Swift XRT follow-up observations of the flat spectrum radio quasar 3C 345

ATel #15328; Federica Giacchino (INFN Roma Tor Vergata & SSDC), Stefano Ciprini (INFN Roma Tov Vergata & SSDC), on behalf of the Fermi Large Area Telescope Collaboration, Francesco Verrecchia (INAF OAR and SSDC)
on 13 Apr 2022; 19:26 UT
Credential Certification: Federica Giacchino (federica.giacchino@roma2.infn.it)

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Following the flaring activity observed (ATel#15313) from a gamma-ray source positionally consistent with the flat spectrum radio quasar 3C 345 (4FGL J1642.9+3948, (Abdollahi et al. 2020, ApJS 247, 33), with radio coordinates (J2000.0), R.A.: 250.745041 deg, Dec.: 39.810276 deg (Johnston et al. 1995, AJ, 110, 880) and redshift of z=0.5928 (Marziani et al. 1996, ApJS, 104, 37) we requested a high-urgency Swift target of opportunity multiple observation.

Swift XRT data were taken in photon counting mode for a net exposure of about 6 ksec split in three observations performed on 2022 April 6 (six days after the Fermi LAT flare announced in ATel#15313, also observed by AGILE in ATel#15314) for 2.5 ksec exposure, and on April 8 for 1.7 ksec and April 10 for 1 ksec exposures. For these three single observations the XRT X-ray spectrum of 3C 345 can be fit by an absorbed power law with the HI column density fixed to the Galactic value of N_HI = 8.88 x 10^19 cm^-2 (Kalberla et al. 2005, A&A, 440, 775), and a power-law variable photon spectral index respectively of 1.41+/-0.09 for April 6, 2.07+/-0.23 for April 8, and 1.52+/-0.18 for April 10 observations.

The unabsorbed X-ray flux in the 0.3-10 keV energy band respectively are (5.9+/-0.6) x 10^-12 erg cm^-2 s^-1 for April 6, (2.4+/-0.8) x 10^-12 erg cm^-2 s^-1 for April 8, and (6.6+/-1.2) x 10^-12 erg cm^-2 s^-1 for April 10 observations. The flux levels of April 6 and 10, 2022 are comparable to previous high X-ray states detected by XRT for the source and reported in Berton et al. 2018, A&A, 614, A148, who included results from 33 XRT observations in the period from November 2008 to June 2016. Instead the flux level of April 8, 2022 is smaller.

Simultaneous Swift-UVOT observations are performed during these three pointings. The observed magnitudes in April 6 are: V =16.35+/-0.06, B=16.71+/-0.04, U=15.93+/-0.04, UVW1=15.89+/-0.05, UVM2=15.86+/-0.04, UVW2=15.01+/-0.04. On April 8 are: B=17.24+/-0.07, U=16.54+/-0.07, UVW1=16.40+/-0.08, UVW2=16.37+/-0.23. On April 10 are: V=17.26+/-0.16, B=17.39+/-0.09, U=16.69+/-0.05, UVW1=16.37+/-0.06, UVM2=16.28+/-0.09, UVW2=16.53+/-0.08. The optical V,B,U magnitudes of 3C 345 are comparable or slightly brighter than the highest optical magnitude ever observed for 3C 345, and comparable to those during October 2009 gamma-ray flare of 3C 345 (ATel#2226) as reported in Berton et al., 2018, A&A, 614, A148.

These preliminary results and the previously known radio-to-X-ray source properties suggest 3C 345 is now an identified gamma-ray blazar. In consideration of the ongoing activity of this source, confirmed also by the Swift observations, we encourage further multiwavelength observations. For this source the Fermi LAT contact persons are F. Schinzel (fschinze[at]nrao.edu) and F. Giacchino (federica.giacchino[at]roma2.infn.it).

We would like to thank the Swift Team for making these observations possible, in particular B. Sbarufatti as the Swift Observatory Duty Scientist.