TITLE:   GCN CIRCULAR
NUMBER:  40165
SUBJECT: EP250416a: refined EP-WXT and EP-FXT analysis
DATE:    25/04/18 01:30:04 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

H. Zhou (PMO, CAS), G.Y. Zhao (SYSU), C. Zhou (HUST), X.L. Chen, K. Chatterjee (YNU) and C.C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:
 
We reduced and analyzed the telemetry data of EP250416a, and all uncertainties reported here are at the 90% confident level. EP250416a became detectable by WXT from ~30 seconds before the trigger time (2025-04-16T17:53:59 UTC, Zhao et al. GCN 40154), and the WXT light curve lasts for ~30 seconds (interupted by the autonomous follow-up observation) with a peak occuring at ~20 seconds after the time when EP250416a became detectable. Lipunov et al. (GCN 40156) and Li et al. (GCN 40157) performed follow-up observations for EP250416a but did not find new optical transient down to i ~20 mag. Later, a possible candidate was detected by Gemini-South (Levan et al. GCN 40160).

The WXT spectrum is fitted by an absorbed powerlaw model, which counts for absorption of the Milky Way and the host galaxy. As suggested by Levan et al. (GCN 40160), the redshift of the host is set to 0.72. The equivalent hydrogen column density of the host, NH, is constrained to be less than ~1.9x10^22 cm^-2 by the WXT spectrum. The photon index of the WXT spectrum is hard, 0.32 (-0.78, +1.00). The mean and peak unabsorbed fluxes (0.5-4 keV) are (5.66+/-1.77)x10^-9 erg/s/cm^2 and (1.92+/-0.76)x10^-8 erg/s/cm^2.

The autonomous EP-FXT follow-up observation starts about 2 minutes after the trigger and lasts for 2 orbits with total on-source time of 3936 seconds. In addition, about 12.3 hours after the trigger, another follow-up observation with the EP-FXT was performed for 2962 seconds.

Results of the autonomous and the follow-up observations are summarized:
+--------------------------------------------------------------------------------+
    T_Start^a [UTC]    | T_mid - T0^b [h] | Exp [s] | Flux (0.5-10 keV) [erg/s/cm^2] 
+--------------------------------------------------------------------------------+
  2025-04-16T17:54:53  |       0.17       |   929   |     (2.56+/-0.17) x 10^-11
  2025-04-16T18:55:35  |       1.49       |  3007   |     (7.86+/-0.48) x 10^-12
  2025-04-17T06:08:14  |      12.68       |  2962   |     (1.64+/-0.20) x 10^-12
+--------------------------------------------------------------------------------+
All fluxes are unabsorbed values derived from FXT-A & FXT-B data.
a. The first and the second rows represent the first and the second orbits of the autonomous observation.
b. Here T0 is the time that EP250416a became detectable, about 30 seconds before the trigger time.

The inferred 0.5-10 keV flux at the XRT epoch is consistent the value derived from the XRT observation (Sbarrato et al. GCN 40163). The model, which is same as the model applied to WXT data, is applied to fit FXT spectra. For the three epochs listed in the above table, the best fitted photon indices are 1.88+/-0.14, 1.98+/-0.15 and 2.23+/-0.38, and the NH of the host are (5.06+/-1.74)x10^21 cm^-2, (5.33+/-1.77)x10^21 cm^-2 and (6.66+/-4.54)x10^21 cm^-2.

The contact TA of EP250416a is Guo-Ying Zhao. Please contact her via email zhaogy28@mail2.sysu.edu.cn if needed.

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).