Enhanced optical activity of the BL Lac S5 1803+784

ATel #17787; 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); Alessandro Nastasi, Sabrina Masiero, Alessio Squilloni (GAL Hassin, Isnello, Italy); Giacomo Bonnoli (INAF - Brera Astronomical Observatory, Milan - Italy).
on 9 May 2026; 22:16 UT
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Credential Certification: Gaia Verna (gaia.verna@unisi.it)

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During the last nights, we observed an enhanced optical activity of the BL Lac S5 1803+784 (R.A. = 270.19035 deg, Decl. = +78.46778 deg (J2000; Johnston et al. 1995, AJ, 110, 880), and redshift z=0.684 (Stickel et al. 1993, A&AS, 98, 393). The same blazar was reported in a flaring state by the Fermi/LAT collaboration a few days ago (ATel#17776). .

The following table reports the brightness recorded in the R band of the Johnson-Cousins photometric systems during the last nights. Quoted uncertainty is statistical only.

Civil date (UT) Mag(R)(dMag) - Observatory  
2026 May 07.865 13.815 (0.019) - Siena 
2026 May 08.849 13.914 (0.005) - GAL Hassin 
 

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 the 1 m Wide-field Mufara Telescope (WMT) at GAL Hassin (Isnello, Italy - IAU MPC observatory code M57).

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.

Brief descriptions of the instrumental setups used for the reported observations are available at the official webpage of the Astronomical Observatory of the University of Siena and on the page of the GAL Hassin Centro Internazionale per le Scienze Astronomiche