Fermi-LAT detection of renewed gamma-ray activity from the FSRQ Ton 599 (4C +29.45)

ATel #17226; S. Wagner (University of Wuerzburg) and Pietro Monti-Guarnieri (University of Trieste and INFN Trieste) on behalf of the Fermi Large Area Telescope Collaboration
on 8 Jun 2025; 19:59 UT
Credential Certification: Sarah Wagner (sarah.wagner@physik.uni-wuerzburg.de)

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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 Ton 599, also known as 4FGL J1159.5+2914 (Abdollahi et al. 2022, ApJS, 260, 53), with coordinates RA = 179.88264 deg, Decl. = 29.24551 deg, (J2000; Johnston et al. 1995 AJ 110, 880) at z = 0.72449 (Adelman-McCarthy et al. 2008, ApJS, 175, 297).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on June 7, 2025 , 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 10 relative to the average flux reported in the fourth data release of the fourth Fermi-LAT catalog (4FGL-DR4, Ballet et al. 2023, arXiv:2307.12546). The corresponding photon index is 2.2 +/- 0.1, and is consistent with the 4FGL-DR4 value of 2.155 +/- 0.006 within the uncertainties. Gamma-ray flaring of Ton 599 has previously been reported in ATels #15859, #14897, #14722, #10931, #8319, and #2795.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. A preliminary light curve for Ton 599 can be accessed via the Fermi-LAT Light-Curve Repository and the Monitored Source List. We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact persons are S. Ciprini (stefano.ciprini@ssdc.asi.it) and D. J. Thompson (David.J.Thompson@nasa.gov).

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.