TITLE:   GCN CIRCULAR
NUMBER:  37178
SUBJECT: EP240802a: Kinder optical, SWIFT-UVOT, and SWIFT-XRT upper limits 6 days after the WXT detection
DATE:    24/08/14 12:51:11 GMT
FROM:    Janet Chen at National Central University <janetstars@gmail.com>

Y. J. Yang, A. Aryan, T.-W. Chen (all NCU), A. K. H. Kong (NTHU), S. Yang (HNAS), C.-C. Ngeow, C.-S. Lin, H.-Y. Hsiao, W.-J. Hou, C.-H. Lai, 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), G. H. Sun, Z. N. Wang (both HNAS), J. Gillanders, 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 continued to observe the field of the fast X-ray transient EP240802a (Wang et al., GCN 37019, Aryan et al., GCN 37021, Leonini et al., GCN 37029, Wang et al., 37033, Zheng et al. 37094) using the 1m LOT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2024arXiv240609270C). EP240802a showed a time coincidence with GRB 240802A (Waratkar et al., GCN 37018, Wang et al., GCN 37022, Frederiks et al., GCN 37079), which was first proposed by Aryan et al., (GCN 35021), and later confirmed by Svinkin et al., (GCN 37026) and Kozyrev et al., (GCN 37078). We triggered LOT again at 15:35 UTC on the 8th of August 2024 (MJD = 60530.650), 6.21 days after the EP trigger, with the hope of a supernova (SN) component emergence.

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. In the stacked frames, we did not find any evidence of a new and uncatalogued source within the 10" error circle of the Follow-up X-ray Telescope (FXT) onboard the Einstein Probe (EP) mission;  hence, no emergence of an SN component 6.21 days post the Wide-field X-ray Telescope (WXT) trigger onboard EP.

Moreover, we utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform the PSF photometry on our stacked frame. The details of the observations and the upper limit (in the AB system) are as follows:

Telescope | Filter | MJD (start) | t-t0 | Exposure | Magnitude | avg. Seeing | med. Airmass
LOT | r | 60530.650 | 6.21 days | 300 sec * 6 | >22.6 | 1".16 | 1.27

We also requested a Swift ToO observation, which was conducted on 2024-08-07 at 06:05:42 UTC, with a total exposure time of approximately 3536 seconds. No significant X-ray source was detected by XRT within the 10” error circle of EP FXT. Using the method described by Kraft et al. (1991), we estimated a 90% confidence level (CL) upper limit for the count rate at 0.00126 c/s. Assuming a photon index of 2 and NH =2.67×10^21 cm^-2 along the line of sight to the source position, WebPIMMS predicts an unabsorbed flux of 6.24×10^-14 ergs cm^-2 s^-1. No optical counterpart consistent with the EP FXT position was detected in the UVOT observation, with a total exposure of 1427 seconds using the w2 filter. An upper limit magnitude of >21.79 (AB system) was obtained.

The presented magnitudes are calibrated using the field stars from the Pan-STARRS1 catalogue and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.56 mag in the direction of the transient (Schlafly & Finkbeiner 2011).

We thank the Swift team for scheduling the ToO observation.