EP241125a

B.-T. Wang (YNAO, CAS), T. C. Zheng (PMO, CAS), W. J. Zhang (NAO, CAS), J. Yang (NJU), H. W. Pan (NAO, CAS) on behalf of the Einstein Probe team
 
We report on the detection of an X-ray transient detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, EP241125a (ID: 11900017164). The transient was first detected with WXT at 2024-11-25 00:06:06 (UTC) and lasted for over 150 seconds. The WXT position of EP241125a is R.A.= 48.561 deg, DEC = 37.677 deg (J2000) with an uncertainty of 2.65 arcmin (radius, 90% C.L. statistical and systematic). It has a peak flux of ~8 x 10^-10 erg/s/cm^2 in the 0.5-4 keV band. The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 1.90 (+1.12/-0.95) (with a column density fixed at the Galactic value of 1.2 x 10^21 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 2.79 (+1.11/-0.86) x 10^(-10) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. No previously known bright X-ray sources are found within the error circle around the source position.
 
We have performed a target of opportunity observation with Swift. The Swift XRT observation began at 2024-11-25 16:16:24 with an exposure time of 3500 seconds in the Photon Counting mode. No X-ray source with a signal-to-noise (S/N) ratio greater than 3 is detected within the positional error circle (2.65 arcmin radius) of the EP source. Further multi-wavelength 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 X-ray observation possible. 

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).

C.-H. Lai, A. Aryan, T.-W. Chen, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), Y. J. Yang, A. Sankar. K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, H.-C. Lin, W.-J. Hou, H.-Y. Hsiao, J.-K. Guo (all NCU), S. Yang, Z. N. Wang, L. L. Fan, G. H. Sun (all HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz and M. Huber (both IfA, Hawaii) report: 

We observed the field of the fast X-ray transient EP241125a (Wang et al., GCN 38318
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 ) using the 1m LOT at the Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2024 arXiv:2406.09270
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 ). The first LOT epoch of observations started at 12:11 UTC on 25th November 2024 (MJD 60638.508), 12.09 hr after the EP-WXT trigger. 

We utilized the astroalign (Beroiz et al., 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al., 2022, ApJ, 935, 167) packages to align and stack the individual frames. We did not find any new and uncataloged optical source in the stacked frames within the 2.65 arcminute error circle of the EP-WXT localization, in comparison to the Pan-STARRS1 3Pi archive images (Chambers et al., 2016 arXiv:1612.05560
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 ). 

Moreover, we utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform aperture photometry on our stacked frames. The details of the observations and measured 3-sigma upper limit (in the AB system) are as follows:

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Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
LOT | r | 60638.508 | 12.09 | 120 * 15 | > 22.9 | 1".13 | 1.20

The presented magnitude was calibrated using the field stars from the Pan-STARRS1 catalog and was not corrected for the expected Galactic foreground extinction corresponding to a reddening of E(B-V) = 0.27 mag in the direction of the transient (Schlafly & Finkbeiner 2011).