Fermi LAT detection of increasing gamma-ray activity of radio quasar 3C 345

ATel #15313; F. Giacchino (INFN Roma Tor Vergata, & ASI, Italy), on behalf of the Fermi Large Area Telescope Collaboration
on 5 Apr 2022; 21:46 UT
Credential Certification: Giovanni La Mura (glamura@lip.pt)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed high-level gamma-ray activity and the spectral hardening from a source positionally consistent with the radio quasar 3C 345 (known as 4FGL J1642.9+3948), with VLBI coordinates (J2000.0), R.A.: 250.745041 deg, Dec.: 39.810276 deg (Johnston et al. 1995, AJ, 110, 880). The redshift of this blazar is z=0.5928 (Marziani et al. 1996, ApJS, 104, 37). 3C 345 is characterised by a strong and extended radio and X-ray jet with an extended radio structure observable from sub-pc to kpc scales, and it is one of the best examples of AGN showing structural and flux variability on parsec scale (for example Lister et al. 2013, AJ, 146, 120).

Preliminary analysis indicates that on 2022 April 4, 3C 345 was in a high state with a daily averaged gamma-ray flux (E>100 MeV) of (1.04 +/- 0.18) e-6 photons cm^-2 s^-1 (statistical uncertainty only), about 20 times greater than the flux reported in the fourth Fermi-LAT source catalog (Abdollahi, S. et al. for the Fermi-LAT collaboration, ApJS 247, 33, 2020). The corresponding photon spectral index (E>100 MeV) of 1.88+/-0.11 (statistical uncertainty only) is smaller than the 4FGL catalog value of 2.38 +/- 0.04.

This is the third time that the Fermi-LAT Collaboration has announced flaring GeV gamma-ray behavior from a source spatially consistent with 3C 345 (ATel #10453 and ATel #2226). This source is a member of the "LAT Monitored Sources" list, with preliminary, uncalibrated estimation of the daily gamma-ray flux observed by Fermi LAT publicly available at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/0FGL_J1641d4p3939. However, we note that an additional gamma-ray source, NRAO 512 (4FGL J1640.4+3945), is nearby 3C 345, and therefore the light curve in the Monitored Source List may be sometimes affected by source confusion depending on the relative fluxes of the two sources. This does not affect the gamma flare reported here, solidly ascribed to 3C 345.

In consideration of the ongoing gamma-ray activity of this source, we encourage multiwavelength observations. Swift ToO observations for this source are scheduled in the next days. The Fermi LAT contact persons for this source are F. Schinzel (fschinze@nrao.edu).

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.