Fermi-LAT detection of hard spectrum gamma-ray flare from FSRQ S4 1800+44
ATel #12553; D. Gasparrini (SSDC/INFN) on behalf of Fermi-LAT collaboration
on 6 Mar 2019; 21:12 UT
Credential Certification: Dario Gasparrini (dario.gasparrini@asdc.asi.it)
Subjects: Gamma Ray, >GeV, AGN, Blazar, Quasar
Referred to by ATel #: 12562
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, ApJS, 218, 23) and 4FGL J1801.5+4404 in the recently released 4FGL (https://fermi.gsfc.nasa.gov/ssc/data/access/lat/8yr_catalog/) 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 2019 March 05, the gamma-ray source was in a high state with a daily averaged flux (E>100 MeV) of (1.8+/-0.2) x 10^-6 photons cm^-2 s^-1 (statistical uncertainty only), more than 70 times greater than the average flux reported in the third Fermi LAT source catalog (3FGL). The corresponding daily averaged photon index of 1.72+/-0.06 (statistical uncertainty only) is notably smaller than the average index of 2.36+/-0.02 in the 4FGL catalog. Previous hard-spectrum enhanced activity was observed by Fermi LAT on 2015 May 19 (ATel #7545) and on 2016 March 14 (ATel #8812). A GCN was issued automatically for this flare (https://gcn.gsfc.nasa.gov/gcn/other/1551887218_fermi.txt)
Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. A preliminary estimation of the daily gamma-ray flux observed by Fermi-LAT will be publicly available (http://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/). 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@gmail.com).
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.
