TITLE: GCN CIRCULAR NUMBER: 38624 SUBJECT: EP241217a: refined EP-WXT analysis and EP-FXT follow-up observations DATE: 24/12/18 16:30:33 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn
H. Zhou (PMO, CAS), S.-F. Zhu (USTC), M. H. Zhang (NAO, CAS), H. Sun (NAO, CAS), C. C. Jin (NAO, CAS) report on behalf of the Einstein Probe team:
Since 2024-12-17T05:34:10 (UTC, about 113 seconds before the trigger time), EP241217a (Zhou et al., GCN 38586) was detectable for with the Wide-field X-ray Telescope (WXT) at low significance and peaked at about 52 seconds before the trigger. The mean and peak WXT count rate of the EP241217a before the slewing of EP is about 0.3 cnt/s and 1 cnt/s, respectively. The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 1.93 (-0.59, +0.71) (with a fixed Milky Way equivalent hydrogen column density of 1.88 x 10^21 cm^-2). The derived average unabsorbed 0.5-4 keV flux is (7.3+/-2.7) x 10^-10 erg/s/cm^2.
In addition to the autonomous follow-up observation, we performed two follow-up observations of EP241217a with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission. The first observation began at 2024-12-17T13:21:55 (UTC, 7.64 h after the trigger of EP241217a), and the exposure time is about 3ks. The second observation began at 2024-12-18T09:57:58 (UTC, 28.37 h after the trigger), but the telemetry data has not been received yet.
Preliminary results of the autonomous (Zhou et al., GCN 38586) and the first follow-up observations are summarized: +--------------------------------------------------------------------------------+ T_Start [UTC] | T_mid - T0 [h] | Exp [s] | Flux (0.5-10 keV) [erg/s/cm^2] +--------------------------------------------------------------------------------+ 2024-12-17T05:38:53 | 1.02 | 3132 | (6.23+/-0.46) x 10^-12 * 2024-12-17T13:21:55 | 8.18 | 2966 | (1.35+/-0.22) x 10^-12 +--------------------------------------------------------------------------------+ All fluxes are observed values derived with FXT-A data. * The autonomous follow-up observation lasts for about 6ks with effective exposure time of 3.1ks. The X-ray flux decays quickly in the autonommous follow-up observation, for the first 911s exposure segment, the mid time is about 10.4 min after the trigger and the 0.5-10keV flux is about 2 x 10^-11 erg/s/cm^2.
The derived temporal decay index is about 0.73+/-0.35, and the estimated flux at the Swift-XRT epoch is (1.52+/-0.27) x 10^-12 erg/s/cm^2, which is consistent with the value reported by Swift-XRT (Williams et al., GCN 38596). Considering the shallow decay observed in the optical band (Izzo et al., GCN 38588; Levan et al., GCN 38587; Fan et al., GCN 38592; Jin et al., GCN 38607; Mohan et al., GCN 38612; Zhu et al., GCN 38613; Bochenek et al., GCN 38615), more follow-up observations are encouraged to monitor the X-ray transient EP241217a with a moderate redshift of 4.59 (Levan et al., GCN 38593).
The time-averaged FXT spectra of the autonomous and the first follow-up observations are fitted by the absorbed powerlaw model with the fixed Milky Way equivalent hydrogen column density nH of 1.88 x 10^21 cm^-2. The best-fitted model shows that the intrinsic absorption is negligible, and the photon index is 1.76+/-0.11 (1.96+/-0.17) for the autonomous (the first) follow-up observation. Our fitting results are consistent with the Swift-XRT results (Williams et al., GCN 38596).
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.
View this GCN Circular online at https://gcn.nasa.gov/circulars/38624. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...