Fermi LAT detection of renewed gamma-ray activity from the FSRQ PKS 2208-137
ATel #17105; P. Monti-Guarnieri (University of Trieste and INFN Trieste), on behalf of the Fermi Large Area Telescope Collaboration
on 24 Mar 2025; 08:09 UT
Credential Certification: Giovanni La Mura (giovanni.lamura@inaf.it)
Subjects: Gamma Ray, >GeV, Request for Observations, AGN, Blazar, Quasar
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 flat-spectrum radio quasar PKS 2208-137, also known as 4FGL J2211.2-1325 (4FGL catalog; The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 332.85041 deg, Dec. = -13.46937 deg (J2000; Truebenbach and Darling 2017, ApJS, 233, 3). This blazar redshift is z = 0.392 (Jones et al. 2009, MNRAS, 399, 683).
Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on March 22, 2025, with a daily averaged gamma-ray flux (E>100MeV) of (1.5+/-0.3) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 70 relative to the average flux reported in the fourth release of the Fourth Fermi-LAT catalog (4FGL-DR4, Ballet et al. 2024, arXiv:2307.12546). This is the highest LAT daily flux ever observed for this source. The corresponding photon index is 2.1+/-0.2, indicating a much harder spectrum than the 4FGL-DR4 value of 2.68+/-0.05. The spectral hardening led to the detection of an 11 GeV photon, associated with the source at a confidence level p > 0.999, on March 22. Previous activity of this source was recently reported on March 10, 2025, in ATel # 17072.
Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. A preliminary light curve for PKS 2208-137 can be accessed via the Fermi-LAT Light Curve Repository at 4FGL_J2211.2-1325. We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact persons are Fausto Casaburo (fausto.casaburo@roma2.infn.it) and Stefano Ciprini (stefano.ciprini@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.
