GCN Circular 36823

Y. J. Yang, A. Aryan (both NCU), S. Yang (HNAS), T.-W. Chen (NCU), H. F. Stevance (Oxford), S. J. Smartt (Oxford/QUB), W.-J. Hou, C.-S. Lin, M.-H. Lee, Y.-C. Pan, H.-Y. Hsiao, C.-C. Ngeow, C.-H. Lai, A. Sankar.K, H.-C. Lin, J.-K. Guo (all NCUIA), 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 (both IfA, University of Hawaii) report: 

We observed the field of the fast X-ray transient EP240703c (Zhang et al., GCN 36818
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) using the 40cm SLT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2024arXiv240609270C). The first SLT epoch of observations started at 14:51 UT on 4th of July 2024 (MJD = 60495.619), 20.6 hrs after the EP trigger and we observed for a total of 4500 seconds on source.  We used the Kinder pipeline (Yang et al. A&A 646, A22) to stack the images and subtract the stacked images with the SkyMapper template images. 

We do not detect any optical counterpart in the errorbox of 3 arcminute around the position of the EP source specified by Zhang et al., (GCN 36818
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). The details of the observations and the corresponding 2.5 sigma upper limit (in the AB system) in our combined image are as follows:

Telescope | Filter | MJD (start) | t-t0 | Exposure | Magnitude | avg. Seeing | med. Airmass
SLT | r | 60495.619 | 20.6 hrs | 300 sec * 15 | > 21.41 | 2".3 | 1.74

The presented magnitudes are calibrated using the field stars from the SkyMapper catalog and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.09 mag in the direction of the burst (Schlafly & Finkbeiner 2011).


The EP team triggered the Swift ToO observations (PI: EP) for EP240703c (Zhang et al., GCN 36818
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). They noticed that there is a previous Swift X-ray observation of a high proper motion star LP 924-17 (Evans+, 2020). The new Swift observation was carried out from 2024-07-04T11:41:37 to 2024-07-04T12:00:53. We obtained some initial results from the quick-look data using the Swift-XRT products provided by the UK Swift Science Data Centre. Applying a simple power-law model with absorption, we derived a column density NH ≈ 5.0 × 10^21 cm^-2, a photon index of approximately 4.2, which indicates a relatively soft spectrum, and an unabsorbed flux in the 0.3-10 keV band of ~ 2 × 10^-11 erg cm^-2 s^-1. 

For the UVOT data, the source is visible in the images across all bands. It is clearly detected in the V band with a magnitude of 17.76 ± 0.20 (stat) ± 0.01 (sys) in the AB system. However, the source cannot be well constrained in the B, U, UVM2, UVW1, and UVW2 bands. Further, we report the PSF photometry measurement on this high proper motion star LP 924-17, which has no significant brightness change during our SLT observations, with an average magnitude of r = 16.88 +/- 0.05 mag.