Fermi LAT Detection of a GeV flare from spectrally hard FSRQ S4 1800+44
ATel #7545; S. Buson (NASA/GSFC/UMBC) on behalf of the Fermi LAT collaboration
on 21 May 2015; 02:59 UT
Credential Certification: Sara Buson (sara.buson@unipd.it)
Subjects: Gamma Ray, >GeV, AGN, Blazar
The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed increasing gamma-ray flux and an unusually hard gamma-ray spectrum from a source positionally consistent with the flat spectrum radio quasar (FSRQ) S4 1800+44 (also known as 3FGL J1801.5+4403, Acero et al. 2015, arXiv:1501.02003) with radio counterpart coordinates (J2000.0), R.A. = 270.3846454 deg, Dec. = 44.0727500 deg (Johnston et al. 1995, AJ, 110, 880). This FSRQ has redshift z=0.663 (Walsh & Carswell, 1982, MNRAS, 200, 191).
Preliminary analysis indicates that on 2015 May 19, the gamma-ray source was in a high state with a daily averaged flux (E>100 MeV) of (0.7+/-0.2) x 10^-6 photons cm^-2 s^-1 (statistical uncertainty only), more than 30 times greater than the average flux reported in the third Fermi LAT source catalog (3FGL). The corresponding daily averaged photon index (E>100 MeV) of 2.0+/-0.2 (statistical uncertainty only) is notably smaller than the average index of 2.70+/-0.09 in the 3FGL catalog.
Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. In consideration of the ongoing activity of this source, we encourage multiwavelength observations. For this source the Fermi LAT contact person is S. Buson (sara.buson@nasa.gov).
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.
