Aims. Radio observations have shown that galaxy clusters are giant reservoirs of cosmic rays (CR). Although a gamma-ray signal from the cluster volume is expected to arise through interactions of CR ...protons with the ambient plasma, a confirming observation is still missing. Methods. We searched for a cumulative gamma-ray emission in the direction of galaxy clusters by analysing a collection of stacked Fermi-LAT count maps. Additionally, we investigated possible systematic differences in the emission between cool-core and non-cool-core cluster populations. Results. Making use of a sample of 53 clusters selected from the HIFLUGCS catalog, we do not detect a significant signal from the stacked sample. The upper limit on the average flux per cluster derived for the total stacked sample is at the level of a few 10-11 ph cm-2 s-1 at a 95% confidence level in the 1–300 GeV band, assuming power-law spectra with photon indices 2.0, 2.4, 2.8, and 3.2. Separate stacking of the cool-core and non-cool-core clusters in the sample lead to similar values of around 5 × 10-11 ph cm-2 s-1 and 2 × 10-11 ph cm-2 s-1, respectively. Conclusions. Under the assumption that decaying π0, produced in collisions between CRs and the ambient thermal gas, are responsible for gamma-ray emission, we set upper limits on the average CR content in galaxy clusters. For the entire cluster population, our upper limit on the gamma-ray flux translates into an upper limit on the average CR-to-thermal energy ratio of 4.6% for a photon index of 2.4, although it is possible for individual systems to exceed this limit. Our 95% upper limits are at the level expected from numerical simulations, which most likely suggests that the injection of CR at cosmological shocks is less efficient than previously assumed.
Context.η Carinae is the colliding wind binary with the highest mass-loss rate in our Galaxy and the only one in which hard X-ray emission has been detected. Aims.η Carinae is therefore a primary ...candidate to search for particle acceleration by probing its gamma-ray emission. Methods. We used the first 21 months of Fermi/LAT data to extract gamma-ray (0.2–100 GeV) images, spectra, and light-curves, then combined them with multi-wavelength observations to model the non-thermal spectral energy distribution. Results. A bright gamma-ray source is detected at the position of η Carinae. Its flux at a few 100 MeV can be modelled by an extrapolation of the hard X-ray spectrum towards higher energies. The spectral energy distribution features two distinct components. The first one extends from the keV to GeV energy range, and features an exponential cutoff at ~1 GeV. It can be understood as inverse Compton scattering of ultraviolet photons by electrons accelerated up to γ ~ 104 in the colliding wind region. The expected synchrotron emission is compatible with the existing upper limit to the non-thermal radio emission. The second component is a hard gamma-ray tail detected above 20 GeV. It could be explained by π0-decay of accelerated hadrons interacting with the dense stellar wind. The ratio of the fluxes of the π0 to inverse Compton components is roughly as predicted by simulations of colliding wind binaries. This hard gamma-ray tail can only be understood if emitted close to the wind collision region. The energy transferred to the accelerated particles (~5% of the collision mechanical energy) is comparable to that of the thermal X-ray emission. Conclusions. The electron spectrum responsible for the keV to GeV emission was modelled and the observed emission above 20 GeV strongly suggests hadronic acceleration in η Carinae. These observations are thus in good agreement with the colliding wind scenario proposed for η Carinae.
We report the serendipitous discovery of transient, point-like high energy γ-ray emission coincident with the position of the suspected BL Lac object 5C 3.178. The source was detected using the Fermi ...Large Area Telescope (LAT) at a significance level of ~8σ during a 200 day period which began on November 17th 2011 (MJD 55 882). The observed γ-ray emission is characterised by a moderate 0.2–300 GeV flux F0.2−300 GeV = (8.22 ± 2.04) × 10-9 ph cm-2 s-1 and a hard power law spectrum with spectral index Γ = 1.76 ± 0.09. These properties appear consistent with the known sub-population of TeV γ-ray-emitting BL Lac objects, implying that the source may be detectable using atmospheric Cherenkov telescope arrays. Moreover, the temporal variability of the source suggests that real-time searches of the Fermi-LAT all-sky dataset for weak emission on ~200 day timescales may represent a rewarding strategy.
OBJECTIVES: Adipocyte is the only cell whose size may vary dramatically in physiological conditions. We hypothesized that increase in fat cell size per se could modulate several signalling pathways ...by changing the relationships between the cell and the extracellular matrix. The aim of the current study was (i) to examine whether within the same fat depot, metabolic functions of adipocyte were modified by cell size and (ii) if such an adaptation exists, to look for an integrin/extracellular-signal-regulated kinases (ERKs) signalling pathway. RESULTS: We isolated two populations of adipocytes with different volumes (67 and 22 10(3) μm3) within the same adipose location. In large compared to small fat cells, fatty acid synthase and lipoprotein lipase activities were increased two- and seven-fold, respectively; GLUT4 protein concentration and leptin expression were increased three-fold; lipolytic capacity was increased four-fold. The integrin/ERK signalling pathway could be the one responsible for the adaptation of adipose functions to cell size. In large compared with small adipocytes, we showed that β1-integrins are present in adipose membranes and at a higher concentration in large than in small cells. In isolated adipocytes, stimulation of β1-integrins with a specific monoclonal antibody results in ERK1 and ERK2 activation. In large compared to small cells, cytoplasmic concentrations of these two mitogen-activated protein kinases were increased two-fold, whereas their activities were increased 10-fold. CONCLUSION: A β1-integrin/ERKs signalling pathway is present in mature adipocyte. Increase in cell size, by modifying the relationships between cell and extracellular matrix, could turn on this pathway. Since ERKs can modulate transcription factors and subsequently modulate gene expression important for adipose function, this pathway could play an important role in the adaptation of adipose functions to cell size.
Dark matter annihilations search in dwarf spheroidal galaxies with fermi Farnier, C.; Nuss, E.; Cohen-Tanugi, J.
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
02/2011, Letnik:
630, Številka:
1
Journal Article
Recenzirano
Launched in June 2008, the Fermi Gamma-ray Telescope includes a pair conversion detector designed for the 20
MeV to
∼
300
GeV
gamma-ray sky study, the Large Area Telescope (LAT). Operating in all-sky ...survey mode, its excellent sensitivity and angular resolution will allow either to discover or constrain a signal coming through the annihilation of dark matter particles. Predicted by cold dark matter scenarios as the largest clumps, dwarf spheroidal galaxies are amongst the most attractive targets for indirect search of dark matter by gamma-ray experiments. We present here an overview of the Fermi LAT Dark Matter and New Physics Working Group efforts in the searches of gamma-ray fluxes coming from WIMP pair annihilations in dwarf spheroidal galaxies.
► Extensive simulations for the design and optimization of CTA were carried out. ► Different sets of simulation tools were checked against each other. ► Large-scale simulations with 275 telescopes ...for later layout selection.► Different analysis methods were applied and compared. ► Resulting sensitivity predictions confirmed the goals of CTA.
The Cherenkov Telescopes Array (CTA) is planned as the future instrument for very-high-energy (VHE) gamma-ray astronomy with a wide energy range of four orders of magnitude and an improvement in sensitivity compared to current instruments of about an order of magnitude. Monte Carlo simulations are a crucial tool in the design of CTA. The ultimate goal of these simulations is to find the most cost-effective solution for given physics goals and thus sensitivity goals or to find, for a given cost, the solution best suited for different types of targets with CTA. Apart from uncertain component cost estimates, the main problem in this procedure is the dependence on a huge number of configuration parameters, both in specifications of individual telescope types and in the array layout. This is addressed by simulation of a huge array intended as a superset of many different realistic array layouts, and also by simulation of array subsets for different telescope parameters. Different analysis methods – in use with current installations and extended (or developed specifically) for CTA – are applied to the simulated data sets for deriving the expected sensitivity of CTA. In this paper we describe the current status of this iterative approach to optimize the CTA design and layout.
Enlarged fat cells exhibit modified metabolic capacities, which could be involved in the metabolic complications of obesity at the whole body level. We show here that sterol regulatory ...element-binding protein 2 (SREBP-2) and its target genes are induced in the adipose tissue of several models of rodent obesity, suggesting cholesterol imbalance in enlarged adipocytes. Within a particular fat pad, larger adipocytes have reduced membrane cholesterol concentrations compared with smaller fat cells, demonstrating that altered cholesterol distribution is characteristic of adipocyte hypertrophy per se. We show that treatment with methyl-beta-cyclodextrin, which mimics the membrane cholesterol reduction of hypertrophied adipocytes, induces insulin resistance. We also produced cholesterol depletion by mevastatin treatment, which activates SREBP-2 and its target genes. The analysis of 40 adipocyte genes showed that the response to cholesterol depletion implicated genes involved in cholesterol traffic (caveolin 2, scavenger receptor BI, and ATP binding cassette 1 genes) but also adipocyte-derived secretion products (tumor necrosis factor alpha, angiotensinogen, and interleukin-6) and proteins involved in energy metabolism (fatty acid synthase, GLUT 4, and UCP3). These data demonstrate that altering cholesterol balance profoundly modifies adipocyte metabolism in a way resembling that seen in hypertrophied fat cells from obese rodents or humans. This is the first evidence that intracellular cholesterol might serve as a link between fat cell size and adipocyte metabolic activity.
This paper presents light curves as well as the first systematic characterization of variability of the 106 objects in the high-confidence Fermi Large Area Telescope Bright AGN Sample (LBAS). Weekly ...light curves of this sample, obtained during the first 11 months of the Fermi survey (2008 August 4-2009 July 4), are tested for variability and their properties are quantified through autocorrelation function and structure function analysis. For the brightest sources, 3 or 4 day binned light curves are extracted in order to determine power density spectra (PDSs) and to fit the temporal structure of major flares. More than 50% of the sources are found to be variable with high significance, where high states do not exceed 1/4 of the total observation range. Variation amplitudes are larger for flat spectrum radio quasars and low/intermediate synchrotron frequency peaked BL Lac objects. Autocorrelation timescales derived from weekly light curves vary from four to a dozen of weeks. Variable sources of the sample have weekly and 3-4 day bin light curves that can be described by 1/f(alpha) PDS, and show two kinds of gamma-ray variability: (1) rather constant baseline with sporadic flaring activity characterized by flatter PDS slopes resembling flickering and red noise with occasional intermittence and (2)-measured for a few blazars showing strong activity-complex and structured temporal profiles characterized by long-term memory and steeper PDS slopes, reflecting a random walk underlying mechanism. The average slope of the PDS of the brightest 22 FSRQs and of the 6 brightest BL Lacs is 1.5 and 1.7, respectively. The study of temporal profiles of well-resolved flares observed in the 10 brightest LBAS sources shows that they generally have symmetric profiles and that their total duration vary between 10 and 100 days. Results presented here can assist in source class recognition for unidentified sources and can serve as reference for more detailed analysis of the brightest gamma-ray blazars.
We present detailed observations of the bright short-hard gamma-ray burst GRB 090510 made with the Gammaray Burst Monitor (GBM) and Large Area Telescope (LAT) on board the Fermi observatory. GRB ...090510 is the first burst detected by the LAT that shows strong evidence for a deviation from a Band spectral fitting function during the prompt emission phase. The time-integrated spectrum is fit by the sum of a Band function with E-peak = 3.9 +/- 0.3 MeV, which is the highest yet measured, and a hard power-law component with photon index -1.62 +/- 0.03 that dominates the emission below approximate to 20 keV and above approximate to 100 MeV. The onset of the high-energy spectral component appears to be delayed by similar to 0.1 s with respect to the onset of a component well fit with a single Band function. A faint GBM pulse and a LAT photon are detected 0.5 s before the main pulse. During the prompt phase, the LAT detected a photon with energy 30.5(-2.6)(+5.8) GeV, the highest ever measured from a short GRB. Observation of this photon sets a minimum bulk outflow Lorentz factor, Gamma greater than or similar to 1200, using simple.. opacity arguments for this GRB at redshift z = 0.903 and a variability timescale on the order of tens of ms for the approximate to 100 keV-few MeV flux. Stricter high confidence estimates imply Gamma greater than or similar to 1000 and still require that the outflows powering short GRBs are at least as highly relativistic as those of long-duration GRBs. Implications of the temporal behavior and power-law shape of the additional component on synchrotron/synchrotron self-Compton, external-shock synchrotron, and hadronic models are considered.
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