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Jóhannesson, Gudlaugur
Publications (5 of 5) Show all publications
Ajello, M., Jóhannesson, G., Zimmer, S. & et al., . (2019). A Search for Cosmic-Ray Proton Anisotropy with the Fermi Large Area Telescope. Astrophysical Journal, 883(1), Article ID 33.
Open this publication in new window or tab >>A Search for Cosmic-Ray Proton Anisotropy with the Fermi Large Area Telescope
2019 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 883, no 1, article id 33Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2019
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-261738 (URN)10.3847/1538-4357/ab3a2e (DOI)000487690200001 ()2-s2.0-85073070536 (Scopus ID)
Note

QC 20191010

Available from: 2019-10-10 Created: 2019-10-10 Last updated: 2020-03-09Bibliographically approved
Ahnen, M. L., Jóhannesson, G., Larsson, S., Yassine, M. & et al., . (2019). MAGIC and Fermi-LAT gamma-ray results on unassociated HAWC sources. Paper presented at ARK CJ, 2017, APJ, V834, PNIL_ wood W. B., 2009, ASTROPHYSICAL JOURNAL, V697, P1071 pez-Coto R., 2017, 35th International Cosmic Ray Conference, Proceedings of Science, V301, P732 min V. P., 1994, ASTROPARTICLE PHYSICS, V2, P137 nin R., 2013, Proc. 33st International Cosmic Ray Conference, Rio de Janeiro, Brasil,. Monthly notices of the Royal Astronomical Society, 485(1), 356-366
Open this publication in new window or tab >>MAGIC and Fermi-LAT gamma-ray results on unassociated HAWC sources
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2019 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 485, no 1, p. 356-366Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
OXFORD UNIV PRESS, 2019
Keywords
pulsars: general, gamma rays: general, astroparticle physics
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-252389 (URN)10.1093/mnras/stz089 (DOI)000466786400024 ()2-s2.0-85072245247 (Scopus ID)
Conference
ARK CJ, 2017, APJ, V834, PNIL_ wood W. B., 2009, ASTROPHYSICAL JOURNAL, V697, P1071 pez-Coto R., 2017, 35th International Cosmic Ray Conference, Proceedings of Science, V301, P732 min V. P., 1994, ASTROPARTICLE PHYSICS, V2, P137 nin R., 2013, Proc. 33st International Cosmic Ray Conference, Rio de Janeiro, Brasil,
Note

QC 20190715

Available from: 2019-07-15 Created: 2019-07-15 Last updated: 2019-10-04Bibliographically approved
Jóhannesson, G. & Bjornsson, G. (2018). Low-energy Electrons in Gamma-Ray Burst Afterglow Models. Astrophysical Journal Letters, 859(1), Article ID Lll.
Open this publication in new window or tab >>Low-energy Electrons in Gamma-Ray Burst Afterglow Models
2018 (English)In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 859, no 1, article id LllArticle in journal (Refereed) Published
Abstract [en]

Observations of gamma-ray burst (GRB) afterglows have long provided the most detailed information about the origin of this spectacular phenomenon. The model that is most commonly used to extract physical properties of the event from the observations is the relativistic fireball model, where ejected material moving at relativistic speeds creates a shock wave when it interacts with the surrounding medium. Electrons are accelerated in the shock wave, generating the observed synchrotron emission through interactions with the magnetic field in the downstream medium. It is usually assumed that the accelerated electrons follow a simple power-law distribution in energy between specific energy boundaries, and that no electron exists outside these boundaries. This Letter explores the consequences of adding a low-energy power-law segment to the electron distribution with energy that contributes insignificantly to the total energy budget of the distribution. The low-energy electrons have a significant impact on the radio emission, providing synchrotron absorption and emission at these long wavelengths. Shorter wavelengths are affected through the normalization of the distribution. The new model is used to analyze the light curves of GRB 990510, and the resulting parameters are compared to a model without the extra electrons. The quality of the fit and the best-fit parameters are significantly affected by the additional model component. The new component is in one case found to strongly affect the X-ray light curves, showing how changes to the model at radio frequencies can affect light curves at other frequencies through changes in best-fit model parameters.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2018
Keywords
gamma-ray burst: general, methods: data analysis, radiation mechanisms: non-thermal
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-230439 (URN)10.3847/2041-8213/aac380 (DOI)000432930600002 ()2-s2.0-85047898747 (Scopus ID)
Note

QC 20180614

Available from: 2018-06-14 Created: 2018-06-14 Last updated: 2018-06-14Bibliographically approved
Ajello, M., Atwood, W. B., Baldini, L., Ballet, J., Barbiellini, G., Bastieri, D., . . . Wood, M. (2017). 3FHL: The Third Catalog of Hard Fermi-LAT Sources. Astrophysical Journal Supplement Series, 232(2), Article ID 18.
Open this publication in new window or tab >>3FHL: The Third Catalog of Hard Fermi-LAT Sources
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2017 (English)In: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 232, no 2, article id 18Article in journal (Refereed) Published
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-216621 (URN)10.3847/1538-4365/aa8221 (DOI)000411892800001 ()2-s2.0-85032822161 (Scopus ID)
Note

QC 20171102

Available from: 2017-11-02 Created: 2017-11-02 Last updated: 2020-03-09Bibliographically approved
Jóhannesson, G., Moskalenko, I. V. & Porter, T. A. (2017). Interstellar gas in 3D, implications for CR propagation and gamma-ray emission. In: Proceedings of Science: . Paper presented at 35th International Cosmic Ray Conference, ICRC 2017, 10 July 2017 through 20 July 2017. Sissa Medialab Srl
Open this publication in new window or tab >>Interstellar gas in 3D, implications for CR propagation and gamma-ray emission
2017 (English)In: Proceedings of Science, Sissa Medialab Srl , 2017Conference paper, Published paper (Refereed)
Abstract [en]

Cosmic-ray (CR) particles propagating in the Galaxy interact in the interstellar medium with gas, losing energy and producing secondary particles via inelastic losses and fragmentation. Observations of the secondaries can be used to constrain the origin and propagation of the CRs, and may also provide signatures of new physics. To date CR propagation models have used the 2D Galactocentric cylindrical symmetry approximation for the spatial distribution of the interstellar gas. This is partly due to difficulties in uniquely determining its true 3D structure. In this contribution a method for determining the 3D spatial distribution of interstellar gas is described and first results using it to develop models that can be used in CR propagation codes like GALPROP are given. Implications for analysis of CR and γ-ray data using 3D spatial models for the interstellar gas are also discussed. 

Place, publisher, year, edition, pages
Sissa Medialab Srl, 2017
Keywords
Cosmic rays, Cosmology, Gases, Spatial distribution, Cylindrical symmetry, Gamma-ray emission, Interstellar gas, Interstellar mediums, New physics, Propagation models, Secondary particles, Spatial models, Gamma rays
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-236856 (URN)2-s2.0-85046058055 (Scopus ID)
Conference
35th International Cosmic Ray Conference, ICRC 2017, 10 July 2017 through 20 July 2017
Note

QC 20181219

Available from: 2018-12-19 Created: 2018-12-19 Last updated: 2018-12-19Bibliographically approved
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