TITLE:   GCN CIRCULAR
NUMBER:  37497
SUBJECT: EP240913a: Swift/XRT detection of an X-ray source within the EP/WXT error circle
DATE:    24/09/14 10:49:22 GMT
FROM:    Dong Xu at NAOC/CAS <dxu@nao.cas.cn>

S.Q. Jiang, Z.P. Zhu, D.Y. Li, C.C. Jin, J. An, X. Liu, S.Y. Fu, D. Xu (NAOC) report on behalf of a large collaboration:

A Swift ToO observation (ID: 16809) of the X-ray transient of EP240913a (Li et al., GCN 37492) was carried out. The observation started at 2024-09-13 19:47:21.024 (UTC) , i.e, 8.13 hr after the EP/WXT trigger, and lasted ~1.65 ks.

An X-ray source is detected in the Swift/XRT image within the EP/WXT error circle (Li et al., GCN 37492) at coordinates

R.A. (J2000) = 01:06:46.69
Dec (J2000) = +16:43:47.20

with an uncertainty of 6.2 arcsec (radius, 90% C.L. statistical and systematic). 

The Swift/XRT spectrum, which can be fitted with an absorbed power-law with a photon index of 1.40 (-0.83, +0.80) modified by Galactic absorption of 5.1 x 10^20 cm^-2, gives an unabsorbed 0.3-10 keV flux of 7.8 (-3.6, +9.7) x 10^(-13) erg/s/cm^2. 

We didn't find any other credible X-ray source within the EP/WXT error circle. And there is no previously known bright X-ray source at the above position. We thus think the source is likely the counterpart of EP240913a, which could be a GRB event (Yin et al., GCN 37493).

We thank the Swift team for performing the ToO observation.