Fermi-LAT detection of renewed gamma-ray activity from the BL Lac S4 0954+65

ATel #17042; Chiara Bartolini (University of Trento & INFN Bari), on behalf of the Fermi Large Area Telescope Collaboration
on 17 Feb 2025; 20:58 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 BL Lac S4 0954+65, also known as 4FGL J0958.7+6534 (4FGL-DR4; Ballet et al. 2024, arXiv:2307.12546), with coordinates R.A. = 149.69686 deg, Decl. = +65.56523 deg (J2000; Johnston et al., 1995, AJ, 110, 880), and redshift z=0.368 (Wills et al., 1992, ApJ, 398, 454).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on February 17, 2025, with a daily averaged gamma-ray flux (E>100MeV) of (2.0+/-0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This flux is 30 times higher than the average flux reported in the fourth Fermi-LAT catalog (4FGL-DR4). The corresponding photon index is 1.91+/-0.07, indicating a harder spectrum than the 4FGL-DR4 value of 2.19+/-0.01. We have reported previous flares from this source in ATels #16994, #15375, #14426, #7093, and #6709.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of these sources will continue. Preliminary light curve for this source can be accessed via the Fermi-LAT Light-Curve Repository at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/LightCurveRepository/source.html?source_name=4FGL_J0958.7+6534. The Fermi-LAT contact person is Felicia McBride (fe@femcbride.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.