Fermi LAT detection of increasing gamma-ray activity from Compact Steep-Spectrum Radio Source 3C 216  

ATel #16024; Federica Giacchino (INFN Sezione Roma TorVergata & ASI Science Data Center, Italy), Giovanni La Mura (LIP, Portugal) and Denis Bernard (LLR, Ecole Polytechnique & CNRS/IN2P3) on behalf of the Fermi Large Area Telescope Collaboration 
on 2 May 2023; 20:11 UT
Credential Certification: Federica Giacchino (federica.giacchino@roma2.infn.it)

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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 compact steep spectrum radio source 3C 216, also known as 4FGL J0910.0+4257 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 137.38957 deg, Decl. = +42.89624 deg (J2000; Petrov et al. 2005, AJ, 129, 1163), and redshift z = 0.670 (Albareti et al. 2017, ApJS, 233, 25).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on May 1, 2023, with a daily averaged gamma-ray flux (E>100MeV) of (1.32+/- 0.15) X 10^-6 photons cm^-2 s^-1(statistical uncertainty only). This corresponds to a flux increase of a factor around 180 relative to the average flux reported in the third release of the fourth Fermi-LAT catalog (4FGL-DR3, Abdollahi et al. 2022, ApJS, 260, 53). This is the highest LAT daily flux ever reported for this source. The corresponding photon index is 2.11+/-0.09, and is smaller than the 4FGL-DR3 value of 2.57+/-0.10, indicating an harder state, index difference of ~0.5.

Previous elevated gamma-ray emission state on December 8, 2022 with a flux of (0.5+/-0.1) X 10^-6 photons cm^-2 s^-1. For this reason a first ATel #15801 on the same source was published.

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 person is Federica Giacchino (federica.giacchino@roma2.infn.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.