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GCN Circular 36022

LXT 240402A: EP-FXT detection of the X-ray afterglow
2024-04-04T09:00:26Z (3 months ago)
EP Team at NAOC/CAS <>
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S. M. Jia (IHEP, CAS), H. Sun (NAOC, CAS), C. K. Li, Y. Chen, W. W. Cui, H. Feng, J. Guan, D. W. Han, W. Li, C. Z. Liu, F. J. Lu, L. M. Song, J. Wang, J. J. Xu, J. Zhang, S. N. Zhang, H. S. Zhao, X. F. Zhao, (IHEP, CAS), E. Kuulkers, A. Santovincenzo (ESA), P. O'Brien (Univ. of Leicester), K. Nandra, A. Rau (MPE), B. Cordier (CEA), X. F. Wu (PMO, CAS), X. P. Xu, Z. X. Ling , Y. Liu , C. Zhang , C. C. Jin , J. W. Hu , D. Y. Li , H. Y. Liu , H. W. Pan , W. Yuan (NAOC, CAS), on behalf of the LEIA and Einstein Probe team

Following the detection of the fast X-ray transient LXT 240402A (Xu et al., GCN 36016) and the associated GRB 240402B (Wang et al., GCN 36017), we performed observations of LXT 240402A with the Follow-up X-ray Telescope (FXT) on board Einstein Probe (EP). The observations began at 2024-04-03T19:45:00, about 35 hours after the LEIA detection. The exposure time was 12 ks. An X-ray source was detected at R.A. = 245.451 deg, DEC = 25.763 deg, with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), 2.4 arcmin away from the LEIA source. 

There is no cataloged X-ray source within the error circle, consistent with this source being the afterglow of LXT240402A. The spectrum can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 4.3e20 cm-2 and a photon index of 1.7(+/-0.1) (90% C.L.). The derived flux in 0.5-10 keV is 7.8(+/-0.8)e-13 erg/s/cm2 (90% C.L.). Further monitoring of this source with FXT is planned for the next few days. 
Please note that EP-FXT is currently undergoing in-flight calibration. The derived source parameters may be subject to larger uncertainties, so please use them with caution. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.
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