Fermi LAT detection of a bright GeV gamma-ray flare from the FSRQ 3C 345

ATel #10453; Stefano Ciprini (ASI SSDC Rome & INFN), Sara Buson (NASA-GSFC) on behalf of the Fermi Large Area Telescope Collaboration
on 2 Jun 2017; 17:50 UT
Credential Certification: Stefano Ciprini (stefano.ciprini@asdc.asi.it)

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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 from a source positionally consistent with the flat spectrum radio quasar 3C 345 (also known as B2 1641+39, S4 1641+39, OS 368, 3FGL J1642.9+3950), 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 peculiarly characterized by a strong and extended radio and X-ray jet with and 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; Schinzel et al. 2012, A&A, 537, 70).

Preliminary analysis indicates that on 2017 May 31, 3C 345 was in a high state with a daily averaged gamma-ray flux (E>100 MeV) of (1.8+/-0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only), more than 25 times greater than its four-year average flux reported in the third Fermi-LAT source catalog (3FGL, Acero et al. 2015, ApJS, 218, 23). The corresponding photon spectral index (E>100 MeV) of 2.0+/-0.1 (statistical uncertainty only) is smaller than the 3FGL catalog value of 2.45+/-0.05.

The peak 6-hour integrated gamma-ray flux (E>100 MeV) of (2.9+/-0.7) X 10^-6 photons cm^-2 s^-1 was reached by 3C 345 on 2017 May 31 in the interval 06:00-12:00 UT, and was about 40 times greater than the average flux reported in the 3FGL catalog. The day after, on 2017 June 1, this blazar was detected continuing in a high state with a daily averaged gamma-ray flux (E>100 MeV) of (1.1+/-0.2) X 10^-6 photons cm^-2 s^-1.

This is the second time that the Fermi-LAT Collaboration has announced flaring GeV gamma-ray behavior from a source spatially consistent with 3C 345 after ATel#2226. This source is a member of the "LAT Monitored Sources", 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 (3FGL J1640.6+3945), is only 0.47deg from 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) and S. Buson (sara.buson@nasa.gov).

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.