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GCN Circular 36819

Subject

EP240703a: Kinder optical follow-up observations

Date

2024-07-04T10:31:37Z (5 months ago)

From

Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>

Via

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A. Aryan (NCUIA), S. Yang (HNAS), T.-W. Chen, Y.-J. Yang, C.-S. Lin, M.-H. Lee, Y.-C. Pan, H.-Y. Hsiao, W.-J. Hou, C.-C. Ngeow, C.-H. Lai, A. Sankar.K, H.-C. Lin, J.-K. Guo (all NCUIA), S. J. Smartt (Oxford/QUB), H. F. Stevance, S. Srivastav, J. Gillanders, L. Rhodes (all Oxford), M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz and M. Huber (IfA, University of Hawaii) report:

We observed the field of the fast X-ray transient EP240703a (Wang et al., GCN 36807

Not found

) 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 12:48 UT on 03rd of July 2024 (MJD = 60494.540), 12.32 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).

We do not detect the presence of the proposed optical transient source at R.A. = 18:15:03.89 and Dec. = -09:42:02.96, as reported by An et al., GCN 36815

Not found

, in the difference and stacked images. Our observations were 1.58 hours before those of the TRT observations and reach 2 magnitudes deeper.

However, we detect a slight enhancement in the brightness of the eclipsing binary, ZTF J181510.36-094110.1 (Chen et al. 2020, ApJS, 249, 18) in the difference image at R.A. = 18:15:10.36 and Dec. = -09:41:10.1, which is close to the reported coordinate of EP240703a being only 0.66 arcmin away.

Morever, we utilized the python based package, AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform the PSF photometry on our stacked frame. The current magnitudes of the eclipsing binary in our stacked frame are r = 19.02 +/- 0.04 mag and i = 18.18 +/- 0.05 mag.

Besides ZTF J181510.36-094110.1, we do not detect any new optical source in the reference and stacked frames. The details of the observations and measured 3-sigma upper limit are as follow:

Telescope | Filter | MJD (start) | t-t0 | Exposure | Magnitude | avg. Seeing | med. Airmass LOT | r | 60494.540 | 12.32 hrs | 300 sec * 6 | > 22.3 | 1".48 | 1.47 LOT | i | 60494.598 | 13.72 hrs | 300 sec * 6 | > 21.8 | 1".34| 1.22

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) = 1.225 mag in the direction of the burst (Schlafly & Finkbeiner 2011).