Fermi-LAT detection of renewed gamma-ray activity from the FSRQ sources PKS 0446+11 and OP 313

ATel #16970; F. Casaburo (INFN Roma 2, ASI-SSDC and Sapienza Universita' di Roma), C. Bartolini (Universita' di Trento and INFN Bari), S. Ciprini (INFN Roma 2 and ASI-SSDC), G. La Mura (INAF-OACa), Pietro Monti-Guarnieri (Universita' di Trieste and INFN Trieste) on behalf of the Fermi Large Area Telescope Collaboration
on 9 Jan 2025; 15:02 UT
Credential Certification: Giovanni La Mura (glamura@lip.pt)

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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 activity from a source positionally consistent with the flat-spectrum radio quasar PKS 0446+11, also known as 4FGL J0449.1+1121 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 72.28196 deg, Dec. = +11.35794 deg (J2000; Johnston et al. 1995, AJ, 110, 880), and redshift z=2.153 (Shaw et al. 2012, ApJ, 748, 49).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on January 5, 2025, with a daily averaged gamma-ray flux (E>100MeV) of (1.5+/-0.5) x 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This value is 24 times larger than 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 2.5+/-0.2, consistent with the 4FGL value of 2.50+/0.02. The Fermi LAT Collaboration has previously reported gamma-ray flaring activity from PKS 0446+11 in ATel #16332.

The LAT has also observed renewed gamma-ray activity from a source positionally consistent with the flat-spectrum radio quasar OP 313, also known as 4FGL J1310.5+3221 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 197.61943 deg, Dec. = +32.34549 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 gamma-ray emission state on January 6, 2025, with a daily averaged gamma-ray flux (E>100MeV) of (2.7+/-0.2) x 10^-6 photons cm^-2 s^-1 (statistical uncertainty only), about 50 times higher than the average flux reported in 4FGL. The corresponding photon index is 1.68+/-0.04, indicating a significantly harder spectral state than the 4FGL value of 2.23+/-0.01. Recent optical flaring activity has also been reported in ATels #16951, #16963, #16964.

The spectral hardening led to the detection of thirteen high-energy photons associated with the source at a confidence level p > 0.999 on January 6. The highest energy event was a 116 GeV photon observed at 19:13:43 UT. A likelihood analysis of the high-energy data over the period from December 26, 00:08:15 UT to January 07, 06:35:42 UT revealed a flux above 10 GeV of (3.16 +/- 0.35) x 10^-8 photons cm^-2 s^-1, a 261-fold increase over the 3FHL catalog value of (1.21 +/- 0.21) x 10^-10 photons cm^-2 s^-1 (Ajello et al., 2017, ApJS, 232, 18). The Fermi-LAT Collaboration has previously reported flaring activity from this source in ATels #6068, #15483, #16356, #16497 and #16940.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of these sources will continue. PKS 0446+11 is being added to the "LAT Monitored Sources" ( http://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/ ), while OP 313 is already on this list, so a preliminary estimation of the daily gamma-ray flux observed by Fermi-LAT will be publicly available for both sources. For PKS 0446+11, the Fermi-LAT contact person is Marcello Giroletti (marcello[DOT]giroletti[AT]inaf[DOT]it), and for OP 313 the contact person is Sara Buson (sara[DOT]buson[AT]astro[DOT]uni-wuerzburg[DOT]de). We encourage multifrequency observations of these sources.

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.