GCN Circular 35725

D. Turpin (CEA Paris-Saclay), C. Adami (LAM), A. de Ugarte Postigo (CNRS/OCA & LAM), E. Le Floc'h, D. Götz, F. Schüssler (CEA Paris-Saclay), B. Schneider (MIT), S. Basa (Pytheas/OHP/LAM), J. T. Palmerio, A. Saccardi, S. D. Vergani (GEPI, Obs. de Paris), report on behalf of a larger collaboration:

We observed the optical afterglow candidate of GRB 240209A (Fermi GBM team, GCN 35343
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) reported by the GOTO team (Belkin et al. GCN 35715
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) and also followed-up by the NOT and Wendelstein telescopes (de Ugarte Postigo et al. GCN 35716
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, Busmann et al. GCN 25724
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) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL instrument. We took two series of r-band and g-band images (3x10min for each band) starting at 2024-02-14 04:00:34 UT. In both single and combined frames we detect the optical source at a high S/N. We performed forced photometry at the position of the source both on the science and difference image (we used PanSTARRS template images to make the subtraction) as the close HyperLEDA 2722973 galaxy might affect our measure. We derived the following magnitudes :
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T-T0 (in days, midtime) | mag | filter | subtracted
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4.893 | 20.06 ± 0.03 mag (AB) | r' | y
4.893 | 19.41 ± 0.02 mag (AB) | r' | n
4.919 | 20.02 ± 0.01 mag (AB) | g' | y
4.919 | 19.78 ± 0.01 mag (AB) | g' | n
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The photometric calibration was performed using nearby stars from the PanSTARRS catalog and magnitudes are not corrected for Galactic extinction and the uncertainties are statistical only.
We note that our residual image magnitudes suggest a fading behavior of the source with respect to the NOT and the Wendelstein r-band epochs while the forced photometry on science images is in very good agreement with them. Further investigations about the potential flux contamination due to the presence of the HyperLEDA 2722973 galaxy will be done.

In addition to our photometric observations, we also performed spectroscopic observations of the source and the HyperLEDA 2722973 galaxy with the blue mode of MISTRAL. A total exposure of 20 minutes was obtained.
The preliminary analysis of the spectrum of the source suggests a rather blue transient (in agreement with the g-r ~ -0.04 we measured  from difference image analysis) but at odds with typical GRB afterglow red colors. Due to the low S/N of the spectrum, we did not identify clear spectral features to estimate a redshift.
The MISTRAL spectrum of the HyperLEDA 2722973 galaxy shows a clear absorption complex that could be identified with the 5175A/5269A lines. This would suggest a redshift of z~0.207, close to the photometric value of z=0.1629+/-0.0150.

Further observation will be taken on Saturday night at OHP (weather permitting) to better characterize the color evolution of this transient.

We acknowledge the excellent support from Observatoire de Haute-Provence, in particular Stéphane Favard for the MISTRAL observations and the SOPHIE observer Magali Deleuil.