TITLE: GCN CIRCULAR NUMBER: 34030 SUBJECT: ZTF23aaoohpy/AT2023lcr: OHP/T193 and OHP/T120 optical follow-up observations DATE: 23/06/20 00:15:55 GMT FROM: Damien Turpin at CEA-Saclay T. Adami (ENS/Saclay, LAM/Pytheas/AMU), C. Adami (LAM/Pytheas/AMU), D. Turpin, E. Le Floc'h (CEA Paris-Saclay), J. T. Palmerio, S. D. Vergani (GEPI, Obs. de Paris), B. Schneider (MIT), S. Basa (LAM), D. Götz (CEA Paris-Saclay), report on behalf of a larger collaboration: We observed the field of ZTF23aaoohpy/AT2023lcr (Swain et al., GCN 34022, Gompertz et al., GCN34023, Kumar et al., GCN34025)using the T193cm and T120cm telescopes at Observatoire de Haute-Provence (France) respectively equipped with the MISTRAL spectro-imager and the T120 CCD camera. A total of 8 exposures were obtained in the imaging mode of MISTRAL in the r-band filter (1x360s +7x600s). Observations were made from 2023-06-19 21:13:47.60 UT to 2023-06-19T22:37:40.86 UT (mid time ~1.85d after the T_GOTO = 2023-06-18 01:27:41 (Gompertz et al., GCN34023)). A total of 16 exposures were also obtained with the T120 CCD camera in the R-band filter (1x300s +8x450s, mid time ~ T_GOTO+1.83d) and V-band (7x600s, mid time ~ T_GOTO+1.88d). In the coadded images, we measured the following magnitudes: ---------------------------------------------------------------------- JD (mid) | Telescope | Filter | Exposure (s) | Magnitude (AB) | ---------------------------------------------------------------------- 2460115.3875 | OHP-T120 | R | 3900 | 20.70 +/- 0.12 | 2460115.413706 | OHP-T193/MISTRAL | r' | 4560 | 20.84 +/- 0.04 | 2460115.440972 | OHP-T120 | V | 4200 | 20.85 +/- 0.07 | ---------------------------------------------------------------------- The photometric calibration was performed using nearby stars from the PS1 catalog and the magnitudes are not corrected for Galactic extinction. Our measurements confirm that the source is fading at a mag rate ~ 0.9 mag/day compared to the last r magnitude reported by ZTF (Swain et al., GCN 34022). The red color of this transient g-r ~ 0.3 and its confirmed fast-fading evolution are consistent with a Gamma-ray Burst afterglow emission. We strongly encourage spectroscopic follow-up of this source in order to identify its astrophysical origin. Additional I-band follow-up observation will be performed at OHP this night. We acknowledge the excellent support from Observatoire de Haute-Provence, in particular Yoann Degot Longhi for the MISTRAL observations, Gavin Coleman the observer using the T193/SOPHIE instrument who allowed us to perform these ToO observations as well as Pascal Gallais, Antoine Claret and Matthias Tornay for the T120 observations.