4,289 research outputs found
Gamma Rays from Clusters and Groups of Galaxies: Cosmic Rays versus Dark Matter
Clusters of galaxies have not yet been detected at gamma-ray frequencies;
however, the recently launched Fermi Gamma-ray Space Telescope, formerly known
as GLAST, could provide the first detections in the near future. Clusters are
expected to emit gamma rays as a result of (1) a population of high-energy
primary and re-accelerated secondary cosmic rays (CR) fueled by structure
formation and merger shocks, active galactic nuclei and supernovae, and (2)
particle dark matter (DM) annihilation. In this paper, we ask the question of
whether the Fermi telescope will be able to discriminate between the two
emission processes. We present data-driven predictions for a large X-ray flux
limited sample of galaxy clusters and groups. We point out that the gamma ray
signals from CR and DM can be comparable. In particular, we find that poor
clusters and groups are the systems predicted to have the highest DM to CR
emission at gamma-ray energies. Based on detailed Fermi simulations, we study
observational handles that might enable us to distinguish the two emission
mechanisms, including the gamma-ray spectra, the spatial distribution of the
signal and the associated multi-wavelength emissions. We also propose optimal
hardness ratios, which will help to understand the nature of the gamma-ray
emission. Our study indicates that gamma rays from DM annihilation with a high
particle mass can be distinguished from a CR spectrum even for fairly faint
sources. Discriminating a CR spectrum from a light DM particle will be instead
much more difficult, and will require long observations and/or a bright source.
While the gamma-ray emission from our simulated clusters is extended,
determining the spatial distribution with Fermi will be a challenging task
requiring an optimal control of the backgrounds.Comment: revised to match resubmitted version, 35 pages, 16 figures: results
unchanged, some discussion added and unnecessary text and figures remove
Constraints on Cold Dark Matter in the Gamma-ray Halo of NGC 253
A gamma-ray halo in a nearby starburst galaxy NGC 253 was found by the
CANGAROO-II Imaging Atmospheric Cherenkov Telescope (IACT). By fitting the
energy spectrum with expected curves from Cold Dark Matter (CDM) annihilations,
we constrain the CDM-annihilation rate in the halo of NGC 253. Upper limits for
the CDM density were obtained in the wide mass range between 0.5 and 50 TeV.
Although these limits are higher than the expected values, it is complementary
important to the other experimental techniques, especially considering the
energy coverage. We also investigate the next astronomical targets to improve
these limits.Comment: 13 pages, 5 figures, aastex.cls, natbib.sty, To appear in ApJ v596n1,
Oct. 10, 200
Oil Transportation in Pipelines with the Existence of Ice
PresentationWhen the ambient temperature is below freezing point, ice may form in the oil transportation pipelines, which can cause flow assurance issues, such as restricting flow path or even plugging the pipeline. Ice plugging was reported to delay the restart of the Poplar pipeline system gathering crude oil from Montana and North Dakota. [1] Ice may also pose threats to the Trans-Alyeska Pipeline System (TAPS). The declining throughput makes the oil get colder much faster. If oil temperature falls below the freezing point, ice forms and leads to flow assurance issues, such as coating critical valves, accumulating in the pipeline, and restricting flow. [2] This paper investigates the fundamentals of ice formation in the pipeline and its effect on the transportation system. A 2-inch diameter carbon steel flow loop was instrumented to measure pressure, temperature, and differential pressure. The experimental results show that ice formation can restrict flow at the low sport in front of the flow meter, the inserted thermocouples, and the perforated plate. Annular ice deposition was found at the pipe wall. The morphology of the deposition is rime ice, indicating the deposition is due to small ice crystals sticking to the pipe surface. It was found that the formation of annular deposition requires a negative temperature gradient. The effect of water cuts and fluid properties on plugging tendency is discussed. The mechanisms for ice deposition along the pipe and plugging at the pipe components are proposed
Editorial: Introducing the Transactions of the International Society for Music Information Retrieval
Clinical and magnetic resonance imaging characteristics of thoracolumbar intervertenral disk extrusions and protrusions in large breed dogs
It has recently been shown that the fat-derived hormone adiponectin has the ability to decrease hyperglycemia and to reverse insulin resistance. However, bacterially produced full-length adiponectin is functionally inactive. Here, we show that endogenous adiponectin secreted by adipocytes is post-translationally modified into eight different isoforms, as shown by two-dimensional gel electrophoresis. Carbohydrate detection revealed that six of the adiponectin isoforms are glycosylated. The glycosylation sites were mapped to several lysines (residues 68, 71, 80, and 104) located in the collagenous domain of adiponectin, each having the surrounding motif of GXKGE(D). These four lysines were found to be hydroxylated and subsequently glycosylated. The glycosides attached to each of these four hydroxylated lysines are possibly glucosylgalactosyl groups. Functional analysis revealed that full-length adiponectin produced by mammalian cells is much more potent than bacterially generated adiponectin in enhancing the ability of subphysiological concentrations of insulin to inhibit gluconeogenesis in primary rat hepatocytes, whereas this insulin-sensitizing ability was significantly attenuated when the four glycosylated lysines were substituted with arginines. These results indicate that full-length adiponectin produced by mammalian cells is functionally active as an insulin sensitizer and that hydroxylation and glycosylation of the four lysines in the collagenous domain might contribute to this activity.link_to_subscribed_fulltex
Spectrum of cosmic rays, produced in supernova remnants
Nonlinear kinetic theory of cosmic ray (CR) acceleration in supernova
remnants is employed to calculate CR spectra. The magnetic field in SNRs is
assumed to be significantly amplified by the efficiently accelerating nuclear
CR component. It is shown that the calculated CR spectra agree in a
satisfactory way with the existing measurements up to the energy eV.
The power law spectrum of protons extends up to the energy eV
with a subsequent exponential cutoff. It gives a natural explanation for the
observed knee in the Galactic CR spectrum. The maximum energy of the
accelerated nuclei is proportional to their charge number . Therefore the
break in the Galactic CR spectrum is the result of the contribution of
progressively heavier species in the overall CR spectrum so that at
eV the CR spectrum is dominated by iron group nuclei. It is shown that this
component plus a suitably chosen extragalactic CR component can give a
consistent description for the entire Galactic CR spectrum.Comment: 4 pages with emulateapj, 3 figures, accepted for publication in the
Astrophysical Journal Letter
- …