The CSS quasar 3C 138 is still in a very bright optical state

ATel #18108; Alessandro Marchini (Astronomical Observatory, DSFTA, University of Siena - Italy); Leonardo Stiaccini, Gaia Verna (Department of Physical Sciences, Earth and Environment (DSFTA), University of Siena - Italy); Giacomo Bonnoli (INAF - Brera Astronomical Observatory, Milan - Italy); Pierfrancesco Basile (PB Observatory - Associazione Cernuschese Astrofili (ACA), Pioltello (MI) - Italy)
on 17 Oct 2026; 22:01 UT
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Credential Certification: Gaia Verna (gaia.verna@unisi.it)

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In the last nights the compact steep-spectrum (CSS) quasar 3C 138 (a.k.a. QSO B0518+165, 4FGL J0521.2+1637) at R.A. 80.292 deg, Dec. +16.629 deg (J2000.0) and z=0.759 (Lynds et al. 1966, ApJ, 144, 1244) reached on 2026 Oct 11.144 an extreme optical magnitude of 15.16 ± 0.03 in the R band of the Johnson-Cousins photometric system. After a few nights, on 2026 Oct 15.162, we found it at 15.39 ± 0.02 (R band), confirming its high optical state. Quoted uncertainties are statistical only.

The source was already observed in an exceptionally bright optical state on 7 Oct 2026 (ATel #18093), and it has been detected in an enhanced state in the HE gamma-ray band on 7 Aug 2026 by Fermi-LAT (ATel #17949).

Observations were conducted from the 0.30 m telescope of the Astronomical Observatory of the University of Siena (Siena, Italy - IAU MPC observatory code K54), and from the 0.13 m telescope of the PB Observatory (Pioltello, Italy).

Any inquiry on these observations can be addressed either to Alessandro Marchini (marchini@unisi.it) or to Gaia Verna (gaia.verna@unisi.it). We will continue monitoring the source in the following nights. Multi-wavelength follow-up is strongly encouraged.