SRG/ART-XC captures 3C 138 in a bright X-ray state
ATel #17645; G. Uskov (1), S. Sazonov (1), I. Lapshov (1), A. Tkachenko (1), A. Lutovinov (1), A. G. Mikhailov (2), Yu. V. Sotnikova (2), T. V. Mufakharov (2, 3) (1 - Space Research Institute of RAS, Russia, 2 - Special Astrophysical Observatory of RAS, Russia, 3 - Xinjiang Astronomical Observatory, China)
on 3 Feb 2026; 19:02 UT
Credential Certification: Sergey Sazonov (sazonov@iki.rssi.ru)
Subjects: X-ray, AGN, Quasar
Following the increased activity of the compact steep-spectrum quasar 3C 138 in the radio, optical and gamma-ray bands over the recent months (ATels #17539, #17540, #17461), we conducted a pointed X-ray observation of this object with the Mikhail Pavlinsky ART-XC telescope on board the SRG observatory on 28-29 January, 2026, with a total duration of 23.3 hours.
The spectrum measured in the 4–30 keV energy band can be approximated by a power law with a photon index of 1.58 ± 0.08 modified by the Galactic absorption (NH = 2.0 × 1021 cm−2). The unabsorbed flux in the observed
4–20 keV energy band is (7.8 ± 0.2) × 10−12 erg s−1 cm−2 (all quoted uncertainties are at the 90% level).
This is a factor of 2–3 higher than during the Swift/XRT observations performed at the end of March, 2025 (ATel #17142). We do not detect any short-term variability during the ART-XC observation.
The ART-XC team plans to continue observations of 3C 138 in the coming months.
