Optical outburst of the blazar 3C 345 with an exceptionally high polarization degree

ATel #16305; J. Otero-Santos (IAA-CSIC Granada, Spain), V. Piirola (Univ. of Turku, Finland), J. Escudero (IAA-CSIC Granada, Spain), I. Agudo (IAA-CSIC Granada, Spain), G. Bonnoli (INAF - Brera Astronomical Observatory, Italy), J. A. Acosta-Pulido (IAC, La Laguna, Spain), D. Morcuende (IAA-CSIC Granada, Spain), V. Casanova (IAA-CSIC Granada, Spain)
on 23 Oct 2023; 12:00 UT
Credential Certification: Jorge Otero-Santos (joteros@iaa.es)

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On October 9th 2023 (MJD 60226), the Fermi-LAT Collaboration reported an increased gamma-ray activity from the flat-spectrum radio quasar 3C 345 (ATel #16274) at redshift z=0.5928 (Marziani et al., 1995, ApJS, 110, 880) and coordinates R.A. = 250.745041 deg, Dec. = 39.810276 deg (J2000; Johnston et al. 1995, AJ, 110, 880). We have followed up this high gamma-ray state with optical photo-polarimetric observations in the R band during the last nights with the Calar Alto Observatory (CAHA) 2.2m and Sierra Nevada Observatory (SNO) T150 (1.5m) and T90 (0.9m) Telescopes. Our photometric monitoring confirms the brightening of the source in the R band up magnitudes comparable to previous optical outbursts observed from this blazar (e.g. 2018, see ATel #15244; 2022, see ATel #15341). We report the following R-band magnitudes:

2023 Oct. 09.808 (MJD 60226.808): 16.54 +/- 0.01 (CAHA 2.2m)
2023 Oct. 10.831 - 10.863 (MJD 60227.831 - 60227.863): 16.30 +/- 0.03 to 16.19 +/- 0.03 (SNO T150)
2023 Oct. 11.793 - 11.902 (MJD 60228.793 - 60228.902): 15.61 +/- 0.03 to 15.30 +/- 0.02 (SNO T150)
2023 Oct. 12.787 - 12.917 (MJD 60229.787 - 60229.917): 16.15 +/- 0.03 to 15.96 +/- 0.03 (SNO T150)

In more recent nights, 2023 Oct. 18.182 and 2023 Oct. 20.180 (MJD 60235.182 and MJD 60237.180), the Sloan-r magnitudes measured with the 1-m telescopes from Las Cumbres Observatory went down to values of 16.23 +/- 0.01 and 16.45 +/- 0.01, respectively.
More remarkably, accompanying this enhanced gamma-ray emission, the measured polarization degree (PD) has also increased, as measured by CAFOS in imaging polarimetric mode (at the CAHA 2.2m Telescope), and the DIPOL-1 polarimeter developed at the University of Turku (installed at the SNO T90 Telescope). We have measured the following polarization degree and polarization angle (PA):

2023 Oct. 09.809 (MJD 60226.809): PD = (29.26 +/- 0.27)%, PA = (47.2 +/- 0.3) deg (CAHA 2.2m)
2023 Oct. 09.860 (MJD 60226.860): PD = (28.35 +/- 0.54)%, PA = (47.3 +/- 0.5) deg (SNO T90)
2023 Oct. 10.859 (MJD 60227.859): PD = (28.10 +/- 0.33)%, PA = (42.2 +/- 0.3) deg (SNO T90)
2023 Oct. 11.843 (MJD 60228.843): PD = (29.94 +/- 0.24)%, PA = (38.1 +/- 0.2) deg (SNO T90)
2023 Oct. 12.847 (MJD 60229.847): PD = (32.09 +/- 0.40)%, PA = (44.0 +/- 0.4) deg (SNO T90)

All quoted uncertainties refer to statistical uncertainties. Further analysis is ongoing. More multiwavelength observations of this source are encouraged during the following days for monitoring this high gamma-ray activity and high polarization period.