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We report on Fermi Large Area Telescope (LAT) and multi-wavelength results onthe 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 MAGICtelescope detected VHE $\gamma$-ray emission from the source, the Fermi-LATdata indicated a significant spectral hardening at GeV energies, with apower-law photon index of $1.8 \pm 0.1$---compared with the 3FGL value(averaged over four years of observation) of $2.34 \pm 0.04$. In contrast,Swift/XRT data showed a softening of the X-ray spectrum, with a photon index of$1.72 \pm 0.08$ (compared with $1.38 \pm 0.03$ averaged during the flare fromMJD 57066 to 57077), possibly indicating a modest contribution of synchrotronphotons by the highest-energy electrons superposed on the inverse Comptoncomponent. Fitting of the quasi-simultaneous ($<1$ day) broadband spectrum witha one-zone synchrotron plus inverse-Compton model revealed that GeV/TeVemission could be produced by inverse-Compton scattering of external photonsfrom the dust torus. We emphasize that a flaring blazar showing high flux of$\gtrsim 1.0 \times 10^{-6}$ photons cm$^{-2}$ s$^{-1}$ ($E>$ 100 MeV) and ahard spectral index of $\Gamma_{\rm GeV} < 2.0$ detected by Fermi-LAT on dailytime scales is a promising target for TeV follow-up by ground-based Cherenkovtelescopes to discover high-redshift blazars, investigate their temporalvariability and spectral features in the VHE band, and also constrain theintensity of the extragalactic background light.

publications of the astronomical society of japan

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