GCN Circular 34043

T.-W. Chen (TUM), S. Yang (HNAS), A.-L. Tsai, W.-P. Chen, W.-J. Hou, H.-Y. Hsiao, C.-C. Ngeow, C.-S. Lin, Y.-C. Pan, H.-C. Lin, and J.-K. Guo (all NCUIA) report:

We observed the field of AT 2023lcr
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, an orphan transient discovered by the ATLAS survey as ATLAS23msn (Tonry et al. TNS Astronomical Transient Report No. 180429; Fulton et al. AstroNote 2023-179). Its fast fading and red nature was identifed and reported by Swain et al. (GCN 34022
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) as ZTF23aaoohpy.

We used the 40cm SLT and 1-m LOT at Lulin Observatory, Taiwan to obtain r and g-band images of AT 2023lcr
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, as part of the Kinder collaboration (Chen et al., AstroNote 2021-92).

The first SLT epoch of observations started at 18:22 UT on 19 of June 2023 (MJD = 60114.765), 1.36 days after the ATLAS discovery. The images were combined from 18 frames with a 300-second exposure time for the r band, taken under seeing conditions of an average of 1.64" and at a median airmass of 2.08. For the LOT observations, the first epoch started later at 19:29 UT on 19 of June 2023 (MJD = 60114.812). The images were combined from 8 frames with a 300-second exposure time for the g band, taken under seeing conditions of an average of 1.50" and at a median airmass of 2.95. We used the Kinder pipeline (Yang et al. A&A 646, A22) to measure the PSF photometry of AT 2023lcr
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 after template subtraction using the SDSS images. We obtained the following preliminary magnitudes (in the AB system):

g = 21.41 +/- 0.16 mag, and
r = 20.94 +/- 0.14 mag.

The given limit is derived based on calibrating against SDSS field stars and is not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.04 mag in the direction of the burst (Schlafly & Finkbeiner 2011).

We collected the light curve evolution of AT 2023lcr
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 from all relevant photometric reports (Swain et al., GCN 34022
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; Gompertz et al. GCN 34023
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; Kumar et al., GCN 34025
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; Fulton et al., AstroNote 2023-179; Adami et al. GCN 34030
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; Perley et al. GCN 34031
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; Jiang et al., GCN 34040
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; Fulton et al., GCN 34042
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) to date, and the light curve plot can be found in the following link (https://photos.app.goo.gl/QpWzJse6NMozERjZ7). The magnitudes obtained from Lulin Observatory are in agreement with the mentioned reports and further support the evidence for a power-law decay in GRB afterglows. Moreover, we have noticed a potential 'plateau' in the g-band light curve, which occurs between the Lulin and PanSTARRS observations.