Fermi-LAT detection of increasing gamma-ray activity from FSRQ S5 0633+73 and renewed gamma-ray activity from FSRQ TXS 2013+370

ATel #16465; Federica Giacchino (INFN-Sezione Roma TorVergata & SSDC-ASI, Italy) on behalf of the Fermi Large Area Telescope Collaboration
on 21 Feb 2024; 20:11 UT
Credential Certification: Federica Giacchino (federica.giacchino@roma2.infn.it)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed gamma-ray activity from a source positionally consistent with the flat-spectrum radio quasar S5 0633+73, also known as 4FGL J0638.6+7320 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 99.991 deg, Decl. = 73.299 deg (J2000; The Fermi-LAT collaboration 2020, ApJS, 247, 33), and redshift z=1.85 (Ajello, M. et al. 2022, ApJS, 263, 24A). It has also observed a renewed gamma-ray activity from a source positionally consistent with the flat-spectrum radio quasar TXS 2013+370, also known as MG2 J201534+3710 and 4FGL J2015.5+3710 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 303.869708 deg, Decl. = 37.183198 deg (J2000; Fomalont et al. 2003, AJ, 126, 2562), and redshift z=0.859 (Shaw et al. 2013, ApJ, 764, 135).

Preliminary analysis indicates that S5 0633+73 was in an elevated gamma-ray emission state on February 19, 2024, with a daily averaged gamma-ray flux (E>100MeV) of (0.4+/-0.1) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 24 relative to the average flux reported in the fourth Fermi-LAT catalog (4FGL). This is the highest LAT daily flux ever observed for this source. The corresponding photon index is 2.43 +/- 0.30, and is slightly harder than the 4FGL value of 3.03+/-0.09 within the uncertainties. Preliminary analysis indicates that TXS 2013+370 was in an elevated gamma-ray emission state on February 19, 2024 with a daily averaged gamma-ray flux (E>100MeV) of (1.9 +/- 0.3) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 15 relative to the average flux reported in the fourth Fermi-LAT catalog (4FGL). The corresponding photon index is 2.02 +/- 0.10 and is slightly harder than the 4FGL value of 2.45 +/- 0.02 within the uncertainties. The Fermi-LAT Collaboration previously announced a flaring gamma-ray activity from TXS 2013+370 on November 7, 2023 (ATel #16327) and an enhanced activity in December 8, 2020 (ATel #14252).

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of these sources will continue. We encourage multifrequency observations of these sources. The flat spectrum radio quasar S5 0633+73 is on the "LAT Monitored Sources" list (https://fermi.gsfc.nasa.gov/ssc/data/access/lat/lcr/source.html?source_name=4FGL_J0638.6+7320) as is the flat spectrum radio quasar TXS 2013+370 (https://fermi.gsfc.nasa.gov/ssc/data/access/lat/lcr/source.html?source_name=4FGL_J2015.5+3710). For these sources the Fermi LAT contact person is Federica Giacchino (federica.giacchino@roma2.infn.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.