GeV) and from HESS (around TeV). We show how the combination of these
data can be used to place robust constraints on DM annihilation scenarios, in a
way which is relatively insensitive to assumptions about the magnetic field
amplitude in this region. Our results are approximately an order of magnitude
more constraining than existing Galactic centre radio and gamma ray limits. For
a DM mass of m_\chi =10 GeV, and an NFW profile, we find that the
velocity-averaged cross-section must be less than a few times 10^-25 cm^3 s^-1.Comment: 14 pages, 9 figures. Version accepted for publication in PRD.
Reference section updated/extended
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