1,199 research outputs found
On The Non Thermal Emission and Acceleration of Electrons in Coma and Other Clusters of Galaxies
Some clusters of galaxies in addition to thermal bremsstrahlung (TB), emit
diffuse radiation from the intercluster medium (ICM) at radio, EUV and hard
x-ray (HXR) ranges. The radio radiation is due to synchrotron by relativistic
electrons, and the inverse Compton (IC) scattering by the cosmic microwave
background radiation of the same electrons is the most natural source for the
HXR and perhaps the EUV emissions. However, simple estimates give a weaker
magnetic field than that suggested by Faraday rotation measurements.
Consequently, non-thermal bremsstrahlung (NTB) and TB have also been suggested
as sources of these emissions. We show that NTB cannot be the source of the
HXRs and that the difficulty with the low magnetic field in the IC model is
alleviated if we take into account the effects of observational bias,
nonisotropic pitch angle distribution and spectral breaks. We derive a spectrum
for the radiating electrons and discuss acceleration scenarios. We show that
continuous and in situ acceleration in the ICM of the background thermal
electrons requires unreasonably high energy input and acceleration of injected
relativistic electrons gives rise to a much flatter spectrum than desired,
unless a large fraction of electrons escape the ICM, in which case one obtains
EUV and HXR emissions extending well beyond the boundaries of the cluster. A
continuous emission by a cooling spectrum resulting from interaction with ICM
of electrons accelerated elsewhere also suffers from similar shortcomings. The
most likely scenario appears to be an episodic injection-acceleration model,
whereby one obtains a time dependent spectrum that for certain phases of its
evolution satisfies all the requirements.Comment: 27 pages, one Table, Four Figures. Latex AAS v5.0. Accepted by Ap
Photocurrent, Rectification, and Magnetic Field Symmetry of Induced Current Through Quantum Dots
We report mesoscopic dc current generation in an open chaotic quantum dot
with ac excitation applied to one of the shape-defining gates. For excitation
frequencies large compared to the inverse dwell time of electrons in the dot
(i.e., GHz), we find mesoscopic fluctuations of induced current that are fully
asymmetric in the applied perpendicular magnetic field, as predicted by recent
theory. Conductance, measured simultaneously, is found to be symmetric in
field. In the adiabatic (i.e., MHz) regime, in contrast, the induced current is
always symmetric in field, suggesting its origin is mesoscopic rectification.Comment: related papers at http://marcuslab.harvard.ed
Matrix and vector models in the strong coupling limit
In this paper we consider matrix and vector models in the large N limit ( matrices and vectors with N^{2} components). For the case of
zero-dimensional model (D=0) it is proved that in the strong coupling limit statistical sums of both models coincide up to a coefficient. This
is also true for D=1.Comment: 8 page
Temperature Dependence of Magnetophonon Resistance Oscillations in GaAs/AlAs Heterostructures at High Filling Factors
The temperature dependence of phonon-induced resistance oscillations has been
investigated in two-dimensional electron system with moderate mobility at large
filling factors at temperature range T = 7.4 - 25.4 K. The amplitude of
phonon-induced oscillations has been found to be governed by quantum relaxation
time which is determined by electron-electron interaction effects. This is in
agreement with results recently obtained in ultra-high mobility two-dimensional
electron system with low electron density [A. T. Hatke et al., Phys. Rev. Lett.
102, 086808 (2009)]. The shift of the main maximum of the magnetophonon
resistance oscillations to higher magnetic fields with increasing temperature
is observed.Comment: 5 pages, 4 figure
Hard X-ray Emission Clumps in the gamma-Cygni Supernova Remnant: an INTEGRAL-ISGRI View
Spatially resolved images of the galactic supernova remnant G78.2+2.1
(gamma-Cygni) in hard X-ray energy bands from 25 keV to 120 keV are obtained
with the IBIS-ISGRI imager aboard the International Gamma-Ray Astrophysics
Laboratory INTEGRAL. The images are dominated by localized clumps of about ten
arcmin in size. The flux of the most prominent North-Western (NW) clump is (1.7
+/- 0.4) 10^{-11} erg/cm^2/s in the 25-40 keV band. The observed X-ray fluxes
are in agreement with extrapolations of soft X-ray imaging observations of
gamma-Cygni by ASCA GIS and spatially unresolved RXTE PCA data. The positions
of the hard X-ray clumps correlate with bright patches of optical line
emission, possibly indicating the presence of radiative shock waves in a
shocked cloud. The observed spatial structure and spectra are consistent with
model predictions of hard X-ray emission from nonthermal electrons accelerated
by a radiative shock in a supernova interacting with an interstellar cloud, but
the powerful stellar wind of the O9V star HD 193322 is a plausible candidate
for the NW source as well.Comment: 5 pages, 5 figures, Astronomy and Astrophysics Letter
On the nature of the hard X-ray source IGR J2018+4043
We found a very likely counterpart to the recently discovered hard X-ray
source IGR J2018+4043 in the multi-wavelength observations of the source field.
The source, originally discovered in the 20-40 keV band, is now confidently
detected also in the 40-80 keV band, with a flux of (1.4 +/- 0.4) x 10(-11) erg
cm(-2) s(-1). A 5 ks Swift observation of the IGR J2018+4043 field revealed a
hard point-like source with the observed 0.5-10 keV flux of 3.4(+0.7)(-0.8) x
10(-12) erg cm(-2) s(-1) (90% confidence level) at alpha = 20h18m38.55s, delta
= +40d41m00.4s (with a 4.2" uncertainty). The combined Swift-INTEGRAL spectrum
can be described by an absorbed power-law model with photon index gamma = 1.3
+/- 0.2 and N_H = 6.1(+3.2)(-2.2) x 10(22) cm(-2). In archival optical and
infrared data we found a slightly extended and highly absorbed object at the
Swift source position. There is also an extended VLA 1.4 GHz source peaked at a
beam-width distance from the optical and X-ray positions. The observed
morphology and multiwavelength spectra of IGR J2018+4043 are consistent with
those expected for an obscured accreting object, i.e. an AGN or a Galactic
X-ray binary. The identification suggests possible connection of IGR J2018+4043
to the bright gamma-ray source GEV J2020+4023 (3EG J2020+4017) detected by COS
B and CGRO EGRET in the gamma-Cygni SNR field.Comment: 5 pages, 3 figures, uses emulateapj styl
Coherent radiation from neutral molecules moving above a grating
We predict and study the quantum-electrodynamical effect of parametric
self-induced excitation of a molecule moving above the dielectric or conducting
medium with periodic grating. In this case the radiation reaction force
modulates the molecular transition frequency which results in a parametric
instability of dipole oscillations even from the level of quantum or thermal
fluctuations. The present mechanism of instability of electrically neutral
molecules is different from that of the well-known Smith-Purcell and transition
radiation in which a moving charge and its oscillating image create an
oscillating dipole.
We show that parametrically excited molecular bunches can produce an easily
detectable coherent radiation flux of up to a microwatt.Comment: 4 page
Light Element Evolution and Cosmic Ray Energetics
Using cosmic-ray energetics as a discriminator, we investigate evolutionary
models of LiBeB. We employ a Monte Carlo code which incorporates the delayed
mixing into the ISM both of the synthesized Fe, due to its incorporation into
high velocity dust grains, and of the cosmic-ray produced LiBeB, due to the
transport of the cosmic rays. We normalize the LiBeB production to the integral
energy imparted to cosmic rays per supernova. Models in which the cosmic rays
are accelerated mainly out of the average ISM significantly under predict the
measured Be abundance of the early Galaxy, the increase in [O/Fe] with
decreasing [Fe/H] notwithstanding. We suggest that this increase could be due
to the delayed mixing of the Fe. But, if the cosmic-ray metals are accelerated
out of supernova ejecta enriched superbubbles, the measured Be abundances are
consistent with a cosmic-ray acceleration efficiency that is in very good
agreement with the current epoch data. We also find that neither the above
cosmic-ray origin models nor a model employing low energy cosmic rays
originating from the supernovae of only very massive progenitors can account
for the Li data at values of [Fe/H] below 2.Comment: latex 19 pages, 2 tables, 10 eps figures, uses aastex.cls natbib.sty
Submitted to the Astrophysical Journa
INTEGRAL detection of hard X-rays from NGC 6334: Nonthermal emission from colliding winds or an AGN?
We report the detection of hard X-ray emission from the field of the
star-forming region NGC 6334 with the the International Gamma-Ray Astrophysics
Laboratory INTEGRAL. The JEM-X monitor and ISGRI imager aboard INTEGRAL and
Chandra ACIS imager were used to construct 3-80 keV images and spectra of NGC
6334. The 3-10 keV and 10-35 keV images made with JEM-X show a complex
structure of extended emission from NGC 6334. The ISGRI source detected in the
energy ranges 20-40 keV and 40-80 keV coincides with the NGC 6334 ridge. The
20-60 keV flux from the source is (1.8+-0.37)*10(-11) erg cm(-2) s(-1).
Spectral analysis of the source revealed a hard power-law component with a
photon index about 1. The observed X-ray fluxes are in agreement with
extrapolations of X-ray imaging observations of NGC 6334 by Chandra ACIS and
ASCA GIS. The X-ray data are consistent with two very different physical
models. A probable scenario is emission from a heavily absorbed, compact and
hard Chandra source that is associated with the AGN candidate radio source NGC
6334B. Another possible model is the extended Chandra source of nonthermal
emission from NGC 6334 that can also account for the hard X-ray emission
observed by INTEGRAL. The origin of the emission in this scenario is due to
electron acceleration in energetic outflows from massive early type stars. The
possibility of emission from a young supernova remnant, as suggested by earlier
infrared observations of NGC 6334, is constrained by the non-detection of 44Ti
lines.Comment: 8 pages, 8 figures, Astronomy and Astrophysics (in press
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