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GCN Circular 35938

X-ray transient EP240315a: Kinder follow-up observations for AT 2024eju with Lulin observatory
2024-03-17T08:26:16Z (4 months ago)
Janet Chen at National Central University <>
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T.-W. Chen (NCUIA), S. Yang (HNAS), M.-H. Lee, H.-Y. Hsiao, C.-H. Lai, A. Sankar.K, C.-C. Ngeow, Y.-C. Pan, C.-S. Lin, W.-J. Hou, H.-C. Lin, J.-K. Guo (all NCUIA), S. Srivastav, S. Smartt (both Oxford) and M. Fulton, T. Moore, C. Angus, A. Aamer (all QUB) report:

We observed the field of AT 2024eju (Srivastav et al., GCN 35932), a possible optical counterpart for the EP240315a (Zhang et al., GCN 35931) found by ATLAS, using the 40cm SLT at Lulin Observatory, Taiwan as part of the Kinder collaboration (Chen et al., AstroNote 2021-92).

The first SLT epoch of observations started at 16:09 UT on 16 of March 2024 (MJD = 60385.673), 0.83 days after the discovery of the X-ray transient. The r-band images were combined from 6 frames, each with a 300-second exposure time, taken under seeing conditions averaging 2.0" and at a median airmass of 1.61. 

We used the Kinder pipeline (Yang et al. A&A 646, A22) to measure the PSF photometry on the source location, after subtracted with the Pan-STARRS1 template image. We obtained the following preliminary upper limit (in the AB system):

r > 21.5 mag (SNR = 2.2 sigma). 

The given limit is derived based on calibrating against Pan-STARRS1 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).

Our r-band limit is deeper than the LT follow-up observations on MJD 60385.848 (Srivastav et al., GCN 35933) and the Nanshan/HMT limit on MJD 60385.69 (Jiang et al., GCN 35935). By direct comparison with the ATLAS o band detection of AT 2024eju, we deduce that the source experienced a rapid fade of more than 1.86 magnitudes within a span of 0.78 days. 

Despite the SNR of the measurement being only 2.2 sigma, which falls below our threshold, we were still able to see a slightly faint source in the subtracted image by eye at the position of AT 2024eju. We thus chose to measure the faint source using a lower threshold and obtained an r-band magnitude of 21.45 +/- 0.22.
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