Fermi-LAT detection of renewed gamma-ray activity from the BL Lac S4 0954+65
ATel #15375; B. Rani (KASI, S. Korea), J. Valverde (UMBC/NASA GSFC), and G. La Mura (LIP, Portugal), on behalf of the Fermi Large Area Telescope Collaboration
on 11 May 2022; 20:00 UT
Credential Certification: Janeth Valverde (valverde@llr.in2p3.fr)
Subjects: Gamma Ray, >GeV, Request for Observations, AGN, Blazar
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 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 149.696855 deg, Decl. = +65.565227 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 May 09, 2022, with a daily averaged gamma-ray flux (E>100MeV) of (1.8+/-0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of more than 30 relative to the average flux reported in the fourth Fermi-LAT catalog (4FGL). This is the highest LAT daily flux ever observed for this source. The corresponding photon index is 2.01+/-0.08, smaller than the 4FGL value of 2.22+/-0.02. Two high-energy photons were detected from the source, with the maximum energy of 37 GeV. The flare triggered the Fermi-LAT Monitor alert system, which issued the GCN/Fermi_LAT_MON Notice (https://gcn.gsfc.nasa.gov/other/1652194322_fermi.txt).
Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue (https://fermi.gsfc.nasa.gov/ssc/data/access/lat/LightCurveRepository/source.html?source_name=4FGL_J0958.7+6534). We encourage multifrequency observations of this source. For this source, 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.
