Fermi-LAT detection of enhanced gamma-ray activity from the BL Lac GB6 J0128+4901

ATel #18066; S. Wagner (IFAE), P. Monti-Guarnieri(University of Trieste and INFN Trieste), Giovanni La Mura (INAF - Astronomical Observatory of Cagliari) on behalf of the Fermi Large Area Telescope Collaboration
on 21 Sep 2026; 17:22 UT
Credential Certification: Sarah Wagner (swagner@ifae.es)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed enhanced gamma-ray activity from a source positionally consistent with the BL Lac GB6 J0128+4901, also known as IVS B0125+487 or 4FGL J0127.9+4857 (Abdollahi et al. 2022, ApJS, 260, 53), with coordinates R.A. = 22.03359 deg, Dec. = +49.01832 deg (J2000; Hunt et al. 2021, AJ, 162, 18), and redshift z=0.067 (Marcha et al. 1996, MNRAS 281, 425).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on September 14, with a daily averaged gamma-ray flux (E>100MeV) of (0.2 +/- 0.1) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 20 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). This is the highest LAT daily flux ever observed for this source. The corresponding photon index is 2.0 +/- 0.2, indicating a significantly harder spectrum than the 4FGL-DR4 value of 2.56 +/- 0.12. Optical flaring was already reported in ATel #18061 .

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 persons are Sarah Wagner (swagner@ifae.es) and Pietro Monti-Guarnieri (pietro.monti-guarnieri@phd.units.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.