TITLE:   GCN CIRCULAR
NUMBER:  37546
SUBJECT: EP240918a: YAHPT Optical Upper Limit
DATE:    24/09/18 16:00:01 GMT
FROM:    Tianrui Sun at Purple Mountain Obs,CAS <trsun@pmo.ac.cn>

Tian-Rui Sun, Jin-Jun Geng, Jian Chen, Yan-Long Hua, Yi-Fang Liang (Purple Mountain Observatory, CAS) and Lei Hu (CMU) report on behalf of the YAHPT team:


Following the detection of EP240918a by Einstein Probe (Zhang et al., GCN 37541), 
we use the Yaoan High Precision Telescope at Yaoan Astronomy Observation Station (Yunnan province, China) to search and follow up.

​
We observed the target position in both Rc and Ic starting from 2024-09-18T13:13:31 with 180s exposure.
We did not detect any possible new optical counterpart with an uncertainty of 20 arcsec in the coadded image.


UT (start)         |5 sigma Upper Limit| Exposure Time | Filter 
2024-09-18T13:13:31| 20.3              | 180*5s        | Rc 
2024-09-18T13:16:34| 19.1              | 180*5s        | Ic 

This non-detection result of the transient is in agreement with the previous circulars (Wang et al., GCN 37544).

We used the USNO B1.0 catalogue (R2mag and Imag) as the magnitude reference for calibration.