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

Abstract

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^-6more » 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:

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 more »; Boschin, Walter « less

Publication Date:

Mon May 23 00:00:00 EDT 2016

Research Org.:

Hiroshima Univ., Higashihiroshima (Japan); NASA Goddard Space Flight Center (GSFC), Greenbelt, MD (United States)

Sponsoring Org.:

USDOE; National Aeronautics and Space Administration (NASA); Japan Society for the Promotion of Science (JSPS) (Japan)

Contributing Org.:

Fermi LAT Collaboration

OSTI Identifier:

1355691

Grant/Contract Number:  

NNH12ZDA001N; NNH13ZDA001N-FERMI; 15K17652

Resource 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

Country of Publication:

United States

Language:

English

Subject:

79 ASTRONOMY AND ASTROPHYSICS; BL Lacertae objects; active galaxies; galaxies jets; gamma rays; X-rays