210,904 research outputs found
SOFT: A synthetic synchrotron diagnostic for runaway electrons
Improved understanding of the dynamics of runaway electrons can be obtained
by measurement and interpretation of their synchrotron radiation emission.
Models for synchrotron radiation emitted by relativistic electrons are well
established, but the question of how various geometric effects -- such as
magnetic field inhomogeneity and camera placement -- influence the synchrotron
measurements and their interpretation remains open. In this paper we address
this issue by simulating synchrotron images and spectra using the new synthetic
synchrotron diagnostic tool SOFT (Synchrotron-detecting Orbit Following
Toolkit). We identify the key parameters influencing the synchrotron radiation
spot and present scans in those parameters. Using a runaway electron
distribution function obtained by Fokker-Planck simulations for parameters from
an Alcator C-Mod discharge, we demonstrate that the corresponding synchrotron
image is well-reproduced by SOFT simulations, and we explain how it can be
understood in terms of the parameter scans. Geometric effects are shown to
significantly influence the synchrotron spectrum, and we show that inherent
inconsistencies in a simple emission model (i.e. not modeling detection) can
lead to incorrect interpretation of the images.Comment: 24 pages, 12 figure
On the origin of >10 GeV photons in gamma-ray burst afterglows
Fermi/LAT has detected long-lasting high-energy photons (>100 MeV) from
gamma-ray bursts (GRBs), with the highest energy photons reaching about 100
GeV. One proposed scenario is that they are produced by high-energy electrons
accelerated in GRB forward shocks via synchrotron radiation. We study the
maximum synchrotron photon energy in this scenario, considering the properties
of the microturbluence magnetic fields behind the shock, as revealed by recent
Particle-in-Cell simulations and theoretical analyses of relativistic
collisionless shocks. Due to the small-scale nature of the micro-turbulent
magnetic field, the Bohm acceleration approximation breaks down at such high
energies. This effect leads to a typical maximum synchrotron photon of a few
GeV at 100 s after the burst and this maximum synchrotron photon energy
decreases quickly with time. We show that the fast decrease of the maximum
synchrotron photon energy leads to a fast decay of the synchrotron flux. The
10-100 GeV photons detected after the prompt phase can not be produced by the
synchrotron mechanism. They could originate from the synchrotron self-Compton
emission of the early afterglow if the circum-burst density is sufficiently
large, or from the external inverse-Compton process in the presence of central
X-ray emission, such as X-ray flares and prompt high-latitude X-ray emission.Comment: 13 pages, 3 figures, accepted by ApJ Letter
A new ordering parameter of spectral energy distributions from synchrotron-self-Compton emitting blazars
The broadband SEDs of blazars exhibit two broad spectral components, which in
leptonic emission models are attributed to synchrotron radiation and
synchrotron self-Compton (SSC) radiation of relativistic electrons. During high
state phases, the high-frequency SSC component often dominates the
low-frequency synchrotron component, implying that the inverse Compton SSC
losses of electrons are at least equal to or greater than the synchrotron
losses of electrons. We calculate from the analytical solution of the kinetic
equation of relativistic electrons, subject to the combined linear synchrotron
and nonlinear synchrotron self-Compton cooling, for monoenergetic injection the
time-integrated total synchrotron and SSC radiation fluences and spectral
energy distributions (SED). Depending on the ratio of the initial cooling
terms, displayed by the injection parameter , we find for , implying complete linear cooling, that the synchrotron peak dominates the
inverse Compton peak and the usual results of the spectra are recovered. For
the SSC peak dominates the synchrotron peak, proving our
assumption that in such a case the cooling becomes initially non-linear. The
spectra also show some unique features, which can be attributed directly to the
non-linear cooling. To show the potential of the model, we apply it to
outbursts of 3C 279 and 3C 454.3, successfully reproducing the SEDs. The
results of our analysis are promising, and we argue that this non-equilibrium
model should be considered in future modeling attempts for blazar flares.Comment: accepted by MNRAS, 32 pages (single column), 7 figure
Klein-Nishina effects on the high-energy afterglow emission of gamma-ray bursts
Extended high-energy(>100MeV) gamma-ray emission that lasts much longer than
the prompt sub-MeV emission has been detected from quite a few gamma-ray bursts
(GRBs) by Fermi Large Area Telescope (LAT) recently. A plausible scenario is
that this emission is the afterglow synchrotron emission produced by electrons
accelerated in the forward shocks. In this scenario, the electrons that produce
synchrotron high-energy emission also undergo inverse-Compton (IC) loss and the
IC scattering with the synchrotron photons should be in the Klein-Nishina
regime. Here we study effects of the Klein-Nishina scattering on the
high-energy synchrotron afterglow emission. We find that, at early times the
Klein-Nishina suppression effect on those electrons that produce the
high-energy emission is usually strong and therefore their inverse-Compton loss
is small with a Compton parameter Y < a few for a wide range of parameter
space. This leads to a relatively bright synchrotron afterglow at high energies
that can be detected by Fermi LAT. As the Klein-Nishina suppression effect
weakens with time, the inverse-Compton loss increases and could dominate over
the synchrotron loss in some parameter space. This will lead to a faster
temporal decay of the high-energy synchrotron emission than what is predicted
by the standard synchrotron model, which may explain the observed rapid decay
of the early high-energy gamma-ray emission in GRB090510 and GRB090902B.Comment: 8 page (emulateapj style), 8 figures, submitted to Ap
A Proton Synchrotron Blazar Model for Flaring in Markarian~501
(abr.) The spectral energy distribution (SED) of blazars typically has a
double-humped appearance usually interpreted in terms of synchrotron
self-Compton models. In proton blazar models, the SED is instead explained in
terms of acceleration of protons and subsequent cascading. We discuss a
variation of the Synchrotron Proton Blazar model, first proposed by M\"ucke &
Protheroe (1999), in which the low energy part of the SED is mainly synchrotron
radiation by electrons co-accelerated with protons which produce the high
energy part of the SED mainly asproton synchrotron radiation. Using a Monte
Carlo/numerical technique to simulate the interactions and subsequent cascading
of the accelerated protons, we are able to fit the observed SED of Markarian
501 during the April 1997 flare. We find that the emerging cascade spectra
initiated by gamma-rays from decay and by from decay
turn out to be relatively featureless. Synchrotron radiation produced by
from decay, and even more importantly by protons, and
subsequent synchrotron-pair cascading, is able to reproduce well the high
energy part of the SED. For this fit we find that synchrotron radiation by
protons dominates the TeV emission, pion photoproduction being less important
with the consequence that we predict a lower neutrino flux than in other proton
blazar models.Comment: 28 pages, 8 Figures, accepted for publication in Astropart.Phy
Thermal synchrotron radiation and its Comptonization in compact X-ray sources
We investigate thermal synchrotron radiation at semi-relativistic and
relativistic temperatures. We find an analytic expression for the
angle-averaged emission coefficient, and show that it is significantly more
accurate that those derived previously. We also present analytic approximations
to the synchrotron turnover frequency. Then, we treat Comptonization of the
synchrotron radiation, and give simple expressions for the spectral shape and
the emitted power. We also consider modifications of the above results by
bremsstrahlung. The importance of Comptonization of synchrotron radiation in
compact X-ray sources is then studied. We consider emission from hot accretion
flows and from active coronae above optically-thick accretion discs in
black-hole binaries and AGNs. Synchrotron Comptonization is found to be, in
general, negligible in luminous sources, except for those with hardest X-ray
spectra and stellar masses. Increasing the black-hole mass results in a strong
reduction of the maximum Eddington ratio possible from to this process. X-ray
spectra of intermediate-luminosity sources, e.g., low-luminosity AGNs, can be
explained by synchrotron Comptonization only in the case of hot accretion
flows. Then, bremsstrahlung emission always dominates X-ray spectra of very
weak sources. Finally, we consider weakly-magnetized neutron stars and find
that synchrotron Comptonization can account for the power-law X-ray spectra
observed in their low states.Comment: 17 pages, 17 postscript figures. Accepted to MNRA
The synchrotron foreground and CMB temperature-polarization cross correlation power spectrum from the first year WMAP data
We analyse the temperature-polarization cross-correlation in the Galactic
synchrotron template that we have recently developed, and between the template
and CMB temperature maps derived from WMAP data. Since the polarized
synchrotron template itself uses WMAP data, we can estimate residual
synchrotron contamination in the CMB angular spectrum. While
appears to be contamined by synchrotron, no evidence for
contamination is found in the multipole range which is most relevant for the
fit of the cosmological optical depth.Comment: Accepted for pubblication on MNRAS Lette
Klein-Nishina Effects on Optically Thin Synchrotron and Synchrotron Self-Compton Spectrum
We present analytic approximations to the optically thin synchrotron and
synchrotron self-Compton (SSC) spectra when Klein-Nishina (KN) effects are
important and pair production and external radiation fields can be neglected.
This theory is useful for analytical treatment of radiation from astrophysical
sources, such as gamma-ray bursts (GRBs), active galactic nuclei and pulsar
wind nebula, where KN effects may be important. We consider a source with a
continuous injection of relativistic electrons with a power-law energy
distribution above some typical injection energy. We find that the
synchrotron-SSC spectra can be described by a broken power-law, and provide
analytic estimates for the break frequencies and power-law indices. In general,
we show that the dependence of the KN cross-section on the energy of the
upscattering electron results in a hardening of the energy distribution of fast
cooling electrons and therefore in a hardening of the observed synchrotron
spectrum. As a result the synchrotron spectrum of fast cooling electrons, below
the typical injection energy, can be as hard as , instead
of the classical when KN effects are neglected. The synchrotron
energy output can be dominated by electrons with energy above the typical
injection energy. We solve self-consistently for the cooling frequency and find
that the transition between synchrotron and SSC cooling can result in a
discontinuous variations of the cooling frequency and the synchrotron and SSC
spectra. We demonstrate the application of our results to theory by applying
them to prompt and afterglow emission models of GRBs
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