Fermi-LAT detection of renewed gamma-ray activity from the FSRQ OP 313, and enhanced gamma-ray activity from the FSRQ ON 393

ATel #16356; Chiara Bartolini (University of Trento & INFN Bari), Federica Giacchino (INFN Sezione Roma TorVergata & ASI Science Data Center), Maria Stella Gisonna (University of Bari), Danilo Elenterio (University of Bari), Graziana Lopetuso (University of Bari) on behalf of the Fermi Large Area Telescope Collaboration
on 1 Dec 2023; 17:12 UT
Credential Certification: Denis Bernard (Denis.bernard@in2p3.fr)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed enhanced gamma-ray activity from a source positionally consistent with the flat-spectrum radio quasar (FSRQ) OP 313 (B2 1308+32), also known as 4FGL J1310.5+3221 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 197.619432 deg, Decl. = +32.345495 deg (J2000; Johnston et al. 1995, AJ, 110, 880), and redshift z=0.997 (Schneider et al. 2010, AJ, 139, 2360).

Preliminary analysis indicates that this source was in an elevated and hard gamma-ray emission state during the past week with a maximum on November 24, 2023, with a daily averaged gamma-ray flux (E>100MeV) of (1.8+/-0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 40 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 1.80+/-0.06 which is significantly harder than the 4FGL value of 2.34+/-0.02. The last time that the Fermi-LAT Collaboration announced flaring gamma-ray activity from OP 313 was June 29, 2022 (ATel #15483).

The Fermi-LAT has also observed enhanced gamma-ray activity from a source positionally consistent with the FSRQ ON 393 (B2 1255+32), also known as 4FGL J1257.8+3228 , with radio coordinates R.A. =194.8714 deg, Decl. = 32.491479 deg (J2000; Petrov et al. 2011, AJ, 142, 89), and redshift z=0.806 (Sloan Digital Sky Survey, 2016, SDSSD).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on November 28, 2023, with a daily averaged gamma-ray flux (E>100MeV) of (0.36+/-0.11) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 20 relative to the average flux reported in the fourth Fermi-LAT catalog (4FGL). The corresponding photon index is 1.98+/-0.18 which is harder than the 4FGL value of 2.32+/-0.04 within the uncertainties.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of these sources will continue. A preliminary light curve for OP 313 can be accessed via the Fermi-LAT Light-Curve Repository at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/LightCurveRepository/source.html?source_name=4FGL_J1310.5+3221. A preliminary light curve for ON 393 can be accessed via the Fermi-LAT Light-Curve Repository at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/LightCurveRepository/source.html?source_name=4FGL_J1257.8+3228. We encourage multifrequency observations of these sources. For OP 313, the Fermi-LAT contact person is Sara Buson (sara.buson at uni-wuerzburg.de). For ON 393, the Fermi-LAT contact person is Chiara Bartolini (chiara.bartolini@ba.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.