Fermi LAT detection of the brightest ever gamma-ray flare from the FSRQ TXS 0646-176
ATel #12327; Roopesh Ojha (NASA/GSFC/UMBC) and Tonia M. Venters (NASA/GSFC) on behalf of the Fermi Large Area Telescope Collaboration.
on 25 Dec 2018; 00:36 UT
Credential Certification: Roopesh Ojha (Roopesh.Ojha@gmail.com)
Subjects: Gamma Ray, >GeV, AGN, Blazar, Quasar
The Large Area Telescope (LAT), one of two instruments on the Fermi Gamma-ray Space Telescope, has observed the brightest ever gamma-ray flare from a source positionally consistent with the flat spectrum radio quasar TXS 0646-176 also known as 3FGL J0648.8-1740 (Acero et al. 2015, ApJS, 218, 23). Its coordinates are R.A. = 102.1187437 deg, Decl. = -17.7348444 deg (J2000; Petrov et al. 2006, AJ, 131, 1872), and it has a redshift z=1.232 (Hewitt & Burbidge 1993, ApJS, 87, 451).
Preliminary analysis indicates that on December 22, TXS 0646-176 reached a daily averaged gamma-ray flux (E>100 MeV) of (3.6+/-0.6)X10^-6 photons cm^-2 s^-1 (uncertainty is statistical only). This corresponds to a flux increase of a factor of about 170 relative to the average flux reported in the third Fermi-LAT catalog (3FGL). This is the highest LAT daily flux ever observed for this source, nearly two and a half times greater than observed during a previous flaring episode in Nov 2018 (ATel #12252). It has a corresponding photon index (E>100 MeV) of 2.4+/-0.2 which is consistent with the 3FGL value of 2.5+\-0.1.
Previous flaring activity has been reported by Fermi LAT in June 2018 (ATel#11706) and July 2015 (ATel#7833). An optical flare was observed in this source on Nov 12 (ATel#12293).
Because Fermi normally 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. Ciprini (stefano.ciprini@ssdc.asi.it).
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.