An exceptional optical flare of the FSRQ OI 280 (PKS 0748+126)

ATel #17739; Alessandro Marchini (Astronomical Observatory, DSFTA, University of Siena - Italy); Luigi Pacciani (INAF-IAPS Roma, Italy); Leonardo Stiaccini, Gaia Verna (Department of Physical Sciences, Earth and Environment (DSFTA), University of Siena - Italy); Simone Leonini, Massimo Conti, Paolo Rosi, Luz Marina Tinjaca Ramirez (Montarrenti Observatory, Sovicille (Siena - Italy)); Giacomo Bonnoli (INAF - Brera Astronomical Observatory, Milan - Italy)
on 4 Apr 2026; 21:35 UT
Credential Certification: Gaia Verna (gaia.verna@unisi.it)

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During the last nights, we observed the flat spectrum radio quasar (FSRQ) OI 280 (R.A. = 07 50 52.05, Dec. = +12 31 04.83; J2000; Charlot et al. 2020, A&A, 644A, 159) at redshift z = 0.889 (Jin et al. 2023, ApJS, 265(1), 25). The observations were scheduled in response to an alert generated by an independent tool for triggering blazar observations (Pacciani (2018) A&A, 615, 56). This tool is based on the gamma-ray monitoring enabled by publicly available Fermi-LAT data. The same blazar was reported in flaring state by the Fermi/LAT collaboration a few days ago (ATel#17731). We witnessed an exceptional optical flare reaching R-magnitude R=14.19 (0.01) on 2026 Apr 04.790.

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 Apr 03.790 14.25 (0.01) - Siena 
2026 Apr 03.796 14.28 (0.01) - Montarrenti 
2026 Apr 03.800 14.25 (0.01) - Montarrenti 
2026 Apr 03.801 14.27 (0.01) - Siena 
2026 Apr 03.813 14.27 (0.01) - Montarrenti 
2026 Apr 03.813 14.27 (0.01) - Siena 
2026 Apr 03.817 14.27 (0.01) - Montarrenti 
2026 Apr 03.821 14.27 (0.01) - Montarrenti 
2026 Apr 03.824 14.26 (0.01) - Siena 
2026 Apr 03.835 14.25 (0.01) - Siena 
2026 Apr 04.790 14.19 (0.01) - Montarrenti			  
2026 Apr 04.798 14.20 (0.01) - 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) and the 0.53 m telescope at Montarrenti Observatory (Sovicille, Italy - C88).

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 Montarrenti Observatory.

Astronomical Observatory of the University of Siena - Official Webpage