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Larsson, Stefan
Publications (7 of 7) Show all publications
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 ()
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: 2019-07-15Bibliographically approved
Ajello, M., Baldini, L., Barbiellini, G., Bastieri, D., Bellazzini, R., Bissaldi, E., . . . Wood, K. (2018). Investigating the Nature of Late-time High-energy GRB Emission through Joint Fermi/Swift Observations. Astrophysical Journal, 863(2), Article ID 138.
Open this publication in new window or tab >>Investigating the Nature of Late-time High-energy GRB Emission through Joint Fermi/Swift Observations
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2018 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 863, no 2, article id 138Article in journal (Refereed) Published
Abstract [en]

We use joint observations by the Swift X-ray Telescope (XRT) and the Fermi Large Area Telescope (LAT) of gamma-ray burst (GRB) afterglows to investigate the nature of the long-lived high-energy emission observed by Fermi LAT. Joint broadband spectral modeling of XRT and LAT data reveals that LAT nondetections of bright X-ray afterglows are consistent with a cooling break in the inferred electron synchrotron spectrum below the LAT and/or XRT energy ranges. Such a break is sufficient to suppress the high-energy emission so as to be below the LAT detection threshold. By contrast, LAT-detected bursts are best fit by a synchrotron spectrum with a cooling break that lies either between or above the XRT and LAT energy ranges. We speculate that the primary difference between GRBs with LAT afterglow detections and the nondetected population may be in the type of circumstellar environment in which these bursts occur, with late-time LAT detections preferentially selecting GRBs that occur in low wind-like circumburst density profiles. Furthermore, we find no evidence of high-energy emission in the LAT-detected population significantly in excess of the flux expected from the electron synchrotron spectrum fit to the observed X-ray emission. The lack of excess emission at high energies could be due to a shocked external medium in which the energy density in the magnetic field is stronger than or comparable to that of the relativistic electrons behind the shock, precluding the production of a dominant synchrotron self-Compton (SSC) component in the LAT energy range. Alternatively, the peak of the SSC emission could be beyond the 0.1-100 GeV energy range considered for this analysis.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2018
Keywords
gamma-ray burst: general
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-234608 (URN)10.3847/1538-4357/aad000 (DOI)000442070500015 ()2-s2.0-85052368293 (Scopus ID)
Funder
Knut and Alice Wallenberg FoundationSwedish National Space BoardSwedish Research Council
Note

QC 20180914

Available from: 2018-09-14 Created: 2018-09-14 Last updated: 2018-09-14Bibliographically approved
Ackermann, M., Ajello, M., Albert, A., Atwood, W. B., Baldini, L., Ballet, J., . . . Rephaeli, Y. (2018). Search for Gamma-Ray Emission from the Coma Cluster with Six Years of Fermi-LAT Data (vol 819, 149, 2016). Astrophysical Journal, 860(1), Article ID 85.
Open this publication in new window or tab >>Search for Gamma-Ray Emission from the Coma Cluster with Six Years of Fermi-LAT Data (vol 819, 149, 2016)
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2018 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 860, no 1, article id 85Article in journal (Refereed) Published
Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2018
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-231711 (URN)10.3847/1538-4357/aac7c3 (DOI)000435366000010 ()2-s2.0-85049235056 (Scopus ID)
Note

QC 20180821

Available from: 2018-08-21 Created: 2018-08-21 Last updated: 2019-08-20Bibliographically approved
Sitarek, J., González, J. B., Ramazani, V. F., Lindfors, E., Pedaletti, G., Tavecchio, F., . . . Collaborations, M.-L. a. (2017). MAGIC observations of variable very-high-energy gamma-ray emission from PKS1510-089 during May 2015 outburst. 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 >>MAGIC observations of variable very-high-energy gamma-ray emission from PKS1510-089 during May 2015 outburst
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2017 (English)In: Proceedings of Science, Sissa Medialab Srl , 2017Conference paper, Published paper (Refereed)
Abstract [en]

PKS1510-089 is a flat spectrum radio quasar located at a redshift of 0.36. It is one of only a few such sources detected in very-high-energy (VHE, >100 GeV) gamma rays. Though PKS1510-089 is highly variable at GeV energies, until recently no variability has been observed in the VHE band. In 2015 May PKS1510-089 showed a high state in optical and in the GeV range. A VHE gamma-ray flare was detected with MAGIC at that time, showing the first instance of VHE gamma-ray flux variability on the time scale of days in this source. We will present the MAGIC results from this observation, discuss their temporal and spectral properties in the multi-wavelength context and present modelling of such emission in the external Compton scenario. 

Place, publisher, year, edition, pages
Sissa Medialab Srl, 2017
Keywords
Cosmic rays, Cosmology, Germanium compounds, Gamma-ray emission, Gamma-ray flux, Multi-wavelengths, Red shift, Spectral properties, Spectrum radio quasar, Time-scales, Very high energies, Gamma rays
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-236847 (URN)2-s2.0-85046057223 (Scopus ID)
Conference
35th International Cosmic Ray Conference, ICRC 2017, 10 July 2017 through 20 July 2017
Funder
Swedish Research CouncilKnut and Alice Wallenberg FoundationSwedish National Space Board
Note

QC 20181220

Available from: 2018-12-20 Created: 2018-12-20 Last updated: 2018-12-20Bibliographically approved
Ahnen, M. L., Larsson, S., Grishina, T. S. & et.al., . (2017). Multiwavelength observations of a VHE gamma-ray flare from PKS 1510-089 in 2015. Astronomy and Astrophysics, 603, Article ID A29.
Open this publication in new window or tab >>Multiwavelength observations of a VHE gamma-ray flare from PKS 1510-089 in 2015
2017 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 603, article id A29Article in journal (Refereed) Published
Abstract [en]

Context. PKS 1510-089 is one of only a few flat spectrum radio quasars detected in the very-high-energy (VHE, > 100 GeV) gamma-ray band. Aims. We study the broadband spectral and temporal properties of the PKS 1510-089 emission during a high gamma-ray state. Methods. We performed VHE gamma-ray observations of PKS 1510-089 with the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes during a long, high gamma-ray state in May 2015. In order to perform broadband modeling of the source, we have also gathered contemporaneous multiwavelength data in radio, IR, optical photometry and polarization, UV, X-ray, and GeV gamma-ray ranges. We construct a broadband spectral energy distribution (SED) in two periods, selected according to VHE gamma-ray state. Results. PKS 1510-089 was detected by MAGIC during a few day-long observations performed in the middle of a long, high optical and gamma-ray state, showing for the first time a significant VHE gamma-ray variability. Similarly to the optical and gamma-ray high state of the source detected in 2012, it was accompanied by a rotation of the optical polarization angle and the emission of a new jet component observed in radio. However, owing to large uncertainty on the knot separation time, the association with the VHE gamma-ray emission cannot be firmly established. The spectral shape in the VHE band during the flare is similar to those obtained during previous measurements of the source. The observed flux variability sets constraints for the first time on the size of the region from which VHE gamma rays are emitted. We model the broadband SED in the framework of the external Compton scenario and discuss the possible emission site in view of multiwavelength data and alternative emission models.

Place, publisher, year, edition, pages
EDP Sciences, 2017
Keywords
galaxies: active; galaxies: jets; gamma rays: galaxies; quasars: individual: PKS 1510-089
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-214605 (URN)10.1051/0004-6361/201629960 (DOI)000406619100048 ()2-s2.0-85021717542 (Scopus ID)
Note

QC 20171020

Available from: 2017-09-18 Created: 2017-09-18 Last updated: 2017-11-03Bibliographically approved
Ackermann, M., Ajello, M., Albert, A., Atwood, W. B., Baldini, L., Ballet, J., . . . Zimmer, S. (2017). The Fermi Galactic Center GeV Excess and Implications for Dark Matter. Astrophysical Journal, 840(1), Article ID 43.
Open this publication in new window or tab >>The Fermi Galactic Center GeV Excess and Implications for Dark Matter
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2017 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 840, no 1, article id 43Article in journal (Refereed) Published
Abstract [en]

The region around the Galactic Center (GC) is now well established to be brighter at energies of a few GeV than what is expected from conventional models of diffuse gamma-ray emission and catalogs of known gamma-ray sources. We study the GeV excess using 6.5 yr of data from the Fermi Large Area Telescope. We characterize the uncertainty of the GC excess spectrum and morphology due to uncertainties in cosmic-ray source distributions and propagation, uncertainties in the distribution of interstellar gas in the Milky Way, and uncertainties due to a potential contribution from the Fermi bubbles. We also evaluate uncertainties in the excess properties due to resolved point sources of gamma rays. The GC is of particular interest, as it would be expected to have the brightest signal from annihilation of weakly interacting massive dark matter (DM) particles. However, control regions along the Galactic plane, where a DM signal is not expected, show excesses of similar amplitude relative to the local background. Based on the magnitude of the systematic uncertainties, we conservatively report upper limits for the annihilation cross-section as a function of particle mass and annihilation channel.

Place, publisher, year, edition, pages
Institute of Physics Publishing, 2017
Keywords
cosmic rays, Galaxy: bulge, Galaxy: center, Galaxy: halo, gamma rays: general, ISM: general
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-216505 (URN)10.3847/1538-4357/aa6cab (DOI)000403223600002 ()2-s2.0-85019104376 (Scopus ID)
Note

Funding details: NNH13ZDA001N, NASA, National Aeronautics and Space Administration; Funding details: NNX14AQ37G, NASA, National Aeronautics and Space Administration; Funding text: This work was partially funded by NASA grants NNX14AQ37G and NNH13ZDA001N.

QC 20171212

Available from: 2017-12-12 Created: 2017-12-12 Last updated: 2017-12-12Bibliographically approved
Abdollahi, S., Larsson, S. & Wood, K. S. (2017). The Second Catalog of Flaring Gamma-Ray Sources from the Fermi All-sky Variability Analysis. Astrophysical Journal, 846(1), Article ID 34.
Open this publication in new window or tab >>The Second Catalog of Flaring Gamma-Ray Sources from the Fermi All-sky Variability Analysis
2017 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 846, no 1, article id 34Article in journal (Refereed) Published
Abstract [en]

We present the second catalog of flaring gamma-ray sources (2FAV) detected with the Fermi All-sky Variability Analysis (FAVA), a tool that blindly searches for transients over the entire sky observed by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. With respect to the first FAVA catalog, this catalog benefits from a larger data set, the latest LAT data release (Pass 8), as well as from an improved analysis that includes likelihood techniques for a more precise localization of the transients. Applying this analysis to the first 7.4 years of Fermi observations, and in two separate energy bands 0.1-0.8 GeV and 0.8-300 GeV, a total of 4547 flares were detected with significance greater than 6s (before trials), on the timescale of one week. Through spatial clustering of these flares, 518 variable gamma-ray sources were identified. Based on positional coincidence, likely counterparts have been found for 441 sources, mostly among the blazar class of active galactic nuclei. For 77 2FAV sources, no likely gamma-ray counterpart has been found. For each source in the catalog, we provide the time, location, and spectrum of each flaring episode. Studying the spectra of the flares, we observe a harder-when-brighter behavior for flares associated with blazars, with the exception of BL Lac flares detected in the low-energy band. The photon indexes of the flares are never significantly smaller than 1.5. For a leptonic model, and under the assumption of isotropy, this limit suggests that the spectrum of freshly accelerated electrons is never harder than p similar to 2.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2017
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-217245 (URN)10.3847/1538-4357/aa8092 (DOI)000408687300011 ()
Note

QC 20171103

Available from: 2017-11-03 Created: 2017-11-03 Last updated: 2017-11-03Bibliographically approved
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