Fermi-LAT detection of enhanced gamma-ray activity from the FSRQ TXS 1235-117

ATel #17349; F. Longo (University of Trieste and INFN Trieste), P. Monti-Guarnieri (University of Trieste and INFN Trieste) and A. Holzmann Airasca (University of Trento & INFN Bari), on behalf of the Fermi Large Area Telescope Collaboration
on 22 Aug 2025; 14:48 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 enhanced gamma-ray activity from a source positionally consistent with the FSRQ TXS 1235-117, also known as 4FGL J1238.5-1201 (Abdollahi et al. 2022, ApJS, 260, 53), with coordinates R.A. = 189.53091 deg, Dec. = -11.99015 deg (J2000; Healey et al. 2007, ApJS, 171, 61), and redshift z=0.293 (Jones et al. 2004, MNRAS 355, 747; Jones et al. 2009, MNRAS, 399, 683).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on August 21, 2025, with a daily averaged gamma-ray flux (E>100MeV) of (0.6+/-0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 30 relative to the average flux reported in the fourth data release of the fourth Fermi-LAT catalog (4FGL-DR4, Ballet et al. 2023, arXiv:2307.12546). This is the highest LAT daily flux ever observed for this source. The corresponding photon index is 2.3+/-0.2 indicating a harder spectral state than the one inferred from the average power-law spectral index of 2.64 +/- 0.05 reported in 4FGL-DR4.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. A preliminary light curve for TXS 1235-117 can be accessed via the Fermi-LAT Light-Curve Repository at 4FGL J1238.5-1201. We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact person is Francesco Longo (Francesco.Longo@ts.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.