Fermi-LAT detection of renewed gamma-ray activity from the TeV BL Lac VER J0521+211

ATel #17895; E. Bronzini (INAF-OAS),  P. Monti-Guarnieri (University of Trieste & INFN Trieste), C. Bartolini (INFN Bari), G. La Mura (INAF-O. A. Cagliari),  on behalf of the Fermi Large Area Telescope Collaboration
on 15 Jul 2026; 21:09 UT
Credential Certification: Ettore Bronzini (ettore.bronzini@inaf.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 TeV BL Lac VER J0521+211, associated with the radio source TXS 0518+211 (a.k.a. RGB J0521+212) and the GeV gamma-ray source 4FGL J0521.7+2112 (Abdollahi et al. 2022, ApJS, 260, 53), with coordinates R.A. = 80.44152 deg, Dec. = +21.21429 deg (J2000; Beasley et al. 2002 ApJS, 141, 13), and at uncertain redshift (Archambault et al. 2013, ApJ, 776, 2, 69; Paiano et al. 2017, ApJ, 837, 2, 144). 

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on 14 July 2026, with a daily averaged gamma-ray flux (E>100MeV) of (0.7+/-0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of about 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 1.9+/-0.2 and is consistent with the 4FGL-DR4 value of 1.95+/-0.01 within the uncertainties. The enhanced state led to the detection of seven high-energy photons associated with the source at a confidence level p > 0.85 between July 01, 2026 and July 14, 2026. The highest energy event was a 80 GeV photon observed on July 14, 2026 at 06:35:23 UT. The Fermi LAT Collaboration has previously reported gamma-ray flaring activity from VER J0521+211 in ATels #5472, #13528.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. This source belongs to the "LAT Monitored Sources" and consequently, a preliminary estimation of the daily gamma-ray flux observed by Fermi-LAT is available at VER J0521+211. The gamma-ray light curve of the source is also available through the Fermi-LAT Light Curve Repository (LCR) at TXS 0518+211. For this source, the Fermi-LAT contact person is Sara Buson (sara.buson@gmail.com).

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.