Browsing by Author "Shiki, Kensei"
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Item 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(Publications of the Astronomical Society of Japan (PASJ), 2016-05-23) Tanaka, Yasuyuki T.; Gonzalez, Josefa Becerra; 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; Ramazani, Vandad Fallah; Reinthal, Riho; Takalo, Leo; Pursimo, Tapio; Boschin, WalterWe 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.Item X-Ray and GeV Gamma-Ray Variability of the Radio Galaxy NGC 1275(IOP, 2018-03-12) Fukazawa, Yasushi; Shiki, Kensei; Tanaka, Yasuyuki; Itoh, Ryosuke; Takahashi, Hiromitsu; Imazato, Fumiya; Dutka, Filippo; Ojha, Roopesh; Nagai, HiroshiWe analyzed Suzaku/XIS data (2006–2015) and Fermi/LAT data (2008–2015) of the gamma-ray emitting radio galaxy NGC 1275. Correlated brightening of the nucleus in both the X-ray and GeV gamma-ray energy bands was found for the period 2013–2015. This is the first evidence of correlated variability between these two energy bands for NGC 1275. We also analyzed Swift/XRT data and found that the X-ray flux increased over several days in 2010, coincidentally with the GeV gamma-ray flare. During the flare, the X-ray spectra were softer, with a photon index of ~2 compared with 1.5–2.1 of the other periods, suggesting the brightening of a synchrotron component. The GeV gamma-ray band also showed a higher flux with a harder spectrum during the 2010 flare. Simultaneous X-ray and GeV gamma-ray flux increase in the flare could be explained by the shock-in-jet scenario. On the other hand, a long-term gradual brightening of radio, X-ray, and GeV gamma-ray flux with a larger gamma-ray amplitude could have an origin other than internal shocks, and some of these possibilities are discussed.