Shiki, Kensei; Desiante, Rachele; Krauß, Felicia; Lindfors, Elina; Carpenter, Bryce; Ojha, Roopesh; Inoue, Yoshiyuki; Finke, Justin D.; Itoh, Ryosuke; Becerra Gonzalez, Josefa; Tanaka, Y. T.; Fukazawa, Yasushi; Longo, Francesco; McEnery, Julie E.; Buson, Sara; Nilsson, Kari; Fallah Ramazani, Vandad; Reinthal, Riho; Takalo, Leo; Pursimo, Tapio; Boschin, W.
Referencia bibliográfica
Publications of the Astronomical Society of Japan, Volume 68, Issue 4, id.51 10 pp.
Fecha de publicación:
8
2016
Número de citas
11
Número de citas referidas
10
Descripción
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-2 s-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.