TITLE: GCN CIRCULAR NUMBER: 37577 SUBJECT: EP240918b and EP240918c: Optical upper limits with Kinder observations DATE: 24/09/20 14:53:35 GMT FROM: Janet Chen at National Central University janetstars@gmail.com
C.-H. Lai, A. Aryan (both NCU), A. K. H. Kong (NTHU), Y. J. Yang, T.-W. Chen, H.-Y. Hsiao, C.-S. Lin (all NCU), J. Gillanders (Oxford), S. Yang (HNAS), S. J. Smartt (Oxford/QUB), A. Sankar. K, W.-J. Hou, C.-C. Ngeow, M.-H. Lee, Y.-C. Pan, H.-C. Lin, J.-K. Guo (all NCU), 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 two fast X-ray transients (FXTs), namely EP240918b and EP240918c reported by Liang et al. (GCN 37555), using the 40cm SLT and 1m LOT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2024arXiv240609270C). The first SLT epoch of observations for EP240918b started at 13:43 UTC on the 19th of September 2024 (MJD = 60572.571), 22.02 hr after the EP-WXT detection. The first LOT epoch of observations for EP240918c started at 13:42 UTC on the 19th of September 2024 (MJD = 60572.571), 19.59 hr after the EP-WXT detection.
We utilized 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 for each FXT. We did not detect any evidence of a new uncataloged source within the EP-WXT error circles reported in Liang et al. (GCN 37555) for both FXTs.
Moreover, we utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform the photometry on our stacked frames. The details of the observations and measured 3-sigma upper limit (in the AB system) for each FXT are as follows:
EP240918b:
Telescope | Filter | MJD (start) | t-t0 | Exposure | Magnitude | avg. Seeing | med. Airmass SLT | r | 60572.571 | 22.02 hr | 7 * 300 s | >19.0 | 2".79 | 2.08
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EP240918c:
Telescope | Filter | MJD (start) | t-t0 | Exposure | Magnitude | avg. Seeing | med. Airmass LOT | r | 60572.571 | 19.59 hr | 5 * 300 s | >18.5 | 2".05 | 2.03
The presented magnitudes for each FXT were calibrated using field stars from the Pan-STARRS1 catalog, and were not corrected for the expected Galactic foreground extinction of E(B-V) = 0.037 and 0.86 mag (Schlafly & Finkbeiner 2011), respectively, in the directions of EP240918b and EP240918c. The sky conditions were unstable during the EP240918c observations, resulting in a shallower limit.
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