TITLE: GCN CIRCULAR NUMBER: 38520 SUBJECT: EP241208a: Optical upper limit with Kinder observations DATE: 24/12/10 10:31:06 GMT FROM: Janet Chen at National Central University janetstars@gmail.com
Z. N. Wang, S. Yang (both HNAS), A. Aryan, T.-W. Chen (both NCU), A. K. H. Kong (NTHU), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), A. Sankar.K, Y.-H. Lee, H.-Y. Miao, Y. J. Yang, M.-H. Lee, W.-J. Hou, C.-C. Ngeow, Y.-C. Pan, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, C.-S. Lin, J.-K. Guo (all NCU), L. L. Fan, G. H. Sun (both HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz and M. Huber (both IfA, Hawaii) report:
We observed the field of the fast X-ray transient EP241208a (Wang et al., GCN 38477, GCN 38513) using the 40cm SLT at the Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2024 arXiv:2406.09270). The first SLT epoch of observations started at 16:13 UTC on 9th November 2024 (MJD 60653.676), 23.62 hrs after the EP-WXT trigger.
We utilize the astroalign (Beroiz et al., 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al., 2022, ApJ, 935, 167) packages to align and stack the individual frames. The plausible host galaxy J083120.19+490500.1, as quoted in Wang et al. (GCN 38513), is clearly seen in our images. Moreover, we used the Kinder pipeline (Yang et al. A&A 646, A22) to subtract the stacked image from the SDSS DR18 (Almeida et al., 2023) template image. We do not find any new source in the difference images within the 10 arcseconds error circle of EP-FXT.
Finally, we utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform aperture photometry on our stacked frames. The details of the observations and measured 3-sigma upper limit (in the AB system) are as follows:
--------------------------------------------------------------------------------------------------- Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass SLT | r | 60653.676 | 23.62| 300 * 24 | > 20.9 | 1".88 | 1.21
The presented magnitude was calibrated using the field stars from the Pan-STARRS1 catalog and was not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_r = 0.09 mag in the direction of the transient (Schlafly & Finkbeiner 2011).
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