2,721 research outputs found
Cosmic-ray Acceleration at Ultrarelativistic Shock Waves: Effects of a "Realistic" Magnetic Field Structure
First-order Fermi acceleration processes at ultrarelativistic shocks are
studied with Monte Carlo simulations. The accelerated particle spectra are
derived by integrating the exact particle trajectories in a turbulent magnetic
field near the shock. ''Realistic'' features of the field structure are
included. We show that the main acceleration process at superluminal shocks is
the particle compression at the shock. Formation of energetic spectral tails is
possible in a limited energy range only for highly perturbed magnetic fields,
with cutoffs occuring at low energies within the resonance energy range
considered. These spectral features result from the anisotropic character of
particle transport in the downstream magnetic field, where field compression
produces effectively 2D perturbations. Because of the downstream field
compression, the acceleration process is inefficient in parallel shocks for
larger turbulence amplitudes, and features observed in oblique shocks are
recovered. For small-amplitude turbulence, wide-energy range particle spectra
are formed and modifications of the process due to the existence of long-wave
perturbations are observed. In both sub- and superluminal shocks, an increase
of \gamma leads to steeper spectra with lower cut-off energies. The spectra
obtained for the ``realistic'' background conditions assumed here do not
converge to the ``universal'' spectral index claimed in the literature. Thus
the role of the first-order Fermi process in astrophysical sources hosting
relativistic shocks requires serious reanalysis.Comment: submitted to Ap
Cosmic Ray Acceleration at Relativistic Shock Waves with a "Realistic" Magnetic Field Structure
The process of cosmic ray first-order Fermi acceleration at relativistic
shock waves is studied with the method of Monte Carlo simulations. The
simulations are based on numerical integration of particle equations of motion
in a turbulent magnetic field near the shock. In comparison to earlier studies,
a few "realistic" features of the magnetic field structure are included. The
upstream field consists of a mean field component inclined at some angle to the
shock normal with finite-amplitude sinusoidal perturbations imposed upon it.
The perturbations are assumed to be static in the local plasma rest frame.
Their flat or Kolmogorov spectra are constructed with randomly drawn wave
vectors from a wide range . The downstream field structure
is derived from the upstream one as compressed at the shock. We present
particle spectra and angular distributions obtained at mildly relativistic sub-
and superluminal shocks and also parallel shocks. We show that particle spectra
diverge from a simple power-law, the exact shape of the spectrum depends on
both the amplitude of the magnetic field perturbations and the wave power
spectrum. Features such as spectrum hardening before the cut-off at oblique
subluminal shocks and formation of power-law tails at superluminal ones are
presented and discussed. At parallel shocks, the presence of finite-amplitude
magnetic field perturbations leads to the formation of locally oblique field
configurations at the shock and the respective magnetic field compressions.
This results in the modification of the particle acceleration process,
introducing some features present in oblique shocks, e.g., particle reflections
from the shock. We demonstrate for parallel shocks a (nonmonotonic) variation
of the particle spectral index with the turbulence amplitude.Comment: revised version (37 pages, 13 figures
Comparison of the results of modified NDVI indicator established on the basis of measurements by hyperspectral spectrometer and digital camera
Due to the lack of readily available imaging remote sensing methods that would determine the vegetation state on a local scale, an attempt was made to calculate the modified NDVI index on the basis of digital camera photography. To verify the effectiveness of the method, the digital camera and hyperspectral spectrometer results were compared. For the analysis material leaves of six different tree species from an urban area were selected and picked in four different phases of the vegetative season. The results prove that there is no significant correlation between the two methods, but they also suggest that further research on the proposed method is necessary.
A Novel Approach in Constraining Electron Spectra in Blazar Jets: The Case of Markarian 421
We report results from the observations of the well studied TeV blazar Mrk
421 with the Swift and the Suzaku satellites in December 2008. During the
observation, Mrk 421 was found in a relatively low activity state, with the
corresponding 2-10 keV flux of erg/s/cm^2. For the purpose
of robust constraining the UV-to-X-ray emission continuum we selected only the
data corresponding to truly simultaneous time intervals between Swift and
Suzaku, allowing us to obtain a good-quality, broad-band spectrum despite a
modest length (0.6 ksec) exposure. We analyzed the spectrum with the parametric
forward-fitting SYNCHROTRON model implemented in XSPEC assuming two different
representations of the underlying electron energy distribution, both well
motivated by the current particle acceleration models: a power-law distribution
above the minimum energy with an exponential cutoff at the
maximum energy , and a modified ultra-relativistic Maxwellian
with an equilibrium energy . We found that the latter implies
unlikely physical conditions within the blazar zone of Mrk 421. On the other
hand, the exponentially moderated power-law electron distribution gives two
possible sets of the model parameters: (i) flat spectrum with low minimum electron energy , and
(ii) steep spectrum with high minimum electron energy
. We discuss different interpretations of
both possibilities in the context of a diffusive acceleration of electrons at
relativistic, sub- or superluminal shocks. We also comment on how exactly the
gamma-ray data can be used to discriminate between the proposed different
scenarios.Comment: 18 pages, 2 figures; accepted for publication in the Astrophysical
Journa
PtrA is required for coordinate regulation of gene expression during phosphate stress in a marine Synechococcus
Previous microarray analyses have shown a key role for the two-component system PhoBR (SYNW0947, SYNW0948) in the regulation of P transport and metabolism in the marine cyanobacterium Synechococcus sp. WH8102. However, there is some evidence that another regulator, SYNW1019 (PtrA), probably under the control of PhoBR, is involved in the response to P depletion. PtrA is a member of the cAMP receptor protein transcriptional regulator family that shows homology to NtcA, the global nitrogen regulator in cyanobacteria. To define the role of this regulator, we constructed a mutant by insertional inactivation and compared the physiology of wild-type Synechcococcus sp. WH8102 with the ptrA mutant under P-replete and P-stress conditions. In response to P stress the ptrA mutant failed to upregulate phosphatase activity. Microarrays and quantitative RT-PCR indicate that a subset of the Pho regulon is controlled by PtrA, including two phosphatases, a predicted phytase and a gene of unknown function psip1 (SYNW0165), all of which are highly upregulated during P limitation. Electrophoretic mobility shift assays indicate binding of overexpressed PtrA to promoter sequences upstream of the induced genes. This work suggests a two-tiered response to P depletion in this strain, the first being PhoB-dependent induction of high-affinity PO4 transporters, and the second the PtrA-dependent induction of phosphatases for scavenging organic P. The levels of numerous other transcripts are also directly or indirectly influenced by PtrA, including those involved in cell-surface modification, metal uptake, photosynthesis, stress responses and other metabolic processes, which may indicate a wider role for PtrA in cellular regulation in marine picocyanobacteria
Drift routes of Cape hake eggs and larvae in the southern Benguela Current system
The aim of this study was to combine observed circulation pattern with data on distribution of hake eggs and larvae in the southern Benguela from a survey in September/October 2005 to investigate drift routes of hake eggs and larvae. Genetic information enabled species-specific information about drift routes of the two hake species (Merluccius capensis and M. paradoxus) to be established. The results showed that both species were transported from spawning areas to nursery areas in the jet current, but differential cross-shelf distribution would most likely lead to species-specific drift routes which could explain why the two species seem to have different nursery areas
Carbon dioxide adsorption and interaction with formation fluids of Jordanian unconventional reservoirs
Shales are mostly unexploited energy resources. However, the extraction and production of their hydrocarbons require innovative methods. Applications involving carbon dioxide in shales could combine its potential use in oil recovery with its storage in view of its impact on global climate. The success of these approaches highly depends on various mechanisms taking place in the rock pores simultaneously. In this work, properties governing these mechanisms are presented at technically relevant conditions. The pendant and sessile drop methods are utilized to measure interfacial tension and wettability, respectively. The gravimetric method is used to quantify CO2 adsorption capacity of shale and gas adsorption kinetics is evaluated to determine diffusion coefficients. It is found that interfacial properties are strongly affected by the operating pressure. The oil-CO2 interfacial tension shows a decrease from approx. 21 mN/m at 0.1 MPa to around 3 mN/m at 20 MPa. A similar trend is observed in brine-CO2 systems. The diffusion coefficient is observed to slightly increase with pressure at supercritical conditions. Finally, the contact angle is found to be directly related to the gas adsorption at the rock surface: Up to 3.8 wt% of CO2 is adsorbed on the shale surface at 20 MPa and 60 °C where a maximum in contact angle is also found. To the best of the author’s knowledge, the affinity of calcite-rich surfaces toward CO2 adsorption is linked experimentally to the wetting
behavior for the first time. The results are discussed in terms of CO2 storage scenarios occurring optimally at 20 MPa
Messung von exhaliertem Kohlenmonoxid bei maschinell beatmeten, kritisch kranken Patienten und deren Korrelation mit Organdysfunktionen
Endogenes Kohlenmonoxid entsteht beim Abbau von Hämoglobin. Im Abbauprozess von Hämoglobin kommt insbesondere einem Enzym eine tragende Rolle zu, der Hämoxygenase 1. Wie kein anderes Enzym reagiert die Hämoxygenase 1 extrem empfindlich auf verschiedene Stresssoren, insbesondere auf Stimuli, die oxidativen Stress verstärken. Die Eigenschaften von Kohlenmonoxid kann man als vasodilatierend, antioxidativ, anitiinflamatorisch und antiproliferativ bezeichnen. In dieser Arbeit wurde bei insgesamt 95 kritisch kranken Patienten, die unter verschiedenen Erkrankungen und ferner allesamt unter einer respiratorischen Insuffizienz mit mindestens 24 stündiger maschineller Beatmung litten, stündlich dass exhalatorische Kohlenmonoxid gemessen. Zum ersten Mal konnte ein bedeutender Zusammenhang zwischen Multiple Organ Dysfunction Score und endogener Kohlenmonoxid- Produktion demonstriert werden. Somit reflektiert die endogene Kohlenmonoxid- Produktion die schwere der akuten Organdysfunktion
No Tradeoff between Coherence and Sub-Poissonianity for Heisenberg-Limited Lasers
The Heisenberg limit to laser coherence -- the number of
photons in the maximally populated mode of the laser beam -- is the fourth
power of the number of excitations inside the laser. We generalize the previous
proof of this upper bound scaling by dropping the requirement that the beam
photon statistics be Poissonian (i.e., Mandel's ). We then show that the
relation between and sub-Poissonianity () is win-win, not a
tradeoff. For both regular (non-Markovian) pumping with semi-unitary gain
(which allows ), and random (Markovian) pumping with
optimized gain, is maximized when is minimized.Comment: This is a companion letter to the manuscript entitled "Optimized
Laser Models with Heisenberg-Limited Coherence and Sub-Poissonian Beam Photon
Statistics", arxiv:2208.14082. 6 pages, 2 figure
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