41 research outputs found
Constraints on Dark Matter Annihilation in Clusters of Galaxies with the Fermi Large Area Telescope
Nearby clusters and groups of galaxies are potentially bright sources of
high-energy gamma-ray emission resulting from the pair-annihilation of dark
matter particles. However, no significant gamma-ray emission has been detected
so far from clusters in the first 11 months of observations with the Fermi
Large Area Telescope. We interpret this non-detection in terms of constraints
on dark matter particle properties. In particular for leptonic annihilation
final states and particle masses greater than ~200 GeV, gamma-ray emission from
inverse Compton scattering of CMB photons is expected to dominate the dark
matter annihilation signal from clusters, and our gamma-ray limits exclude
large regions of the parameter space that would give a good fit to the recent
anomalous Pamela and Fermi-LAT electron-positron measurements. We also present
constraints on the annihilation of more standard dark matter candidates, such
as the lightest neutralino of supersymmetric models. The constraints are
particularly strong when including the fact that clusters are known to contain
substructure at least on galaxy scales, increasing the expected gamma-ray flux
by a factor of ~5 over a smooth-halo assumption. We also explore the effect of
uncertainties in cluster dark matter density profiles, finding a systematic
uncertainty in the constraints of roughly a factor of two, but similar overall
conclusions. In this work, we focus on deriving limits on dark matter models; a
more general consideration of the Fermi-LAT data on clusters and clusters as
gamma-ray sources is forthcoming.Comment: accepted to JCAP, Corresponding authors: T.E. Jeltema and S. Profumo,
minor revisions to be consistent with accepted versio
DEVELOPMENT of the MODEL of GALACTIC INTERSTELLAR EMISSION for STANDARD POINT-SOURCE ANALYSIS of FERMI LARGE AREA TELESCOPE DATA
Most of the celestial \u3b3 rays detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope originate from the interstellar medium when energetic cosmic rays interact with interstellar nucleons and photons. Conventional point-source and extended-source studies rely on the modeling of this diffuse emission for accurate characterization. Here, we describe the development of the Galactic Interstellar Emission Model (GIEM), which is the standard adopted by the LAT Collaboration and is publicly available. This model is based on a linear combination of maps for interstellar gas column density in Galactocentric annuli and for the inverse-Compton emission produced in the Galaxy. In the GIEM, we also include large-scale structures like Loop I and the Fermi bubbles. The measured gas emissivity spectra confirm that the cosmic-ray proton density decreases with Galactocentric distance beyond 5 kpc from the Galactic Center. The measurements also suggest a softening of the proton spectrum with Galactocentric distance. We observe that the Fermi bubbles have boundaries with a shape similar to a catenary at latitudes below 20\ub0 and we observe an enhanced emission toward their base extending in the north and south Galactic directions and located within \u2dc4\ub0 of the Galactic Center
The On-orbit Calibrations for the Fermi Large Area Telescope
The Large Area Telescope (LAT) on--board the Fermi Gamma ray Space Telescope
began its on--orbit operations on June 23, 2008. Calibrations, defined in a
generic sense, correspond to synchronization of trigger signals, optimization
of delays for latching data, determination of detector thresholds, gains and
responses, evaluation of the perimeter of the South Atlantic Anomaly (SAA),
measurements of live time, of absolute time, and internal and spacecraft
boresight alignments. Here we describe on orbit calibration results obtained
using known astrophysical sources, galactic cosmic rays, and charge injection
into the front-end electronics of each detector. Instrument response functions
will be described in a separate publication. This paper demonstrates the
stability of calibrations and describes minor changes observed since launch.
These results have been used to calibrate the LAT datasets to be publicly
released in August 2009.Comment: 60 pages, 34 figures, submitted to Astroparticle Physic
Redução da poda hibernal e aumento da produção de pereiras 'Hosui' pelo uso de prohexadiona cálcio
Multi-wavelength observations of blazar AO 0235+164 in the 2008-2009 flaring state
The blazar AO 0235+164 (z = 0.94) has been one of the most active objects observed by Fermi Large Area Telescope (LAT) since its launch in Summer 2008. In addition to the continuous coverage by Fermi, contemporaneous observations were carried out from the radio to γ-ray bands between 2008 September and 2009 February. In this paper, we summarize the rich multi-wavelength data collected during the campaign (including F-GAMMA, GASP-WEBT, Kanata, OVRO, RXTE, SMARTS, Swift, and other instruments), examine the cross-correlation between the light curves measured in the different energy bands, and interpret the resulting spectral energy distributions in the context of well-known blazar emission models. We find that the γ-ray activity is well correlated with a series of near-IR/optical flares, accompanied by an increase in the optical polarization degree. On the other hand, the X-ray light curve shows a distinct 20 day high state of unusually soft spectrum, which does not match the extrapolation of the optical/UV synchrotron spectrum. We tentatively interpret this feature as the bulk Compton emission by cold electrons contained in the jet, which requires an accretion disk corona with an effective covering factor of 19% at a distance of 100 R g. We model the broadband spectra with a leptonic model with external radiation dominated by the infrared emission from the dusty torus. © 2012. The American Astronomical Society. All rights reserved
The spectral energy distribution of fermi bright blazars
We have conducted a detailed investigation of the broadband spectral properties of the γ-ray selected blazars of the Fermi LAT Bright AGN Sample (LBAS). By combining our accurately estimated Fermi γ-ray spectra with Swift, radio, infra-red, optical, and other hard X-ray/γ-ray data, collected within 3 months of the LBAS data taking period, we were able to assemble high-quality and quasi-simultaneous spectral energy distributions (SED) for 48 LBAS blazars. The SED of these γ-ray sources is similar to that of blazars discovered at other wavelengths, clearly showing, in the usual log ν-log ν Fν representation, the typical broadband spectral signatures normally attributed to a combination of low-energy synchrotron radiation followed by inverse Compton emission of one or more components. We have used these SED to characterize the peak intensity of both the low- and the high-energy components. The results have been used to derive empirical relationships that estimate the position of the two peaks from the broadband colors (i.e., the radio to optical, αro, and optical to X-ray, αox, spectral slopes) and from the γ-ray spectral index. Our data show that the synchrotron peak frequency (νSpeak) is positioned between 1012.5 and 1014.5 Hz in broad-lined flat spectrum radio quasars (FSRQs) and between 10 13 and 1017 Hz in featureless BL Lacertae objects. We find that the γ-ray spectral slope is strongly correlated with the synchrotron peak energy and with the X-ray spectral index, as expected at first order in synchrotron-inverse Compton scenarios. However, simple homogeneous, one-zone, synchrotron self-Compton (SSC) models cannot explain most of our SED, especially in the case of FSRQs and low energy peaked (LBL) BL Lacs. More complex models involving external Compton radiation or multiple SSC components are required to reproduce the overall SED and the observed spectral variability. While more than 50% of known radio bright high energy peaked (HBL) BL Lacs are detected in the LBAS sample, only less than 13% of known bright FSRQs and LBL BL Lacs are included. This suggests that the latter sources, as a class, may be much fainter γ-ray emitters than LBAS blazars, and could in fact radiate close to the expectations of simple SSC models. We categorized all our sources according to a new physical classification scheme based on the generally accepted paradigm for Active Galactic Nuclei and on the results of this SED study. Since the LAT detector is more sensitive to flat spectrum γ-ray sources, the correlation between νSpeak and γ-ray spectral index strongly favors the detection of high energy peaked blazars, thus explaining the Fermi overabundance of this type of sources compared to radio and EGRET samples. This selection effect is similar to that experienced in the soft X-ray band where HBL BL Lacs are the dominant type of blazars. © 2010 The American Astronomical Society
The use of Brazilian vegetable oils in nanoemulsions: an update on preparation and biological applications
Insights into the high-energy γ-ray emission of Markarian 501 from extensive multifrequency observations in the Fermi era
We report on the γ-ray activity of the blazar Mrk 501 during the first 480 days of Fermi operation. We find that the average Large Area Telescope (LAT) γ-ray spectrum of Mrk 501 can be well described by a single power-law function with a photon index of 1.78 ± 0.03. While we observe relatively mild flux variations with the Fermi-LAT (within less than a factor of two), we detect remarkable spectral variability where the hardest observed spectral index within the LAT energy range is 1.52 ± 0.14, and the softest one is 2.51 ± 0.20. These unexpected spectral changes do not correlate with the measured flux variations above 0.3 GeV. In this paper, we also present the first results from the 4.5 month long multifrequency campaign (2009 March 15-August 1) on Mrk 501, which included the Very Long Baseline Array (VLBA), Swift, RXTE, MAGIC, and VERITAS, the F-GAMMA, GASP-WEBT, and other collaborations and instruments which provided excellent temporal and energy coverage of the source throughout the entire campaign. The extensive radio to TeV data set from this campaign provides us with the most detailed spectral energy distribution yet collected for this source during its relatively low activity. The average spectral energy distribution of Mrk 501 is well described by the standard one-zone synchrotron self-Compton (SSC) model. In the framework of this model, we find that the dominant emission region is characterized by a size ≲0.1 pc (comparable within a factor of few to the size of the partially resolved VLBA core at 15-43 GHz), and that the total jet power (≃1044 erg s-1) constitutes only a small fraction (∼10-3) of the Eddington luminosity. The energy distribution of the freshly accelerated radiating electrons required to fit the time-averaged data has a broken power-law form in the energy range 0.3 GeV-10 TeV, with spectral indices 2.2 and 2.7 below and above the break energy of 20 GeV. We argue that such a form is consistent with a scenario in which the bulk of the energy dissipation within the dominant emission zone of Mrk 501 is due to relativistic, proton-mediated shocks. We find that the ultrarelativistic electrons and mildly relativistic protons within the blazar zone, if comparable in number, are in approximate energy equipartition, with their energy dominating the jet magnetic field energy by about two orders of magnitude. © 2011. The American Astronomical Society
Microstructural Modification By Laser Surface Remelting And Its Effect On The Corrosion Resistance Of An Al-9 Wt%si Casting Alloy
Aluminum alloys with silicon as a major alloying element constitute a class of materials, which provides the most significant part of all shaped castings manufactured. Such alloys have a wide range of applications in the automotive and aerospace industries. The literature presents contradictory results and no satisfactory explanations concerning to resulting microstructures provided by laser surface remelting (LSR) and its effect on the electrochemical behavior of Al-Si alloys. The aim of this study was to investigate the effect of microstructural refinement by LSR on corrosion resistance of an Al-9 wt%Si casting alloy. As-cast samples were subjected to a continuous 1 kW CO2 laser. Corrosion resistance has been analyzed by an electrochemical impedance spectroscopy (EIS) technique and polarization curves carried out in both 0.5 M NaCl and 0.5 M H2SO4 solutions at 25 °C. An equivalent circuit has also been proposed and impedance parameters were simulated by the ZView® software. It was found that the structural modification provided by the LSR process induces a decreasing effect on the corrosion resistance when compared to that of the untreated sample. © 2007 Elsevier B.V. All rights reserved.254927632770Rooy, E.L., (1988) Metals Handbook, 15. , ASM International, Materials Park, Ohio p. 743Bokota, A., Iskierka, S., (1996) Acta Mater., 44, p. 445Cheung, N., Ierardi, M.C.F., Garcia, A., Vilar, R., (2000) Lasers Eng., 10, p. 275Gu, B.W., Ma, T.C., Brown, S.K., Mannik, L., (1994) Mater. Sci. Technol., 10, p. 425Kou, S., Hsu, S.C., Mehrabian, R., (1981) Metall. Trans., 12 B, p. 33Steen, P.H., Ehrhard, P., Schüssler, A., (1994) Metall. Mater. Trans., 25 A, p. 427Chong, P.H., Liu, Z., Skeldon, P., Thompson, G.E., (2003) Appl. Surf. Sci., 208-209, p. 399Xianqing, Y., Chengjun, Z., Xuefeng, S., Manping, H., Jianguo, M., (2007) Appl. Surf. Sci., 253, p. 4409Kurz, W., Fisher, D.J., (1992) Fundamentals of Solidification, , Trans Tech Publications, SwitzerlandMunitz, A., (1985) Metall. Trans., 16 B, p. 149Zimmermann, M., Carrard, M., Kurz, W., (1989) Acta Metall., 37, p. 3305Liu, Z., Chong, P.H., Skeldon, A.P., Hilton, P.A., Spencer, J.T., Quayle, B., (2006) Surf. Coat. Technol., 200, p. 5514Osório, W.R., Spinelli, J.E., Freire, C.M., Garcia, A., (2007) Mater. Manuf. Proc., 22, p. 341Goulart, P.R., Osório, W.R., Spinelli, J.E., Garcia, A., (2007) Mater. Manuf. Proc., 22, p. 328Osório, W.R., Goulart, P.R., Garcia, A., (2008) Mater. Lett., 62, p. 365Virtanen, S., Böhni, H., Busin, R., Marchione, T., Pierantoni, M., Blank, E., (1994) Corros. Sci., 36, p. 1625Opera, B.K., Andriyakhin, V.M., Volgin, V.I., Bandurkin, V.V., (1985) Prot. Met., 21, p. 74Wong, T.T., Liang, G.Y., (1997) J. Mater. Proc. Technol., 63, p. 930Wong, T.T., Liang, G.Y., Tang, C.Y., (1997) J. Mater. Proc. Technol., 66, p. 172Osório, W.R., Goulart, P.R., Santos, G.A., Moura Neto, C., Garcia, A., (2006) Metall. Mater. Trans., 37 A, p. 2525Gudic, S., Radosevic, J., Kliskic, M., (2002) Electrochim. Acta, 47, p. 3009Assis, S.L., Wolynec, S., Costa, I., (1815) Electrochim. Acta, 51 (2006)Osório, W.R., Spinelli, J.E., Ferreira, I.L., Garcia, A., (2007) Electrochim. Acta, 52, p. 3265Osório, W.R., Garcia, L.R., Goulart, P.R., Garcia, A., (2007) Mater. Chem. Phys., 106, p. 343Osório, W.R., Cheung, N., Spinelli, J.E., Goulart, P.R., Garcia, A., (2007) J. Solid State Electrochem., 11, p. 1421Osório, W.R., Freire, C.M., Garcia, A., (2005) J. Alloys Compd., 397, p. 17