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

Author/Creator ORCID

Date

2016-05-23

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Citation of Original Publication

Yasuyuki T. Tanaka, Josefa Becerra Gonzalez, Ryosuke Itoh, Justin D. Finke, Yoshiyuki Inoue, Roopesh Ojha, Bryce Carpenter, Elina Lindfors, Felicia Krauß, Rachele Desiante, Kensei Shiki, Yasushi Fukazawa, Francesco Longo, Julie E. McEnery, Sara Buson, Kari Nilsson, Vandad Fallah Ramazani, Riho Reinthal, Leo Takalo, Tapio Pursimo, Walter Boschin, 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, Publications of the Astronomical Society of Japan, Volume 68, Issue 4, August 2016, 51, https://doi.org/10.1093/pasj/psw049

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Abstract

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. 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.