Fermi-LAT detection of renewed gamma-ray activity from FSRQ S4 0805+41
ATel #17146; Federica Giacchino (INFN Roma Tor Vergata & SSDC ASI), Giovanni La Mura (INAF-OAC) on behalf of the Fermi Large Area Telescope Collaboration
on 14 Apr 2025; 20:13 UT
Credential Certification: Federica Giacchino (federica.giacchino@roma2.infn.it)
Subjects: Gamma Ray, >GeV, AGN, Blazar, Quasar
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 the flat spectrum radio quasar S4 0805+41, also known as 4FGL J0809.3+4053 (4FGL-DR4; Ballet et al. 2024, arXiv:2307.12546), 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 April 13, with a daily averaged gamma-ray flux (E>100MeV) of (0.4+/-0.1) x 10^-6 photons cm^-2 s^-1 (statistical uncertainty only), corresponding to a flux increase of a factor of about 40 relative to the average flux reported in 4FGL-DR4. The photon index is 2.01 +/- 0.22, corresponding to a significantly harder spectrum than the 4FGL-DR4 catalog value of 2.63 +/- 0.08. The spectral hardening resulted in the detection of one high-energy (E = 11 GeV) photon, having a probability of being associated with the source p > 0.999. The Fermi-LAT Collaboration has previously reported enhanced activity from this source in ATel #15704.
Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of S4 0805+41 will continue. This source has an entry in the Fermi LAT light curve repository (https://fermi.gsfc.nasa.gov/ssc/data/access/lat/LightCurveRepository/source.html?source_name=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.
