Optical follow-up of the gamma-ray flare of the blazar OP 313 reveals a bright state with high polarization degree

ATel #16360; J. Otero-Santos (IAA - CSIC Granada, Spain), V. Piirola (Univ. of Turku, Finland), L. Pacciani (INAF - IAPS Roma, Italy), J. Escudero (IAA - CSIC Granada, Spain), I. Agudo (IAA - CSIC Granada, Spain), G. Bonnoli (INAF - Brera Astronomical Observatory, Italy), D. Morcuende (IAA - CSIC Granada, Spain), V. Casanova (IAA - CSIC Granada, Spain), F. J. Aceituno (IAA - CSIC Granada, Spain), A. Sota (IAA - CSIC Granada, Spain), A. Marchini (Astronomical Observatory, University of Siena - Italy), L. Stiaccini, (Department of Physical Sciences, Earth and Environment (DSFTA), University of Siena - Italy), S. Leonini (Montarrenti Observatory, Sovicille (Siena), Italy), M. Conti (Montarrenti Observatory, Sovicille (Siena), Italy), P. Rosi (Montarrenti Observatory, Sovicille (Siena), Italy), L. M. Tinjaca Ramirez (Montarrenti Observatory, Sovicille (Siena), Italy)
on 4 Dec 2023; 18:53 UT
Credential Certification: Jorge Otero-Santos (joteros@iaa.es)

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The Flat Spectrum Radio Quasar (FSRQ) OP 313 (also known as B2 1308+32), with coordinates R.A. = 197.619 deg, Decl. = +32.345 deg (J2000; Johnston et al. 1995, AJ, 110, 880), and redshift z=0.997 (Schneider et al. 2010, AJ, 139, 2360), has experienced a significant gamma-ray enhanced activity as reported by the Fermi-LAT Collaboration on December 1st, 2023 (see ATel #16356). A flux increase of a factor 40 with respect to the average flux of the source, and a hardening of the spectral index were reported. Reacting to an alert generated by an independent tool for triggering blazar observations based on the gamma-ray monitoring allowed by Fermi-LAT publicly available data (Pacciani L., (2018) A&A, 615, 56) we had already started since November 26th R-band photometric observations with the T150 1.5m telescope at the Sierra Nevada Observatory (OSN, Granada, Spain), the 0.30m telescope of the Astronomical Observatory of the University of Siena (IAU code K54) and the 0.53m telescope of the Montarrenti Observatory (IAU code C88), both located in Siena (Italy). This follow-up confirms the brightening of the source also in the optical band, with a ~1.6 optical magnitude increase since our last monitoring measurement on 2023 November 7.210, reaching values comparable to other outbursts observed for this blazar (e.g. in May-July 2022, see ATels #15399,#15441,#15447,#15459,#15474,#15476,#15480,#15496,#15515). The results of the R-band photometry are shown in the table below:

Civil Date [UT] - Mag (dMag) - Instrument
2023 Nov. 07.210 15.84 (0.02) SNO T150
2023 Nov. 27.189 14.20 (0.04) SNO T150
2023 Nov. 29.175 14.60 (0.02) Siena
2023 Nov. 29.189 14.53 (0.06) SNO T150
2023 Dec. 04.187 14.83 (0.03) Montarrenti
2023 Dec. 04.194 14.85 (0.03) Siena


We take also the opportunity to report that in occasion of the bright flare in July 2022, a follow-up observation performed with the 0.9m telescope T90 at SNO, not analyzed at that time, allowed to measure an optical magnitude R=12.54 (0.05) on 2022 July 1.981, consistent with the almost simultaneous measurement at R=12.61 (0.02) obtained on 2022 July 1.985 at Rozhen NAO (Bulgaria) and reported in ATel #15496, and also with the R=12.59 (0.01) measured on 2022 July 4.800 from SAO RAS (ATel #15515). The current R-band optical state is therefore exceptional for sure, but still 2 magnitudes dimmer than that extraordinary event.

In addition, polarimetric follow-up has also been carried on by means of the DIPOL-1 polarimeter, developed at the University of Turku (Finland) and installed at SNO T90. We observe that, accompanying the bright optical and gamma-ray emission, the optical polarization degree (PD) has also increased up to PD~22%. The resulting polarization properties are reported below:

Civil Date(UT) - PD [%] (dPD [%]) - PA [deg] (dPA [deg]) - Instrument
2023 Nov. 27.226 21.78 (0.56) -31.69 (0.65) SNO T90
2023 Nov. 29.207 14.25 (0.64) -35.53 (1.03) SNO T90


Any inquiry on these observations can be addressed either to Jorge Otero Santos (joteros@iaa.es) or to Giacomo Bonnoli (giacomo.bonnoli@inaf.it). All quoted uncertainties refer to statistical uncertainties. Further analysis of the data is ongoing. We will continue monitoring the source in the following nights. More multi-wavelength observations of this source are encouraged during this high gamma-ray activity and high polarization period.