| 1. |
- Ahnen, M. L., et al.
(author)
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MAGIC and Fermi-LAT gamma-ray results on unassociated HAWC sources
- 2019
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In: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 485:1, s. 356-366
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Journal article (peer-reviewed)abstract
- The HAWC Collaboration released the 2HWC catalogue of TeV sources, in which 19 show no association with any known high-energy (HE; E greater than or similar to 10 GeV) or very-high-energy (VHE; E greater than or similar to 300 GeV) sources. This catalogue motivated follow-up studies by both the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) and Fermi-LAT (Large Area Telescope) observatories with the aim of investigating gamma-ray emission over a broad energy band. In this paper, we report the results from the first joint work between High Altitude Water Cherenkov (HAWC), MAGIC, and Fermi-LAT on three unassociated HAWC sources: 2HWC J2006+341, 2HWC J1907+084*, and 2HWC J1852+013*. Although no significant detection was found in the HE and VHE regimes, this investigation shows that a minimum 1 degrees extension (at 95 per cent confidence level) and harder spectrum in the GeV than the one extrapolated from HAWC results are required in the case of 2HWC J1852+013*, whilst a simply minimum extension of 0.16 degrees (at 95 per cent confidence level) can already explain the scenario proposed by HAWC for the remaining sources. Moreover, the hypothesis that these sources are pulsar wind nebulae is also investigated in detail.
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| 2. |
- Ajello, M., et al.
(author)
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A Decade of Gamma-Ray Bursts Observed by Fermi-LAT : The Second GRB Catalog
- 2019
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In: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 878:1
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Journal article (peer-reviewed)abstract
- The Large Area Telescope (LAT) aboard the Fermi spacecraft routinely observes high-energy emission from gamma-ray bursts (GRBs). Here we present the second catalog of LAT-detected GRBs, covering the first 10 yr of operations, from 2008 to 2018 August 4. A total of 186 GRBs are found; of these, 91 show emission in the range 30-100 MeV (17 of which are seen only in this band) and 169 are detected above 100 MeV. Most of these sources were discovered by other instruments (Fermi/GBM, Swift/BAT, AGILE, INTEGRAL) or reported by the Interplanetary Network (IPN); the LAT has independently triggered on four GRBs. This catalog presents the results for all 186 GRBs. We study onset, duration, and temporal properties of each GRB, as well as spectral characteristics in the 100 MeV-100 GeV energy range. Particular attention is given to the photons with the highest energy. Compared with the first LAT GRB catalog, our rate of detection is significantly improved. The results generally confirm the main findings of the first catalog: the LAT primarily detects the brightest GBM bursts, and the high-energy emission shows delayed onset as well as longer duration. However, in this work we find delays exceeding 1 ks and several GRBs with durations over 10 ks. Furthermore, the larger number of LAT detections shows that these GRBs not only cover the high-fluence range of GBM-detected GRBs but also sample lower fluences. In addition, the greater number of detected GRBs with redshift estimates allows us to study their properties in both the observer and rest frames. Comparison of the observational results with theoretical predictions reveals that no model is currently able to explain all results, highlighting the role of LAT observations in driving theoretical models.
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| 3. |
- Ajello, M., et al.
(author)
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A Search for Cosmic-Ray Proton Anisotropy with the Fermi Large Area Telescope
- 2019
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 883:1
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Journal article (peer-reviewed)abstract
- The Fermi Large Area Telescope (LAT) has amassed a large data set of primary cosmic-ray protons throughout its mission. In fact, it is the largest set of identified cosmic-ray protons ever collected at this energy. The LAT' s wide field of view and full-sky survey capabilities make it an excellent instrument for studying cosmic-ray anisotropy. As a space-based survey instrument, the LAT is sensitive to anisotropy in both R.A. and decl., while ground-based observations only measure the anisotropy in R.A. We present the results of the first-ever proton anisotropy search using Fermi LAT. The data set was collected over eight years and consists of approximately 179 million protons above 78 GeV, enabling it to probe dipole anisotropy below an amplitude of 10(-3), resulting in the most stringent limits on the decl. dependence of the dipole to date. We measure a dipole amplitude delta = 3.9 +/- 1.5 x 10(-4) with a p-value of 0.01 (pretrials) for protons with energy greater than 78 GeV. We discuss various systematic effects that could give rise to a dipole excess and calculate upper limits on the dipole amplitude as a function of minimum energy. The 95% confidence level upper limit on the dipole amplitude is delta(UL) = 1.3 x 10(-3) for protons with energy greater than 78 GeV and delta(UL )= 1.2 x 10(-3) for protons with energy greater than 251 GeV.
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| 4. |
- Jóhannesson, Gudlaugur, et al.
(author)
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Cosmic-ray propagation in light of recent HAWC observations of pulsar wind nebula
- 2019
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In: Proceedings of Science. - : Sissa Medialab Srl.
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Conference paper (peer-reviewed)abstract
- Recent observations made with the High Altitude Water Cherenkov (HAWC) telescope of pulsar wind nebula (PWN) suggest that the diffusion in their vicinity is characterised with a lower diffusion constant than that predicted for propagation in the interstellar medium from observations of cosmic-ray (CR) fluxes at Earth. In this contribution, it is shown that models with the slow diffusion region localised about PWN can successfully explain the HAWC observations of the Geminga PWN and still retain consistency with other CR measurements. Parameter exploration shows that the size of the smaller diffusion zone has implications for the both the PWN emission at lower energies observable by the Fermi Large Area Telescope and the predicted positron flux at the Earth. Unless the Geminga PWN is unique, there are likely many small regions with slow diffusion throughout the Milky Way. The consequences for the propagation of CRs and the resulting interstellar emissions from across the Galaxy are discussed and explored.
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| 5. |
- Johannesson, Gudlaugur, et al.
(author)
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Cosmic-Ray Propagation in Light of the Recent Observation of Geminga
- 2019
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In: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 879:2
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Journal article (peer-reviewed)abstract
- The High Altitude Water Cherenkov (HAWC) telescope recently observed extended emission around the Geminga and PSR. B0656+14 pulsar wind nebulae (PWNe). These observations have been used to estimate cosmic-ray (CR) diffusion coefficients near the PWNe that appear to be more than two orders of magnitude smaller than the diffusion coefficients typically derived for the interstellar medium from measured abundances of secondary species in CRs. Two-zone diffusion models have been proposed as a solution to this discrepancy, where the slower diffusion zone (SDZ) is confined to a small region around the PWN. Such models are shown to successfully reproduce the HAWC observations of the Geminga PWN while retaining consistency with other CR data. It is found that the size of the SDZ influences the predicted positron flux and the spectral shape of the extended gamma-ray emission at lower energies that can be observed with the Fermi Large Area Telescope. If the two observed PWNe are not unique, then it is likely that there are similar pockets of slow diffusion around many CR sources elsewhere in the Milky Way. The consequences of such a picture for Galactic CR propagation is explored.
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| 6. |
- Moskalenko, Igor V., et al.
(author)
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GalProp code for galactic cosmic ray propagation and associated photon emissions
- 2019
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In: Proceedings of Science. - : Sissa Medialab Srl.
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Conference paper (peer-reviewed)abstract
- The last decade brought spectacular advances in the astrophysics of cosmic rays (CRs) and γ-ray astronomy. These observations pose a considerable challenge to conventional astrophysics thus leaving an ample discovery space for new phenomena. Understanding the conventional astrophysical backgrounds is vital in moving to the new territory. The state-of-the-art CR propagation code called GALPROP is designed to address exactly this challenge. Having 23 years of development behind it, the GALPROP code has become a de-facto standard in astrophysics of CR, diffuse γ-rays, and searches of new physics. The GALPROP code uses information from astronomy, particle, and nuclear physics to predict CRs, grays, synchrotron emission and its polarization in a self-consistent manner - it provides the modeling code unifying the many results of individual measurements in physics and astronomy spanning in energy coverage, types of instrumentation, and the nature of detected species. The range of physical validity of the GALPROP code covers sub-keV - PeV energies for particles and from micro-eV - PeV for photons. The GALPROP framework includes the code and independently developed datasets, such as interstellar gas (H2, H I, H II), radiation and magnetic fields distributions as well as the nuclear and particle production cross sections. The code and the datasets are public and are extensively used by many experimental collaborations, and by thousands of individual researchers worldwide for interpretation of their data and for making predictions. We will present latest updates to the GALPROP framework that improve its accuracy and capabilities and will discuss its applications. As always, the latest release of the code and datasets is available through the WebRun, a service to the scientific community enabling easy use of the GALPROP via web browsers.
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| 7. |
- Riley, A. H., et al.
(author)
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Possible Detection of Gamma-Rays from Epsilon Eridani
- 2019
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In: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 878:1
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Journal article (peer-reviewed)abstract
- We use the Fermi Large Area Telescope gamma-ray observatory to search for gamma-ray emission from four nearby, debris disk-hosting main-sequence stars: τ Ceti, Eridani, Fomalhaut, and Vega. For three stars (τ Ceti, Fomalhaut, and Vega), we establish upper limits that are consistent with theoretical expectations. For Eridani, we find a possible spatially coincident source with a soft energy spectrum of dN/dE ∼ E -3.6. However, at this stage we are unable to rule out that this emission is due to a more extended feature in the diffuse background. In the interpretation that the emission is due to Eridani, the >100 MeV gamma-ray luminosity is ∼1027 erg s-1 ≃ 3 ×10-7 L o, which is ∼1010 times the gamma-ray luminosity from the disk of the quiet Sun. We find ≲2σ evidence of source variability over a ∼7 yr timescale. In the interpretation that the gamma-ray emission is from Eridani itself, we consider two possible models: (1) cosmic-ray collisions with solid bodies in the debris disk, which extends out ∼60 au from the host star, and (2) emission from the stellar activity. For the former model, assuming a total disk mass consistent with infrared measurements, we find that the size distribution of bodies is steeper than expected for a collisional cascade state. If confirmed as being associated with Eridani, this would be the first indication of gamma-ray emission from the vicinity of a main-sequence star other than the Sun.
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