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GCN Circular 36057

EP240408a: Swift follow-up observation
2024-04-10T08:13:41Z (a month ago)
EP Team at NAOC/CAS <>
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J. W. Hu, D. H. Zhao, Y. Liu, Z. X. Ling, C. Zhang, C. C. Jin, H. Q. Cheng, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, M. H. Huang, D. Y. Li, T. Y. Lian, H. Y. Liu, M. J. Liu, Z. Z. Lv, X. Mao, H. W. Pan, X. Pan, H. Sun, 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 (NAOC, 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 up on the new X-ray flare EP240408a detected by Einstein Probe (Hu et al., GCN 36053), we performed a target of opportunity observation with Swift. The Swift XRT observation began at 2024-04-10T02:57:40 (UTC) with an exposure time of 966 seconds in the Photon Counting mode, about 33 hours after the burst detected by EP-WXT. An X-ray source was detected at R.A. = 158.851 deg, DEC = -35.747 deg, with an uncertainty of 3.68 arcsec (radius, 90% C.L. statistical and systematic), 32 arcsec away from the position of EP240408a. The spectrum can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 6.23 x 10^20 cm^-2 and a photon index of 1.9(-0.2, +0.2). The derived average unabsorbed 0.5-4.0 keV flux is 1.2(-0.1,+0.3) x 10^-11 erg/s/cm2 (90% C.L.). There is no catalogued X-ray source within the error circle, suggesting that this XRT source is most likely associated with EP240408a. No significant counterpart was detected in the UVOT image within the error circle of the XRT position. 

Further follow-up observations are encouraged to identify the nature of this X-ray transient. 

We thank the Neil Gehrels Swift Observatory team for making the ToO observation possible. 
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