Fermi-LAT detection of renewed gamma-ray activity from the FSRQ TXS 0646-176

ATel #14786; I. Mereu (INFN Perugia) on behalf of the Fermi-LAT Collaboration
on 14 Jul 2021; 18:13 UT
Credential Certification: Isabella Mereu (mereuisabella@gmail.com)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed renewed gamma-ray flaring activity from a source positionally consistent with the flat-spectrum radio quasar TXS 0646-176, also known as 4FGL J0648.4-1743 (The Fermi-LAT collaboration 2020, ApJS, 247, 1), with coordinates R.A. = 102.1187437 deg, Decl. = -17.7348444 deg (J2000; Petrov et al. 2006, AJ, 131, 1872), and redshift z=1.232 (Murdoch et al. 1984, PASA, 5, 341).

Preliminary analysis indicates that this source went into a high-flux state on July 12, 2021, reaching a peak daily averaged gamma-ray flux (E>100MeV) of (2.6±0.3) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). The latter corresponds to a flux increase of a factor of about 76 relative to the average flux reported in the fourth Fermi-LAT catalog (4FGL). The corresponding photon index is 2.2±0.1, and is smaller than the 4FGL value of 2.57+/-0.05. In the six hour interval 6-12 UTC the averaged gamma-ray flux (E>100MeV) reached the value of (5.0±0.9) X 10^-6 photons cm^-2 s^-1, 133 times the 4FGL value. Previous flaring episodes were reported in ATel #7833, ATel #11706 and ATel #12327.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. This source is part 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/). We encourage further multifrequency observations of this source. 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.