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Fermi LAT Detection of a Hard Spectrum Gamma-ray Flare from the FSRQ S4 0954+65

ATel #7093; Roopesh Ojha (NASA/GSFC/UMBC), Bryce Carpenter (CUA/NASA/GSFC), and Yasuyuki Tanaka (Hiroshima University) on behalf of the Fermi Large Area Telescope Collaboration
on 17 Feb 2015; 19:15 UT
Credential Certification: Roopesh Ojha (Roopesh.Ojha@gmail.com)

Subjects: Gamma Ray, >GeV, TeV, AGN, Blazar, Quasar

Referred to by ATel #: 7684, 8445

The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope has observed an unusually hard spectrum gamma-ray flare from the flat spectrum radio quasar (FSRQ) S4 0954+65 (3FGL J0958.6+6534; RA: 149.696855, Dec: +65.565227, J2000, Johnston et al. 1995, AJ, 110, 880) at z = 0.368 (Wills et al. 1992, ApJ, 398, 454). S4 0954+65 had a photon index (E>100MeV) of 2.1, 1.9, 2.1, 1.9, 1.7, and 1.9 on 2015 Feb 9, 10, 11, 12, 13, and 14, respectively, with typical statistical errors of +/-0.1. The respective fluxes were 0.6, 0.5, 0.5, 1.0. 1.0, and 1.0 x 10^-6 photons cm^-2 s^-1 with typical errors of +/-0.2. The weekly averaged flux for Feb 9 - 16 is 0.78 +/- 0.06 x 10^-6 photons cm^2 s^-1 with a photon index of 1.94 +/- 0.05. The source continues to be active.

This is consistent with the detection of very high energy (VHE; E>100 GeV) emission from this source by MAGIC on the night of 2015 Feb 13/14 (ATel #7080). LAT last detected a flare from this source on 2014 Nov 15 when it had a photon index of 2.20 +/-0.16 (Atel #6709), which is a more typical value for a blazar detected by LAT and is similar to its 3FGL (Acero et al. 2015, arXiv:1501.02003) value of 2.38 +/- 0.04. S4 0954+65 is also in an exceptionally high optical state (ATels #7083; #7057; #7055; #7046, #7001; #6996). This source is included in the catalog of gamma-ray sources detected by LAT at energies above 10 GeV (1FHL; Ackermann et al. 2013, ApJS, 209, 34).

This source is one of the "LAT Monitored Sources" and consequently a preliminary estimation of the daily gamma-ray flux observed by Fermi LAT is publicly available (http://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/S4_0954p65)

Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. We encourage further multifrequency observations of this source. For this source the Fermi LAT contact person is Felicia Krauss (felicia.krauss@sternwarte.uni-erlangen.de).

The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.