Fermi-LAT detection of renewed gamma-ray activity from the FSRQ ON 393

ATel #17812; F. Casaburo (INAF - OAR, INFN - Roma Tor Vergata, Sapienza University of Roma), G. La Mura (INAF-OAC), and S. Ciprini (INFN - Roma Tor Vergata, ASI - SSDC), on behalf of the Fermi Large Area Telescope Collaboration
on 25 May 2026; 18:04 UT
Credential Certification: Giovanni La Mura (giovanni.lamura@inaf.it)

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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 ON 393, also known as 4FGL J1257.8+3228 (The Fermi-LAT collaboration 2020, ApJS, 247, 1), with radio coordinates R.A. = 194.48847 deg, Decl. = 32.49148 deg (J2000; Petrov et al. 2011, AJ, 142, 89), and redshift z=0.806 (Albareti et al. 2017, ApJS, 233(2), 25).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on May 24, 2026, with a daily averaged gamma-ray flux (E>100MeV) of (0.5 +/- 0.1) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of more than 20 relative to the average flux reported in the fourth data release of the fourth Fermi-LAT source catalog (4FGL-DR4, Ballet et al. 2023, arXiv:2307.12546). The corresponding photon index is 2.0 +/- 0.2, indicating a significantly harder spectrum than the 4FGL-DR4 value of 2.32 +/- 0.03. The Fermi-LAT Collaboration has previously reported flaring activity from this source in ATel #16356.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. A preliminary light curve for ON 393 can be accessed via the Fermi-LAT Light Curve Repository at 4FGL J1257.8+3228. We encourage multifrequency observations of this source. For this source 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.