GCN Circular 34030

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
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 (Swain et al., GCN 34022
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, 
Gompertz et al., GCN34023
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, Kumar et al., GCN34025
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)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
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)). 
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
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).
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.