Fermi-LAT detection of renewed gamma-ray activity from FSRQ S4 0805+41

ATel #17341; A. Holzmann Airasca (University of Trento & INFN Bari) and P. Monti-Guarnieri (University of Trieste and INFN Trieste), on behalf of the Fermi Large Area Telescope Collaboration
on 18 Aug 2025; 19:38 UT
Credential Certification: Chiara Bartolini (chiara.bartolini-1@unitn.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 S4 0805+41, also known as 4FGL J0809.3+4053 (Abdollahi et al. 2022, ApJS, 260, 53), with radio coordinates R.A. = 122.23605 deg, Decl. = 40.87914 deg (J2000; Johnston et al. 1995, AJ, 110, 880), and redshift z=1.419 (Xu et al. 1994, AJ, 108, 395; Schneider et al. 2010, AJ, 139, 2360).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on 2025 August 17, with a daily averaged gamma-ray flux (E>100MeV) of (0.3+/-0.1) x 10^-6 photons cm^-2 s^-1 (statistical uncertainty only), corresponding to a flux increase of a factor of about 30 relative to the average flux reported in the fourth data release of the fourth Fermi-LAT catalog (4FGL-DR4; Ballet et al. 2024, arXiv:2307.12546). The photon index is 1.9 +/- 0.2, indicating a significantly harder spectral state than the one inferred from the average power-law spectral index of 2.63 +/- 0.08 reported in 4FGL-DR4. The Fermi-LAT Collaboration has previously reported enhanced activity from this source in ATels #17146 and #15704.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of S4 0805+41 will continue. A preliminary light curve for this source can be accessed via the Fermi-LAT Light Curve repository at 4FGL J0809.3+4053. We encourage multifrequency observations of this gamma-ray source. For this source, the Fermi-LAT contact person is C. C. Cheung (teddy.cheung@nrl.navy.mil).

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.