TITLE: GCN CIRCULAR NUMBER: 36723 SUBJECT: EP240618a: Swift/XRT upper limit DATE: 24/06/21 10:21:36 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn
W. Chen, H. Sun (NAOC, CAS), H. Zhou (PMO, CAS), W. D. Zhang, D. Y. Li, Z. X. Ling, Y. Liu, C. Zhang, C. C. Jin, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, H. Y. Liu, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, W. X. Wang, Y. L. Wang, Q. Y. Wu, X. P. Xu, Y. F. Xu, H. N. Yang, W. Yuan, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), Y. Chen, S. M. Jia, S. N. Zhang (IHEP, CAS), E. Kuulkers, A. Santovincenzo (ESA), P. O'Brien (Univ. of Leicester), K. Nandra, A. Rau (MPE), B. Cordier (CEA) on behalf of the Einstein Probe team
Following the detection of the fast X-ray transient EP240618a by the Einstein Probe (Sun et al., GCN 36690), we triggered a Swift target of opportunity observation (ObsID: 00016670001). The XRT onboard Swift began the observation at 2024-06-20T15:56:06 UTC, about 58 hours after the WXT detection, with an exposure time of 2.0 ks in the Photon Counting mode.
No significant X-ray source was detected within the 30 arcsec region around the reported FXT position. Assuming an absorbed power-law model with a Galactic column density of 1.98 x 10^21 cm^-2 and a photon index of 2, the estimated flux upper limit in 0.3-10 keV at the 90% confidence level is 1.2 x 10^-13 ergs/cm^2/s.
We thank the Neil Gehrels Swift Observatory team for making the X-ray observation possible.
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