Fermi-LAT detection of enhanced gamma-ray activity from the BL Lac Object GB6 J1203+5819
ATel #17451; C. C. Cheung (NRL), on behalf of the Fermi Large Area Telescope Collaboration
on 22 Oct 2025; 22:02 UT
Credential Certification: Teddy Cheung (Teddy.Cheung@nrl.navy.mil)
Subjects: Gamma Ray, >GeV, AGN, Blazar
The Large Area Telescope (LAT), one of the two instruments onboard the Fermi Gamma-ray Space Telescope, has observed enhanced gamma-ray activity from a source positionally consistent with the BL Lac object GB6 J1203+5819 (SDSS J120355.34+581945.5; Plotkin et al. 2008, AJ, 135, 2453), also known as 4FGL J1203.7+5821 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with radio coordinates R.A. = 180.98056 deg, Dec. = 58.32933 deg (J2000; Petrov & Kovalev, 2025, ApJS, 276, 38). The four available SDSS spectra give different values for the redshift (see: https://skyserver.sdss.org/DR19/VisualTools/explore/summary?sId=1479536432610568192).
Preliminary analysis indicates that this source was in an elevated gamma-ray emission state from October 13.0-20.0, 2025, with a weekly averaged gamma-ray flux (E>100MeV) of (5.8 +/- 1.7) X 10^-8 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase by a factor of more than 10 relative to the average flux reported in the fourth Fermi-LAT catalog Data-Release 4 (4FGL-DR4, Ballet et al. 2023, arXiv, arXiv:2307.12546). The corresponding photon index is 1.9 +/- 0.2, indicating a harder spectrum than the 4FGL-DR4 value of 2.42 +/- 0.11. The transient was identified thanks to the method implemented within the Fermi-LAT Collaboration known as "Fermi All-sky Variability Analysis" that searches the sky for high-energy transients on weekly time scales (Ackermann et al. 2013, ApJ, 771, 57; Abdollahi et al. 2017, ApJ, 846, 34).
Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. We encourage multifrequency observations of this 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.
