The CSS quasar 3C 138 reached an exceptionally high magnitude in the R and V bands

ATel #18142; 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); Pierfrancesco Basile, Emanuele Russo (PB Observatory, Pioltello (MI) - Italy and Osservatorio Astronomico Civico "Gabriele Barletta" - Associazione Cernuschese Astrofili (ACA), Cernusco sul Naviglio (MI) - Italy); Giacomo Bonnoli (INAF - Brera Astronomical Observatory, Milan - Italy).
on 4 Nov 2026; 16:13 UT
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Credential Certification: Gaia Verna (gaia.verna@unisi.it)

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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) was recently reported to be flaring in the R-band (ATel #18108, #18093).

Over the last nights, 3C 138 reached an exceptionally high magnitude of 14.94 in the R-band and 15.60 in the V-band.

The following table reports the average brightness recorded each night in the R-band and V-band of the Johnson-Cousins photometric system over the last weeks. Quoted uncertainty is statistical only.

 
 
Civil Date (UT)  Mag   (Err)  Band - Observatory 
2026 Oct. 26.064 R= 15.28 (0.04) - Cernusco S.N. 
2026 Oct. 27.046 R= 15.32 (0.02) - Siena 
2026 Oct. 27.120 R= 15.33 (0.08) - Pioltello 
2026 Nov. 01.137 R= 14.94 (0.09) - Siena 
2026 Nov. 02.066 R= 15.13 (0.03) - Pioltello 
2026 Nov. 02.985 R= 15.31 (0.02) - Cernusco S.N. 
2026 Nov. 03.922 R= 14.98 (0.03) - Siena 
2026 Oct. 27.049 V= 15.94 (0.04) - Siena 
2026 Nov. 03.913 V= 15.60 (0.04) - Siena 
 

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), the 0.13 m telescope of the PB Observatory (Pioltello, Italy) and the 0.40 m telescope of the Osservatorio Astronomico Civico "Gabriele Barletta" (Cernusco sul Naviglio, 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.

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