GCN Circular 36839
Subject
EP240708a: Optical upper limits with Kinder observations
Date
2024-07-09T20:13:05Z (3 months ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
Web form
C.-H. Lai, A. Aryan, T.-W. Chen, Y. J. Yang (all NCU), Y.-L. Wu (NTNU), C.-S. Lin (NCU), S. Yang (HNAS), W.-J. Hou, H.-Y. Hsiao, C.-C. Ngeow, A. Sankar.K, M.-H. Lee, Y.-C. Pan, H.-C. Lin, J.-K. Guo (all NCU), S. J. Smartt (Oxford/QUB), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, J. Gillanders (Oxford), A. K. H. Kong (NTHU), M. Fulton, T. Moore, C. Angus and A. Aamer (all QUB) report:
We observed the field of the fast X-ray transient EP240708a (Chen et al., GCN 36838) using the One-meter Telescope (LOT) at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2024arXiv240609270C). The first LOT epoch of observations started at 16:38 UT on 09th of July 2024 (MJD = 60500.693), 17.16 hrs after the EP trigger.
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. We subtracted the stacked images with the Pan-STARRS1 template image using hotpants (Becker A., 2015, ascl.soft. ascl:1504.004). The subtracted image did not provide any evidence of a new source. Moreover, we utilized the python based package, AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform the PSF photometry on our stacked frame.
We did not detect any optical counterpart in the errorbox of 3 arcminute around the reported position of the EP240708a in the stacked and subtracted frames. The details of the observations and measured 3-sigma upper limit (in the AB system) are as follow:
Telescope | Filter | MJD (start) | t-t0 | Exposure | Magnitude | avg. Seeing | med. Airmass
LOT | r | 60500.693 | 17.16 hrs | 300 sec * 6 | > 22.6 | 1".86 | 2.16
LOT | g | 60500.719 | 17.78 hrs | 300 sec * 6 | > 22.4 | 1".67| 1.82
The presented magnitudes are calibrated using the field stars from the Pan-STARRS1 catalog and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.03 mag in the direction of the transient (Schlafly & Finkbeiner 2011).