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Title: A significant hardening and rising shape detected in the MeV/GeV νF ν spectrum from the recently discovered very-high-energy blazar S4 0954+65 during the bright optical flare in 2015 February

In this paper, we report on Fermi Large Area Telescope (LAT) and multi-wavelength results on the recently discovered very-high-energy (VHE, E > 100 GeV) blazar S4 0954+65 (z = 0.368) during an exceptionally bright optical flare in 2015 February. During the time period (2015 February 13/14, or MJD 57067) when the MAGIC telescope detected VHE γ-ray emission from the source, the Fermi-LAT data indicated a significant spectral hardening at GeV energies, with a power-law photon index of 1.8 ± 0.1—compared with the 3FGL (The Fermi LAT 4-Year Point Source Catalog) value (averaged over four years of observation) of 2.34 ± 0.04. In contrast, Swift X-Ray Telescope data showed a softening of the X-ray spectrum, with a photon index of 1.72 ± 0.08 (compared with 1.38 ± 0.03 averaged during the flare from MJD 57066 to 57077), possibly indicating a modest contribution of synchrotron photons by the highest-energy electrons superposed on the inverse Compton component. Fitting of the quasi-simultaneous (<1 d) broad-band spectrum with a one-zone synchrotron plus inverse-Compton model revealed that GeV/TeV emission could be produced by inverse-Compton scattering of external photons from the dust torus. Finally, we emphasize that a flaring blazar showing high flux of ≳1.0 × 10more » -6 photons cm -2s -1 (E > 100 MeV) and a hard spectral index of ΓGeV < 2.0 detected by Fermi-LAT on daily timescales is a promising target for TeV follow-up by ground-based Cherenkov telescopes to discover high-redshift blazars, investigate their temporal variability and spectral features in the VHE band, and also constrain the intensity of the extragalactic background light.« less
Authors:
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Publication Date:
Grant/Contract Number:
NNH12ZDA001N; NNH13ZDA001N-FERMI; 15K17652
Type:
Accepted Manuscript
Journal Name:
Publications of the Astronomical Society of Japan
Additional Journal Information:
Journal Volume: 68; Journal Issue: 4; Journal ID: ISSN 0004-6264
Research Org:
Hiroshima Univ., Higashihiroshima (Japan); NASA Goddard Space Flight Center (GSFC), Greenbelt, MD (United States)
Sponsoring Org:
USDOE; National Aeronautic and Space Administration (NASA); Japan Society for the Promotion of Science (JSPS) (Japan)
Contributing Orgs:
Fermi LAT Collaboration
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; BL Lacertae objects; active galaxies; galaxies jets; gamma rays; X-rays
OSTI Identifier:
1355691

Tanaka, Yasuyuki T., Becerra Gonzalez, Josefa, Itoh, Ryosuke, Finke, Justin D., Inoue, Yoshiyuki, Ojha, Roopesh, Carpenter, Bryce, Lindfors, Elina, Krauß, Felicia, Desiante, Rachele, Shiki, Kensei, Fukazawa, Yasushi, Longo, Francesco, McEnery, Julie E., Buson, Sara, Nilsson, Kari, Fallah Ramazani, Vandad, Reinthal, Riho, Takalo, Leo, Pursimo, Tapio, and Boschin, Walter. A significant hardening and rising shape detected in the MeV/GeV νFν spectrum from the recently discovered very-high-energy blazar S4 0954+65 during the bright optical flare in 2015 February. United States: N. p., Web. doi:10.1093/pasj/psw049.
Tanaka, Yasuyuki T., Becerra Gonzalez, Josefa, Itoh, Ryosuke, Finke, Justin D., Inoue, Yoshiyuki, Ojha, Roopesh, Carpenter, Bryce, Lindfors, Elina, Krauß, Felicia, Desiante, Rachele, Shiki, Kensei, Fukazawa, Yasushi, Longo, Francesco, McEnery, Julie E., Buson, Sara, Nilsson, Kari, Fallah Ramazani, Vandad, Reinthal, Riho, Takalo, Leo, Pursimo, Tapio, & Boschin, Walter. A significant hardening and rising shape detected in the MeV/GeV νFν spectrum from the recently discovered very-high-energy blazar S4 0954+65 during the bright optical flare in 2015 February. United States. doi:10.1093/pasj/psw049.
Tanaka, Yasuyuki T., Becerra Gonzalez, Josefa, Itoh, Ryosuke, Finke, Justin D., Inoue, Yoshiyuki, Ojha, Roopesh, Carpenter, Bryce, Lindfors, Elina, Krauß, Felicia, Desiante, Rachele, Shiki, Kensei, Fukazawa, Yasushi, Longo, Francesco, McEnery, Julie E., Buson, Sara, Nilsson, Kari, Fallah Ramazani, Vandad, Reinthal, Riho, Takalo, Leo, Pursimo, Tapio, and Boschin, Walter. 2016. "A significant hardening and rising shape detected in the MeV/GeV νFν spectrum from the recently discovered very-high-energy blazar S4 0954+65 during the bright optical flare in 2015 February". United States. doi:10.1093/pasj/psw049. https://www.osti.gov/servlets/purl/1355691.
@article{osti_1355691,
title = {A significant hardening and rising shape detected in the MeV/GeV νFν spectrum from the recently discovered very-high-energy blazar S4 0954+65 during the bright optical flare in 2015 February},
author = {Tanaka, Yasuyuki T. and Becerra Gonzalez, Josefa and Itoh, Ryosuke and Finke, Justin D. and Inoue, Yoshiyuki and Ojha, Roopesh and Carpenter, Bryce and Lindfors, Elina and Krauß, Felicia and Desiante, Rachele and Shiki, Kensei and Fukazawa, Yasushi and Longo, Francesco and McEnery, Julie E. and Buson, Sara and Nilsson, Kari and Fallah Ramazani, Vandad and Reinthal, Riho and Takalo, Leo and Pursimo, Tapio and Boschin, Walter},
abstractNote = {In this paper, we report on Fermi Large Area Telescope (LAT) and multi-wavelength results on the recently discovered very-high-energy (VHE, E > 100 GeV) blazar S4 0954+65 (z = 0.368) during an exceptionally bright optical flare in 2015 February. During the time period (2015 February 13/14, or MJD 57067) when the MAGIC telescope detected VHE γ-ray emission from the source, the Fermi-LAT data indicated a significant spectral hardening at GeV energies, with a power-law photon index of 1.8 ± 0.1—compared with the 3FGL (The Fermi LAT 4-Year Point Source Catalog) value (averaged over four years of observation) of 2.34 ± 0.04. In contrast, Swift X-Ray Telescope data showed a softening of the X-ray spectrum, with a photon index of 1.72 ± 0.08 (compared with 1.38 ± 0.03 averaged during the flare from MJD 57066 to 57077), possibly indicating a modest contribution of synchrotron photons by the highest-energy electrons superposed on the inverse Compton component. Fitting of the quasi-simultaneous (<1 d) broad-band spectrum with a one-zone synchrotron plus inverse-Compton model revealed that GeV/TeV emission could be produced by inverse-Compton scattering of external photons from the dust torus. Finally, we emphasize that a flaring blazar showing high flux of ≳1.0 × 10-6 photons cm-2s-1 (E > 100 MeV) and a hard spectral index of ΓGeV < 2.0 detected by Fermi-LAT on daily timescales is a promising target for TeV follow-up by ground-based Cherenkov telescopes to discover high-redshift blazars, investigate their temporal variability and spectral features in the VHE band, and also constrain the intensity of the extragalactic background light.},
doi = {10.1093/pasj/psw049},
journal = {Publications of the Astronomical Society of Japan},
number = 4,
volume = 68,
place = {United States},
year = {2016},
month = {5}
}