| 11. |
- Abdo, A. A., et al.
(författare)
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FERMI-LAT OBSERVATIONS OF THE GEMINGA PULSAR
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 720:1, s. 272-283
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Tidskriftsartikel (refereegranskat)abstract
- We report on the Fermi-LAT observations of the Geminga pulsar, the second brightest non-variable GeV source in the gamma-ray sky and the first example of a radio-quiet gamma-ray pulsar. The observations cover one year, from the launch of the Fermi satellite through 2009 June 15. A data sample of over 60,000 photons enabled us to build a timing solution based solely on gamma-rays. Timing analysis shows two prominent peaks, separated by Delta phi = 0.497 +/- 0.004 in phase, which narrow with increasing energy. Pulsed gamma-rays are observed beyond 18 GeV, precluding emission below 2.7 stellar radii because of magnetic absorption. The phase-averaged spectrum was fitted with a power law with exponential cutoff of spectral index Gamma = (1.30 +/- 0.01 +/- 0.04), cutoff energy E-0 = (2.46 +/- 0.04 +/- 0.17) GeV, and an integral photon flux above 0.1 GeV of (4.14 +/- 0.02 +/- 0.32) x 10(-6) cm(-2) s(-1). The first uncertainties are statistical and the second ones are systematic. The phase-resolved spectroscopy shows a clear evolution of the spectral parameters, with the spectral index reaching a minimum value just before the leading peak and the cutoff energy having maxima around the peaks. The phase-resolved spectroscopy reveals that pulsar emission is present at all rotational phases. The spectral shape, broad pulse profile, and maximum photon energy favor the outer magnetospheric emission scenarios.
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| 12. |
- Abdo, A. A., et al.
(författare)
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Gamma-ray light curves and variability of bright fermi-detected blazars
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 722:1, s. 520-542
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 13. |
- Abdo, A. A., et al.
(författare)
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OBSERVATIONS OF MILKY WAY DWARF SPHEROIDAL GALAXIES WITH THE FERMI-LARGE AREA TELESCOPE DETECTOR AND CONSTRAINTS ON DARK MATTER MODELS
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 712:1, s. 147-158
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Tidskriftsartikel (refereegranskat)abstract
- We report on the observations of 14 dwarf spheroidal galaxies (dSphs) with the Fermi Gamma-Ray Space Telescope taken during the first 11 months of survey mode operations. The Fermi telescope, which is conducting an all-sky gamma-ray survey in the 20 MeV to > 300 GeV energy range, provides a new opportunity to test particle dark matter models through the expected gamma-ray emission produced by pair annihilation of weakly interacting massive particles (WIMPs). Local Group dSphs, the largest galactic substructures predicted by the cold dark matter scenario, are attractive targets for such indirect searches for dark matter because they are nearby and among the most extreme dark matter dominated environments. No significant gamma-ray emission was detected above 100 MeV from the candidate dwarf galaxies. We determine upper limits to the gamma-ray flux assuming both power-law spectra and representative spectra from WIMP annihilation. The resulting integral flux above 100 MeV is constrained to be at a level below around 10(-9) photons cm(-2) s(-1). Using recent stellar kinematic data, the gamma-ray flux limits are combined with improved determinations of the dark matter density profile in eight of the 14 candidate dwarfs to place limits on the pair-annihilation cross section of WIMPs in several widely studied extensions of the standard model, including its supersymmetric extension and other models that received recent attention. With the present data, we are able to rule out large parts of the parameter space where the thermal relic density is below the observed cosmological dark matter density and WIMPs (neutralinos here) are dominantly produced non-thermally, e. g., in models where supersymmetry breaking occurs via anomaly mediation. The gamma-ray limits presented here also constrain some WIMP models proposed to explain the Fermi and PAMELA e(+)e(-) data, including low-mass wino-like neutralinos and models with TeV masses pair annihilating into muon-antimuon pairs.
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| 14. |
- Abdo, A. A., et al.
(författare)
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SPECTRAL PROPERTIES OF BRIGHT FERMI-DETECTED BLAZARS IN THE GAMMA-RAY BAND
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 710:2, s. 1271-1285
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Tidskriftsartikel (refereegranskat)abstract
- The gamma-ray energy spectra of bright blazars of the LAT Bright AGN Sample LBAS) are investigated using Fermi-LAT data. Spectral properties hardness, curvature, and variability) established using a data set accumulated over 6 months of operation are presented and discussed for different blazar classes and subclasses: flat spectrum radio quasars (FSRQs), low-synchrotron peaked BLLacs (LSP-BLLacs), intermediate-synchrotron peaked BLLacs (ISP-BLLacs), and high-synchrotron peaked BLLacs (HSP-BLLacs). The distribution of photon index G, obtained from a power-law fit above 100 MeV) is found to correlate strongly with blazar subclass. The change in spectral index from that averaged over the 6 months observing period is < 0.2-0.3 when the flux varies by about an order of magnitude, with a tendency toward harder spectra when the flux is brighter for FSRQs and LSP-BLLacs. A strong departure from a single power-law spectrum appears to be a common feature for FSRQs. This feature is also present for some high-luminosity LSP-BLLacs, and a small number of ISP-BLLacs. It is absent in all LBAS HSP-BLLacs. For 3C 454.3 and AO 0235+164, the two brightest FSRQ source and LSP-BLLac source, respectively, a broken power law (BPL) gives the most acceptable of power law, BPL, and curved forms. The consequences of these findings are discussed.
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| 15. |
- Abdo, A. A., et al.
(författare)
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Spectrum of the Isotropic Diffuse Gamma-Ray Emission Derived from First-Year Fermi Large Area Telescope Data
- 2010
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 104:10
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Tidskriftsartikel (refereegranskat)abstract
- We report on the first Fermi Large Area Telescope (LAT) measurements of the so-called "extra-galactic" diffuse gamma-ray emission (EGB). This component of the diffuse gamma-ray emission is generally considered to have an isotropic or nearly isotropic distribution on the sky with diverse contributions discussed in the literature. The derivation of the EGB is based on detailed modeling of the bright foreground diffuse Galactic gamma-ray emission, the detected LAT sources, and the solar gamma-ray emission. We find the spectrum of the EGB is consistent with a power law with a differential spectral index gamma = 2.41 +/- 0.05 and intensity I(>100 MeV) = (1.03 +/- 0.17) x 10(-5) cm(-2) s(-1) sr(-1), where the error is systematics dominated. Our EGB spectrum is featureless, less intense, and softer than that derived from EGRET data.
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| 16. |
- Abdo, A. A., et al.
(författare)
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SUZAKU OBSERVATIONS OF LUMINOUS QUASARS : REVEALING THE NATURE OF HIGH-ENERGY BLAZAR EMISSION IN LOW-LEVEL ACTIVITY STATES
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 716:1, s. 835-849
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Tidskriftsartikel (refereegranskat)abstract
- We present the results from the Suzaku X-ray observations of five flat-spectrum radio quasars (FSRQs), namely PKS 0208-512, Q 0827+243, PKS 1127-145, PKS 1510-089, and 3C 454.3. All these sources were additionally monitored simultaneously or quasi-simultaneously by the Fermi satellite in gamma rays and the Swift UVOT in the UV and optical bands, respectively. We constructed their broadband spectra covering the frequency range from 10(14) Hz up to 10(25) Hz, and those reveal the nature of high-energy emission of luminous blazars in their low-activity states. The analyzed X-ray spectra are well fitted by a power-law model with photoelectric absorption. In the case of PKS 0208-512, PKS 1127-145, and 3C 454.3, the X-ray continuum showed indication of hardening at low energies. Moreover, when compared with the previous X-ray observations, we see a significantly increasing contribution of low-energy photons to the total X-ray fluxes when the sources are getting fainter. The same behavior can be noted in the Suzaku data alone. A likely explanation involves a variable, flat-spectrum component produced via inverse-Compton emission, plus an additional, possibly steady soft X-ray component prominent when the source gets fainter. This soft X-ray excess is represented either by a steep power-law (photon indices Gamma similar to 3-5) or a blackbody-type emission with temperatures kT similar to 0.1-0.2 keV. We model the broadband spectra of the five observed FSRQs using synchrotron self-Compton and/or external-Compton radiation models. Our modeling suggests that the difference between the low-and high-activity states in luminous blazars is due to the different total kinetic power of the jet, most likely related to varying bulk Lorentz factor of the outflow within the blazar emission zone.
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| 17. |
- Abdo, A. A., et al.
(författare)
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THE FERMI-LAT HIGH-LATITUDE SURVEY : SOURCE COUNT DISTRIBUTIONS AND THE ORIGIN OF THE EXTRAGALACTIC DIFFUSE BACKGROUND
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 720:1, s. 435-453
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Tidskriftsartikel (refereegranskat)abstract
- This is the first of a series of papers aimed at characterizing the populations detected in the high-latitude sky of the Fermi-LAT survey. In this work, we focus on the intrinsic spectral and flux properties of the source sample. We show that when selection effects are properly taken into account, Fermi sources are on average steeper than previously found (e.g., in the bright source list) with an average photon index of 2.40 +/- 0.02 over the entire 0.1-100 GeV energy band. We confirm that flat spectrum radio quasars have steeper spectra than BL Lacertae objects with an average index of 2.48 +/- 0.02 versus 2.18 +/- 0.02. Using several methods, we build the deepest source count distribution at GeV energies, deriving that the intrinsic source (i.e., blazar) surface density at F-100 >= 10(-9) ph cm(2) s(-1) is 0.12(-0.02)(+0.03) deg(-2). The integration of the source count distribution yields that point sources contribute 16(+/- 1.8)% (+/- 7% systematic uncertainty) of the GeV isotropic diffuse background. At the fluxes currently reached by LAT, we can rule out the hypothesis that pointlike sources (i.e., blazars) produce a larger fraction of the diffuse emission.
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| 18. |
- Ackermann, M., et al.
(författare)
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Constraints on dark matter annihilation in clusters of galaxies with the Fermi large area telescope
- 2010
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Ingår i: Journal of Cosmology and Astroparticle Physics. - : IOP Publishing. - 1475-7516. ; :5, s. 25-
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Tidskriftsartikel (refereegranskat)abstract
- Nearby clusters and groups of galaxies are potentially bright sources of highenergy 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 similar to 200GeV, 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 gammaray flux by a factor of similar to 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.
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| 19. |
- Ackermann, M., et al.
(författare)
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FERMI GAMMA-RAY SPACE TELESCOPE OBSERVATIONS OF GAMMA-RAY OUTBURSTS FROM 3C 454.3 IN 2009 DECEMBER AND 2010 APRIL
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 721:2, s. 1383-1396
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Tidskriftsartikel (refereegranskat)abstract
- The flat spectrum radio quasar 3C 454.3 underwent an extraordinary outburst in 2009 December when it became the brightest gamma-ray source in the sky for over 1 week. Its daily flux measured with the Fermi-Large Area Telescope at photon energiesE > 100 MeV reached F-100 = 22 +/- 1 x 10(6) photon cm(-2) s(-1), representing the highest daily flux of any blazar ever recorded in high-energy. -rays. It again became the brightest source in the sky in 2010 April, triggering a pointed-mode observation by Fermi. The correlated. -ray temporal and spectral properties during these exceptional events are presented and discussed. The main results show flux variability over time scales less than 3 hr and very mild spectral variability with an indication of gradual hardening preceding major flares. The light curves during periods of enhanced activity in 2008 July-August and 2010 December show strong resemblance, with a flux plateau of a few days preceding the major flare. No consistent loop pattern emerged in the. -ray spectral index versus the flux plane as would be expected in acceleration and cooling scenarios. The maximum energy of a photon from 3C 454.3 is approximate to 20 GeV and a minimum Doppler factor of approximate to 13 is derived. The gamma-ray spectrum of 3C 454.3 shows a significant spectral break between approximate to 2 and 3 GeV that is very weakly dependent on the flux state, even when the flux changes by an order of magnitude.
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| 20. |
- Ackermann, M., et al.
(författare)
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Fermi LAT observations of cosmic-ray electrons from 7 GeV to 1 TeV
- 2010
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Ingår i: PHYSICAL REVIEW D. - 1550-7998. ; 82:9, s. 092004-
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of our analysis of cosmic-ray electrons using about 8 x 10(6) electron candidates detected in the first 12 months on-orbit by the Fermi Large Area Telescope. This work extends our previously published cosmic-ray electron spectrum down to 7 GeV, giving a spectral range of approximately 2.5 decades up to 1 TeV. We describe in detail the analysis and its validation using beam-test and on-orbit data. In addition, we describe the spectrum measured via a subset of events selected for the best energy resolution as a cross-check on the measurement using the full event sample. Our electron spectrum can be described with a power law proportional to E-3.08+/-0.05 with no prominent spectral features within systematic uncertainties. Within the limits of our uncertainties, we can accommodate a slight spectral hardening at around 100 GeV and a slight softening above 500 GeV.
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