396 research outputs found
Synchrotron and Synchrotron Self-Compton Spectral Signatures and Blazar Emission Models
We find that energy losses due to synchrotron self-Compton (SSC) emission in
blazar jets can produce distinctive signatures in the time-averaged synchrotron
and SSC spectra of these objects. For a fairly broad range of particle
injection distributions, SSC-loss dominated synchrotron emission exhibits a
spectral dependence . The presence or absence of this
dependence in the optical and ultraviolet spectra of flat spectrum radio
quasars such as 3C~279 and in the soft X-ray spectra of high frequency BL Lac
objects such as Mrk 501 gives a robust measure of the importance of SSC losses.
Furthermore, for partially cooled particle distributions, spectral breaks of
varying sizes can appear in the synchrotron and SSC spectra and will be related
to the spectral indices of the emission below the break. These spectral
signatures place constraints on the size scale and the non-thermal particle
content of the emitting plasma as well as the observer orientation relative to
the jet axis.Comment: 4 pages, 1 figure, LaTeX2e, emulateapj5.sty, accepted for publication
in Ap
Spectral Energy Distributions of Gamma Ray Bursts Energized by External Shocks
Sari, Piran, and Narayan have derived analytic formulas to model the spectra
from gamma-ray burst blast waves that are energized by sweeping up material
from the surrounding medium. We extend these expressions to apply to general
radiative regimes and to include the effects of synchrotron self-absorption.
Electron energy losses due to the synchrotron self-Compton process are also
treated in a very approximate way. The calculated spectra are compared with
detailed numerical simulation results. We find that the spectral and temporal
breaks from the detailed numerical simulation are much smoother than the
analytic formulas imply, and that the discrepancies between the analytic and
numerical results are greatest near the breaks and endpoints of the synchrotron
spectra. The expressions are most accurate (within a factor of ~ 3) in the
optical/X-ray regime during the afterglow phase, and are more accurate when
epsilon_e, the fraction of swept-up particle energy that is transferred to the
electrons, is <~ 0.1. The analytic results provide at best order-of-magnitude
accuracy in the self-absorbed radio/infrared regime, and give poor fits to the
self-Compton spectra due to complications from Klein-Nishina effects and
photon-photon opacity.Comment: 16 pages, 7 figures, ApJ, in press, 537, July 1, 2000. Minor changes
in response to referee report, corrected figure
Synchrotron and SSC Emission and the Blast-Wave Model of Gamma-Ray Bursts
We investigate the dynamics and radiation from a relativistic blast-wave
which decelerates as it sweeps up ambient matter. The bulk kinetic energy of
the blast-wave shell is converted into internal energy by the process of
accreting external matter. If it takes the form of non-thermal electrons and
magnetic fields, then this internal energy will be emitted as synchrotron and
synchrotron self-Compton radiation. We perform analytic and numerical
calculations for the deceleration and radiative processes and present
time-resolved spectra throughout the evolution of the blast-wave. We also
examine the dependence of the burst spectra and light curves on various
parameters describing the magnetic field and non-thermal electron
distributions. We find that for bursts such as GRB~910503, GRB~910601 and
GRB~910814, the spectral shapes of the prompt gamma-ray emission at the peaks
in strongly constrain the magnetic fields in these bursts to be
well below (\la 10^{-2}) the equipartition values. These calculations are
also considered in the context of the afterglow emission from the recently
detected gamma-ray burst counterparts.Comment: 27 pages, 5 figures, submitted to Ap
Blazar synchrotron emission of instantaneously power-law injected electrons under linear synchrotron, non-linear SSC, and combined synchrotron-SSC cooling
The broadband SEDs of blazars show two distinct components which in leptonic
models are associated with synchrotron and SSC emission of highly relativistic
electrons. In some sources the SSC component dominates the synchrotron peak by
one or more orders of magnitude implying that the electrons mainly cool by
inverse Compton collisions with their self-made synchrotron photons. Therefore,
the linear synchrotron loss of electrons, which is normally invoked in emission
models, has to be replaced by a nonlinear loss rate depending on an energy
integral of the electron distribution. This modified electron cooling changes
significantly the emerging radiation spectra. It is the purpose of this work to
apply this new cooling scenario to relativistic power-law distributed
electrons, which are injected instantaneously into the jet. We will first solve
the differential equation of the volume-averaged differential number density of
the electrons, and then discuss their temporal evolution. Since any non-linear
cooling will turn into linear cooling after some time, we also calculated the
electron number density for a combined cooling scenario consisting of both the
linear and non-linear cooling. For all cases, we will also calculate
analytically the emerging optically thin synchrotron fluence spectrum which
will be compared to a numerical solution. For small normalized frequencies f <
1 the fluence spectra show constant spectral indices. We find for linear
cooling a_SYN = 1/2, and for non-linear cooling a_SSC = 3/2. In the combined
cooling scenario we obtain for the small injection parameter b_1 = 1/2, and for
the large injection parameter b_2 = 3/2, which becomes b_1 = 1/2 for very small
frequencies, again. This is the same behaviour as for monoenergetically
injected electrons.Comment: 24 pages, 25 figures, submitted to A&
High Energy Radiation Generated at Boundary Shear Layers of Relativistic Jets
A simple model of cosmic ray electron acceleration at the jet boundary
(Ostrowski 2000) yields a power-law particle energy distribution of
ultrarelativistic electrons with an energy cut-off growing with time and
finally, a growing particle bump at the energy, where energy gains equal
radiation losses. For such electron distribution, in tens-of-kpc scale jets, we
derived the observed time varying spectra of synchrotron and inverse-Compton
radiation, including comptonization of synchrotron and cosmic microwave
background photons. Slowly varying spectral index along the jet in the
`low-frequency' spectral range is a natural consequence of boundary layer
acceleration. Variations of the high energy bump of the electron distribution
can give rise to anomalous behaviour in X-ray band in comparison to the lower
frequencies.Comment: 7 pages, 1 figure included, accepted for publication by PASA Volume
19, Number 1: Special Issue on AGN Variability Across the Electromagnetic
Spectru
Association of tumor necrosis factor genetic polymorphism with chronic atrophic gastritis and gastric adenocarcinoma in Chinese Han population.
0.05). However, TNF-beta Ncol*1/2 and d2/d6 genotypes did not relate to age, gender, grade of differentiation and clinicopathologic stage in patients with gastric adenocarcinoma. The frequency of TNFa6b5c1 haplotype homozygote was significantly lower in patients with gastric adenocarcinoma than in healthy controls (1.79% vs 15.85%, P=0.006). CONCLUSION: TNFa10 allele may be a risk factor for chronic atrophic gastritis. TNF-beta Ncol*1/2 and d2/d6 genotypes are associated with the susceptibility to gastric adenocarcinoma, whereas TNFa6b5c1 haplotype homozygote may contribute to the resistance against gastric adenocarcinoma
Temporal and Spectral Variabilities of High Energy Emission from Blazars Using Synchrotron Self-Compton Models
Multiwavelength observations of blazars such as Mrk 421 and Mrk 501 show that
they exhibit strong short time variabilities in flare-like phenomena. Based on
the homogeneous synchrotron self-Compton (SSC) model and assuming that time
variability of the emission is initiated by changes in the injection of
nonthermal electrons, we perform detailed temporal and spectral studies of a
purely cooling plasma system. One important parameter is the total injected
energy E and we show how the synchrotron and Compton components respond as E
varies. We discuss in detail how one could infer important physical parameters
using the observed spectra. In particular, we could infer the size of the
emission region by looking for exponential decay in the light curves. We could
also test the basic assumption of SSC by measuring the difference in the rate
of peak energy changes of synchrotron and SSC peaks. We also show that the
trajectory in the photon-index and flux plane evolves clockwise or
counter-clockwise depending on the value of E and observed energy bands.Comment: 35 pages, 18 figures, accepted by the Astrophysical Journa
Stuck in the Past? Rumination-Related Memory Integration
Memories connected to ruminative concerns repetitively capture attention, even in situations designed to alter them. However, recent research on memory updating suggests that memory for benign substitutes (e.g., reinterpretations) might be facilitated by integration with the ruminative memories. As a first approach, two experiments (Ns = 72) mimicked rumination-related memories with rumination-themed stimuli and an imagery task. College undergraduates screened for ruminative status first studied and imaged ruminative cue-target word pairs, and then in a second phase they studied the same cues re-paired with benign targets (along with new and repeated pairs). On the test of cued recall of benign targets, they judged whether each recalled word had been repeated or changed across the two phases (or was new in the second phase). When target changes were not remembered, recall of benign targets revealed proactive interference that was insensitive to ruminative status. However, when participants remembered change and the ruminative targets, their recall of benign targets was facilitated, particularly if they identified as ruminators (Experiment 1). When the test simply asked for recall of either or both targets (Experiment 2), ruminators recalled both targets more frequently than did others. These outcomes suggest that ruminative memories might provide bridges to remembering associated benign memories, such as reinterpretations, under conditions consistent with everyday ruminative retrieval
Interleukin (IL)-1 gene polymorphisms: relevance of disease severity associated alleles with IL-1beta and IL-1ra production in multiple sclerosis.
BACKGROUND: Multiple sclerosis (MS) is an autoimmune disorder, with a considerable genetic influence on susceptibility and disease course. Cytokines play an important role in MS pathophysiology, and genes encoding various cytokines are logical candidates to assess possible associations with MS susceptibility and disease course. We previously reported an association of a combination of polymorphisms in the interleukin (IL)-1B and IL-1 receptor antagonist (IL-1RN) genes (i.e. IL-1RN allele 2+/IL-1B(+3959)allele 2-) with disease severity in MS. Extending this observation, we investigated whether IL-1beta and IL-1ra production differed depending on carriership of this gene combination. METHODS: Twenty MS patients and 20 controls were selected based upon carriership of the specific combination. In whole blood, in vitro IL-1beta and IL-1ra production was determined by enzyme-linked immunosorbent-assay after 6 and 24 h of stimulation with lipopolysaccharide. RESULTS: Carriers of the specific combination produced more IL-1ra, especially in MS patients, although not significantly. IL-1ra production was significantly higher in individuals homozygous for IL-1RN allele 2. In patients, Il-1ra production was higher and IL-1beta production lower compared with controls. In primary progressive patients, the IL-1beta /IL-1ra ratio was significantly lower than in relapsing-remitting patients. CONCLUSION: Our results suggest higher in vitro IL-1ra production in carriers of IL-1RN allele 2, with an indication of an allelic dose-effect relationship
Glacial flour dust storms in the Gulf of Alaska : hydrologic and meteorological controls and their importance as a source of bioavailable iron
Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 38 (2011): L06602, doi:10.1029/2010GL046573.Iron is an essential micronutrient that limits primary productivity in much of the ocean, including the Gulf of Alaska (GoA). However, the processes that transport iron to the ocean surface are poorly quantified. We combine satellite and meteorological data to provide the first description of widespread dust transport from coastal Alaska into the GoA. Dust is frequently transported from glacially-derived sediment at the mouths of several rivers, the most prominent of which is the Copper River. These dust events occur most frequently in autumn, when coastal river levels are low and riverbed sediments are exposed. The dust plumes are transported several hundred kilometers beyond the continental shelf into iron-limited waters. We estimate the mass of dust transported from the Copper River valley during one 2006 dust event to be between 25–80 ktons. Based on conservative estimates, this equates to a soluble iron loading of 30–200 tons. We suggest the soluble Fe flux from dust originating in glaciofluvial sediment deposits from the entire GoA coastline is two to three times larger, and is comparable to the annual Fe flux to GoA surface waters from eddies of coastal origin. Given that glaciers are retreating in the coastal GoA region and in other locations, it is important to examine whether fluxes of dust are increasing from glacierized landscapes to the ocean, and to assess the impact of associated Fe on marine ecosystems.We appreciate support from the USGS
CMGP, NCCWSC, the Mendenhall postdoc program, the Woods Hole
PEP intern program, and from NASA‐IDS
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