Fermi-LAT detection of renewed gamma-ray activity from the BL Lac S4 0954+65 and from the FSRQ PKS 0235-618
ATel #16994; Denis Bernard (LLR, Ecole Polytechnique & CNRS / IN2P3), on behalf of the Fermi Large Area Telescope Collaboration
on 25 Jan 2025; 17:03 UT
Credential Certification: Denis Bernard (Denis.bernard@in2p3.fr)
Subjects: Gamma Ray, >GeV, AGN, Blazar, Quasar
Referred to by ATel #: 17005
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. = 149.69686 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 Jan 24, 2025, with a daily averaged gamma-ray flux (E>100MeV) of (1.5+/-0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This flux is 20 times higher than the average flux reported in the fourth Fermi-LAT catalog (4FGL). The corresponding photon index is 2.05+/-0.10, indicating a harder spectrum than the 4FGL value of 2.19+/-0.01. We have reported previous flares from this source in ATels #15375, #14426, #7093 and #6709.
The LAT has also observed renewed gamma-ray activity from a source positionally consistent with the flat-spectrum radio quasar PKS 0235-618 , also known as 4FGL J0236.8-6136 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 39.22186 deg, Dec. = -61.60422 (J2000; Xu et al., 2019, ApJS, 242, 5), and redshift z=0.465 (Healey, S. et al. 2008, ApJS, 175, 97).
Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on Jan 24, 2025, with a daily averaged gamma-ray flux (E>100MeV) of (0.36+/-0.12) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This flux is more than 20 times higher than the average flux reported in 4FGL. The corresponding photon index is 1.7+/-0.2, indicating a harder spectrum than the 4FGL value of 2.29+/-0.03. We have reported a previous flare from this source in ATel #2669.
Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of these sources will continue. Preliminary light curves for these sources can be accessed via the Fermi-LAT Light-Curve Repository at S4 0954+65 and PKS 0235-618. We encourage multifrequency observations of these sources. The Fermi-LAT contact persons are Felicia McBride (fe@femcbride.com) for S4 0954+65 and Sara Cutini (sara.cutini@asdc.asi.it) for PKS 0235-618.
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.