EP250125a

A. Aryan, T.-W. Chen, Y.-H. Lee, W.-J. Hou, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), S. J. Smartt (Oxford/QUB), J. Gillanders (Oxford), A. Sankar.K, Y. J. Yang, M.-H. Lee, H.-Y. Miao, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-Y. Hsiao, H.-C. Lin, 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 EP250125a (Wu et al., GCN 39028
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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 16:39 UTC on 25th Januaryr 2025 (MJD 60700.694), 14.04 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.
In the stacked frame, we did not detect the optical counterpart candidate suggested by Levan et al., (GCN 39027
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 ) and Ferro et al., (GCN39031
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 ). Considering the decay rate by two above-mentioned optical detections, probably, the optical counterpart candidate faded beyond the detection limit in our observations.

We utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform PSF photometry on our stacked frame. The details of the observations and measured 3-sigma upper limit (in the AB system) are as follows:
---------------------------------------------------------------------------------------------------Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass 
LOT | r | 60700.694 | 14.04 | 300 * 6 | >22.5 | 1".47 | 1.83

The presented magnitudes were calibrated using the field stars from the Pan-STARRS1 catalog (Chambers et al., 2016 arXiv:1612.05560
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 ) and were not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_r = 0.112 mag in the direction of the transient (Schlafly & Finkbeiner 2011).   

M. Ferro, P. D’Avanzo, R. Brivio, S. Covino, D. Fugazza (INAF-OAB) report on behalf of the REM team:

We observed the field of the X-ray transient EP250125a, detected by Einstein Probe (Wu et al., GCN 39028
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 ) and Swift/XRT (Kennea et al., GCN 39029
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 ) with the REM 60cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the g, r, i, z, J, H, K bands, starting on 2025 January 25 at 02:44:32 UT (i.e. 493 seconds after the EP trigger), and lasting for about 3 hours.
From preliminary photometry we detect the counterpart in the optical images at the position reported by Levan et al. (GCN 39027
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 ), with the following magnitudes:

r = 19.3 +- 0.3 (AB; calibrated against the Pan-STARRS catalogue)
at a mid-time of t - t0 = 765 s after the trigger,

H > 15.8 (Vega; calibrated against the 2MASS catalogue) 
at a mid-time of t - t0 = 641 s after the trigger.

J. A. Kennea (PSU), K. L. Page, P. A. Evans (Leicester) and B. Sbarufatti (INAF) report on behalf of the Swift XRT Team.

At 03:26UT Swift performed a series of Target-of-Opportunity observations of the Einstein Probe transient EP250125a (GCN #39028
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 ), approximately 49 minutes after the EP trigger. In the XRT data we detect a bright X-ray source at the following coordinates: RA/Dec(J2000) = 175.36337, -21.71347

RA (J2000): 11h 41m 27.21s,
Dec (J2000): -21d 42' 48.5",

with an estimated error radius of 3.8 arc-seconds (90% confidence). This lies 2.1 arc-seconds from the reported EP FXT position of the transient, and 3.9 arc-seconds from the proposed optical afterglow of the source (GCN #39029
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 ). 

We note that the source is fading with a 3-sigma significance. The peak flux seen by XRT is 4.3 (±1.0) ×10^-12 erg cm^-1 s^-1 (0.3-10 keV), dropping to a flux of 4.8 (±1.1) x 10^-13 erg cm^-1 s^-1 (0.3-10 keV) at a second observation taken at T0+8.9 hours. 

Q.-Y. Wu (NAOC, CAS), J. Yang (NJU), X.-Y. Zhou (PRIC) and C.-C. Jin (NAOC, CAS) report on behalf of the Einstein Probe team:

We report on the detection of an X-ray transient, designated EP250125a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, which triggered the on-board processing unit at 2025-01-25T02:37:31 (UTC) (trigger ID: 01709130802). The transient event started at 2025-01-25T02:36:19 (UTC) and lasted for 74 s before the observation was interrupted by the autonomous follow-up observation. The WXT position of the source is R.A. = 175.364 deg, DEC = -21.708 deg (J2000) with an uncertainty of 2.3 arcmin in radius (90% C.L. statistical and systematic). The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic equivalent hydrogen column density of 4.15 x 10^20 cm^-2 and a photon index of 0.8 (-0.5/+0.5). The derived average unabsorbed 0.5-4 keV flux is 1.8 (-0.5/+0.7) x 10^(-9) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

An autonomous observation was performed by the Follow-up X-ray Telescope (FXT) about two minutes later. An uncatalogued source was detected at R.A. = 175.3639, DEC = -21.7138 (J2000) with an uncertainty of about 20 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the optical conterpart reported by Levan et al., GCN 39027
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 . The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic equivalent hydrogen column density of 4.15 x 10^20 cm^-2 and a photon index of 2.0 (-0.1/+0.1). The derived average unabsorbed 0.5-10 keV flux is 5.4 (-0.6/+0.6) x 10^(-12) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

More information on this source will be updated when the full telemetry data is received. Further follow-up observations are encouraged to identify the nature of this X-ray transient.  

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). 

A. J. Levan (Radboud Univ. and Warwick Univ.), J. A. Quirola-Vasquez (Radboud Univ.), F. E. Bauer (PUC), D. B. Malesani (DAWN/NBI and Radboud Univ.), A. Martin-Carrillo (UCD), P. G. Jonker (Radboud Univ.) report on behalf of a larger collaboration.

We observed the field of the EP transient 01709130802 (EP250125a, trigger time: 2025-01-25 02:37:31.749 UT) using the Gemini South telescope equipped with the GMOS-S camera and spectrograph. A set of images in the r and z bands were secured, starting on 2025 25.294 UT (4.43 hr after the trigger).

A point-like object not present in the archival Pan-STARRS images of the field is detected in both filters at coordinates (J2000), well within the 3 arcmin-radius WXT error circle:

RA = 11:41:27.39
Dec = -21:42:51.5

For this object, we measure an AB magnitude r = 21.60 +- 0.03, calibrated against nearby objects from the Pan-STARRS catalog.

We suggest this object is the optical counterpart of EP250125a.

A spectrum of the source was taken with the B480 grism, starting at 08:39 UT. In the first 600 s exposure, the source is well detected across the spectral range from 4300 to 8200 AA (preliminary wavelength calibration). A strong absorption trough is visible at ~4730 AA, which we interpret as a DLA. From the detection of several narrow absorption features, including those from Si II, C II, Si IV, Al II, and Fe II we determine z = 2.89, which we suggest to be the redshift of EP250125a.

We thank the Gemini staff, in particular Karleyne Silva, for masterfully executing the requested observations