| 11. |
- Ackermann, M., et al.
(author)
-
Determination of the point-spread function for the fermi large area telescope from on-orbit data and limits on pair halos of active galactic nuclei
- 2013
-
In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 765:1, s. 54-
-
Journal article (peer-reviewed)abstract
- The Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope is a pair-conversion telescope designed to detect photons with energies from approximate to 20 MeV to > 300 GeV. The pre-launch response functions of the LAT were determined through extensive Monte Carlo simulations and beam tests. The point-spread function (PSF) characterizing the angular distribution of reconstructed photons as a function of energy and geometry in the detector is determined here from two years of on-orbit data by examining the distributions of gamma rays from pulsars and active galactic nuclei (AGNs). Above 3 GeV, the PSF is found to be broader than the pre-launch PSF. We checked for dependence of the PSF on the class of gamma-ray source and observation epoch and found none. We also investigated several possible spatial models for pair-halo emission around BL Lac AGNs. We found no evidence for a component with spatial extension larger than the PSF and set upper limits on the amplitude of halo emission in stacked images of low-and high-redshift BL Lac AGNs and the TeV blazars 1ES0229 + 200 and 1ES0347-121.
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| 12. |
- Ackermann, M., et al.
(author)
-
Fermi establishes classical novae as a distinct class of gamma-ray sources
- 2014
-
In: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 345:6196, s. 554-558
-
Journal article (peer-reviewed)abstract
- A classical nova results from runaway thermonuclear explosions on the surface of a white dwarf that accretes matter from a low-mass main-sequence stellar companion. In 2012 and 2013, three novae were detected in gamma rays and stood in contrast to the first gamma-ray-detected nova V407 Cygni 2010, which belongs to a rare class of symbiotic binary systems. Despite likely differences in the compositions and masses of their white dwarf progenitors, the three classical novae are similarly characterized as soft-spectrum transient gamma-ray sources detected over 2- to 3-week durations. The gamma-ray detections point to unexpected high-energy particle acceleration processes linked to the mass ejection from thermonuclear explosions in an unanticipated class of Galactic gamma-ray sources.
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| 13. |
- Ackermann, M., et al.
(author)
-
HIGH-ENERGY GAMMA-RAY EMISSION FROM SOLAR FLARES : SUMMARY OF FERMI LARGE AREA TELESCOPE DETECTIONS AND ANALYSIS OF TWO M-CLASS FLARES
- 2014
-
In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 787:1, s. 15-
-
Journal article (peer-reviewed)abstract
- We present the detections of 18 solar flares detected in high-energy gamma-rays (above 100 MeV) with the Fermi Large Area Telescope (LAT) during its first 4 yr of operation. This work suggests that particle acceleration up to very high energies in solar flares is more common than previously thought, occurring even in modest flares, and for longer durations. Interestingly, all these flares are associated with fairly fast coronal mass ejections (CMEs). We then describe the detailed temporal, spatial, and spectral characteristics of the first two long-lasting events: the 2011 March 7 flare, a moderate (M3.7) impulsive flare followed by slowly varying gamma-ray emission over 13 hr, and the 2011 June 7 M2.5 flare, which was followed by gamma-ray emission lasting for 2 hr. We compare the Fermi LAT data with X-ray and proton data measurements from GOES and RHESSI. We argue that the gamma-rays are more likely produced through pion decay than electron bremsstrahlung, and we find that the energy spectrum of the proton distribution softens during the extended emission of the 2011 March 7 flare. This would disfavor a trapping scenario for particles accelerated during the impulsive phase of the flare and point to a continuous acceleration process at play for the duration of the flares. CME shocks are known for accelerating the solar energetic particles (SEPs) observed in situ on similar timescales, but it might be challenging to explain the production of gamma-rays at the surface of the Sun while the CME is halfway to the Earth. A stochastic turbulence acceleration process occurring in the solar corona is another likely scenario. Detailed comparison of characteristics of SEPs and gamma-ray-emitting particles for several flares will be helpful to distinguish between these two possibilities.
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| 14. |
- Ackermann, M., et al.
(author)
-
Inferred Cosmic-Ray Spectrum from Fermi Large Area Telescope gamma-Ray Observations of Earth's Limb
- 2014
-
In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 112:15, s. 151103-
-
Journal article (peer-reviewed)abstract
- Recent accurate measurements of cosmic-ray (CR) species by ATIC-2, CREAM, and PAMELA reveal an unexpected hardening in the proton and He spectra above a few hundred GeV, a gradual softening of the spectra just below a few hundred GeV, and a harder spectrum of He compared to that of protons. These newly discovered features may offer a clue to the origin of high-energy CRs. We use the Fermi Large Area Telescope observations of the gamma-ray emission from Earth's limb for an indirect measurement of the local spectrum of CR protons in the energy range similar to 90 GeV-6 TeV (derived from a photon energy range 15 GeV-1 TeV). Our analysis shows that single power law and broken power law spectra fit the data equally well and yield a proton spectrum with index 2.68 +/- 0.04 and 2.61 +/- 0.08 above similar to 200 GeV, respectively.
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| 15. |
- Ackermann, M., et al.
(author)
-
Multiwavelength observations of GRB 110731A : GeV emission from onset to afterglow
- 2013
-
In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 763:2, s. 71-
-
Journal article (peer-reviewed)abstract
- We report on the multiwavelength observations of the bright, long gamma-ray burst GRB 110731A, by the Fermi and Swift observatories, and by the MOA and GROND optical telescopes. The analysis of the prompt phase reveals that GRB 110731A shares many features with bright Large Area Telescope bursts observed by Fermi during the first three years on-orbit: a light curve with short time variability across the whole energy range during the prompt phase, delayed onset of the emission above 100 MeV, extra power-law component and temporally extended high-energy emission. In addition, this is the first GRB for which simultaneous GeV, X-ray, and optical data are available over multiple epochs beginning just after the trigger time and extending for more than 800 s, allowing temporal and spectral analysis in different epochs that favor emission from the forward shock in a wind-type medium. The observed temporally extended GeV emission is most likely part of the high-energy end of the afterglow emission. Both the single-zone pair transparency constraint for the prompt signal and the spectral and temporal analysis of the forward-shock afterglow emission independently lead to an estimate of the bulk Lorentz factor of the jet Γ ∼ 500-550.
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| 16. |
- Ackermann, M., et al.
(author)
-
The fermi large area telescope on orbit : Event classification, instrument response functions, and calibration
- 2012
-
In: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 203:1, s. 4-
-
Journal article (peer-reviewed)abstract
- The Fermi Large Area Telescope (Fermi-LAT, hereafter LAT), the primary instrument on the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view, high-energy γ-ray telescope, covering the energy range from 20MeV to more than 300GeV. During the first years of the mission, the LAT team has gained considerable insight into the in-flight performance of the instrument. Accordingly, we have updated the analysis used to reduce LAT data for public release as well as the instrument response functions (IRFs), the description of the instrument performance provided for data analysis. In this paper, we describe the effects that motivated these updates. Furthermore, we discuss how we originally derived IRFs from Monte Carlo simulations and later corrected those IRFs for discrepancies observed between flight and simulated data. We also give details of the validations performed using flight data and quantify the residual uncertainties in the IRFs. Finally, we describe techniques the LAT team has developed to propagate those uncertainties into estimates of the systematic errors on common measurements such as fluxes and spectra of astrophysical sources.
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| 17. |
- Ackermann, M., et al.
(author)
-
The Imprint of the Extragalactic Background Light in the Gamma-Ray Spectra of Blazars
- 2012
-
In: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 338:6111, s. 1190-1192
-
Journal article (peer-reviewed)abstract
- The light emitted by stars and accreting compact objects through the history of the universe is encoded in the intensity of the extragalactic background light (EBL). Knowledge of the EBL is important to understand the nature of star formation and galaxy evolution, but direct measurements of the EBL are limited by galactic and other foreground emissions. Here, we report an absorption feature seen in the combined spectra of a sample of gamma-ray blazars out to a redshift of z similar to 1.6. This feature is caused by attenuation of gamma rays by the EBL at optical to ultraviolet frequencies and allowed us to measure the EBL flux density in this frequency band.
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| 18. |
- Ackermann, M., et al.
(author)
-
THE SPECTRUM AND MORPHOLOGY OF THE FERMI BUBBLES
- 2014
-
In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 793:1, s. 64-
-
Journal article (peer-reviewed)abstract
- The Fermi bubbles are two large structures in the gamma-ray sky extending to 55 degrees above and below the Galactic center. We analyze 50 months of Fermi Large Area Telescope data between 100MeV and 500 GeV above 10 degrees in Galactic latitude to derive the spectrum and morphology of the Fermi bubbles. We thoroughly explore the systematic uncertainties that arise when modeling the Galactic diffuse emission through two separate approaches. The gamma-ray spectrum is well described by either a log parabola or a power law with an exponential cutoff. We exclude a simple power law with more than 7 sigma significance. The power law with an exponential cutoff has an index of 1.9 +/- 0.2 and a cutoff energy of 110 +/- 50 GeV. We find that the gamma-ray luminosity of the bubbles is 4.4(-0.9)(+2.4) x 10(37) erg s(-1). We confirm a significant enhancement of gamma-ray emission in the southeastern part of the bubbles, but we do not find significant evidence for a jet. No significant variation of the spectrum across the bubbles is detected. The width of the boundary of the bubbles is estimated to be 3.4(-2.6)(+3.7) deg. Both inverse Compton (IC) models and hadronic models including IC emission from secondary leptons fit the gamma-ray data well. In the IC scenario, synchrotron emission from the same population of electrons can also explain the WMAP and Planck microwave haze with a magnetic field between 5 and 20 mu G.
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| 19. |
- Ackermann, M., et al.
(author)
-
The spectrum of isotropic diffuse gamma-ray emission between 100 MeV and 820 GeV
- 2015
-
In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 799:1, s. 86-
-
Journal article (peer-reviewed)abstract
- The gamma-ray sky can be decomposed into individually detected sources, diffuse emission attributed to the interactions of Galactic cosmic rays with gas and radiation fields, and a residual all-sky emission component commonly called the isotropic diffuse gamma-ray background (IGRB). The IGRB comprises all extragalactic emissions too faint or too diffuse to be resolved in a given survey, as well as any residual Galactic foregrounds that are approximately isotropic. The first IGRB measurement with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi) used 10 months of sky-survey data and considered an energy range between 200 MeV and 100 GeV. Improvements in event selection and characterization of cosmic-ray backgrounds, better understanding of the diffuse Galactic emission (DGE), and a longer data accumulation of 50 months allow for a refinement and extension of the IGRB measurement with the LAT, now covering the energy range from 100 MeV to 820 GeV. The IGRB spectrum shows a significant high-energy cutoff feature and can be well described over nearly four decades in energy by a power law with exponential cutoff having a spectral index of 2.32 +/- 0.02 and a break energy of (279 +/- 52) GeV using our baseline DGE model. The total intensity attributed to the IGRB is (7.2 +/- 0.6) x 10(-6) cm(-2) s(-1) sr(-1) above 100 MeV, with an additional +15%/-30% systematic uncertainty due to the Galactic diffuse foregrounds.
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| 20. |
- Ajello, M., et al.
(author)
-
Impulsive and long duration high-energy gamma-ray emission from the very bright 2012 march 7 solar flares
- 2014
-
In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 789:1, s. 20-
-
Journal article (peer-reviewed)abstract
- The Fermi Large Area Telescope (LAT) detected gamma-rays up to 4 GeV from two bright X-class solar flares on 2012 March 7, showing both an impulsive and temporally extended emission phases. The gamma-rays appear to originate from the same active region as the X-rays associated with these flares. The >100 MeV gamma-ray flux decreases monotonically during the first hour (impulsive phase) followed by a slower decrease for the next 20 hr. A power law with a high-energy exponential cutoff can adequately describe the photon spectrum. Assuming that the gamma rays result from the decay of pions produced by accelerated protons and ions with a power-law spectrum, we find that the index of that spectrum is similar to 3, with minor variations during the impulsive phase. During the extended phase the photon spectrum softens monotonically, requiring the proton index varying from similar to 4 to >5. The >30 MeV proton flux observed by the GOES satellites also shows a flux decrease and spectral softening, but with a harder spectrum (index similar to 2-3). Based on these observations, we explore the relative merits of prompt or continuous acceleration scenarios, hadronic or leptonic emission processes, and acceleration at the solar corona or by the fast coronal mass ejections. We conclude that the most likely scenario is continuous acceleration of protons in the solar corona that penetrate the lower solar atmosphere and produce pions that decay into gamma rays. However, acceleration in the downstream of the shock cannot be definitely ruled out.
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