Swift XRT follow-up observations of 3C138

ATel #17142; Federica Giacchino (INFN Roma Tor Vergata & SSDC ASI),  Pietro Monti-Guarnieri (University of Trieste and INFN Trieste), Giovanni La Mura (INAF-OAC) on behalf of the Fermi Large Area Telescope Collaboration
on 12 Apr 2025; 20:14 UT
Credential Certification: Federica Giacchino (federica.giacchino@roma2.infn.it)

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Following the flaring activity observed (ATel #17107) from a gamma-ray source positionally consistent with the compact steep spectrum quasar 3C 138 also known as 4FGL J0521.2+1637 (4FGL-DR4; Ballet et al. 2024, arXiv:2307.12546), with radio coordinates R.A. = 80.29119 deg, Decl. = +16.63946 deg (J2000; Truebenbach and Darling 2017, ApJS, 233, 3), and redshift z= 0.759 (Lynds et al. 1966, ApJ, 144, 1244), we requested a medium-urgency Swift target of opportunity multiple observation.

Swift XRT data were taken in photon counting mode for a total net exposure of approximately 3.2 ksec split in three observations performed on March 27, 2025 (three days after the Fermi LAT flare announced in ATel#17107) for 1.8 ksec exposure, on March 29, 2024 for 1.38 ksec and on March 31, 2024 for 0.2 ksec. For these three single observations the XRT X-ray spectrum of 3C 138 can be fitted by an absorbed single power law with the HI column density fixed to the Galactic value of N_HI = 10^22 cm^-2 (Dickey & Lockman 1990) and a power-law variable photon spectral index respectively of 1.77+/-0.40 for March 27 and 1.68+/-0.37 for March 29. The 0.2ks XRT exposure on March 31 did not yield enough counts to estimate the spectral properties. The unabsorbed X-ray fluxes in the 0.4-10 keV energy band respectively are (3.1+/-0.1)x10^-12 erg cm^-2s^-1 for March 27 and (3.9+/-0.2)x10^-12 erg cm^-2s^-1 for March 29 observations. The flux level of both observations is larger than previous X-ray detected by Chandra for the source and reported in Wang et al. (2016, ApJS, 224, 40W).

Simultaneous Swift-UVOT observations have been performed. The observed magnitude on March 27 is: UVW2=18.20+/- 0.07; on March 29 UVW1=17.52+/-0.05. No previous observations in these bands have been found.

In consideration of the ongoing activity of this source, confirmed also by the Swift observations and radio observations ATel #17077 and #17104, we encourage further multiwavelength observations. For this source, the Fermi-LAT contact persons are Ettore Bronzini (ettore.bronzini@inaf.it) and C.C. Cheung (Teddy.Cheung@nrl.navy.mil).

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