Fermi-LAT detection of enhanced gamma-ray activity from TXS 0946+181 and 3C 273

ATel #16698; S. Wagner (University of Wuerzburg), J. Valverde (UMBC/NASA GSFC), I. Mereu (INFN Perugia); on behalf of the Fermi Large Area Telescope Collaboration
on 9 Jul 2024; 19:53 UT
Credential Certification: Sarah Wagner (sarah.wagner@physik.uni-wuerzburg.de)

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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 flat spectrum radio quasar TXS 0946+181, also known as MG1 J094941+1752, GB6 J0949+1752 and 4FGL J0949.2+1749 in the fourth Fermi-LAT catalog Data Release 4 (4FGL-DR4; Ballet et al. 2024, arXiv:2307.12546), with coordinates R.A. = 147.41568 deg, Dec = +17.88039 deg (J2000; Petrov & Taylor 2011, AJ, 142, 89). The redshift of TXS 0946+181 is z=0.693 (Schneider et al. 2010, AJ, 139, 2360).

Preliminary analysis indicates that TXS 0946+181 was in an elevated gamma-ray emission state on July 7, 2024, with a daily averaged gamma-ray flux (E>100MeV) of (10 +/- 2) x 10^-7 photons cm^-2 s^-1 (statistical uncertainties only), corresponding to a flux increase of a factor of about 150 with respect to the average flux reported in the 4FGL-DR4 Catalog. The corresponding photon index is 2.5+/-0.4, consistent with the 4FGL value of 2.5+/-0.1. After the recent first gamma-ray flare of TXS 0946+181 reported in ATel #16671, this is the second time that the Fermi LAT Collaboration reports enhanced gamma-ray activity for this source.

The Fermi LAT also has observed enhanced gamma-ray activity from 3C 273 (J2000, R.A. = 187.27791 deg, Dec. = 2.05239 deg, Johnston et al. 1995, AJ, 110, 880), also known as 4FGL J1229.0+0202 in 4FGL-DR4 with the redshift z = 0.158 (Strauss et al. 1992, ApJS, 83, 29).

Preliminary analysis indicates that 3C 273 was in an elevated gamma-ray emission state on July 7, 2024, with a daily averaged gamma-ray flux (E>100MeV) of (1.6 ± 0.3) x 10^-6 photons cm^-2 s^-1 (statistical uncertainties only), corresponding to a flux increase of only a factor of about 7 with respect to the average flux reported in the 4FGL-DR4 Catalog but significantly higher than recently, as can be seen in the light curve repository. The corresponding photon index is 2.5+/-0.4, consistent with the 4FGL value of 2.69 +/- 0.01. Gamma-ray activity of 3C 273, the prototype of QSOs, detected with Fermi-LAT has previously been reported in ATel #1707 (Sept 2008), ATel #2168 (in Aug 2009), and ATel #2200 (Sept 2009).

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of both TXS 0946+181 and 3C 273 will continue. We encourage multifrequency observations of these gamma-ray sources. For TXS 0946+181, the Fermi-LAT contact persons are F. Casaburo (fausto.casaburo[at]roma2.infn.it), F. Giacchino (federica.giacchino[at]roma2.infn.it), and S. Ciprini (stefano.ciprini[at]roma2.infn.it). For 3C 273, the Fermi-LAT contact persons are Werner Collmar (wec[at]mpe.mpg.de) and Gino Tosti (tosti[at]pg.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