EP250402a

Jonathan Quirola-Vásquez (Radboud), Peter G. Jonker (Radboud),  Franz Bauer (PUC), Antonio Martin-Carrillo (UCD), Andrew J. Levan (Radboud), Daniele B. Malesani (DAWN/ NBI and Radboud), Javi Sánchez-Sierras (Radboud), Agnes van Hoof (Radboud), Jennifer Chacon (PUC), Joyce van Dalen (Radboud) report on behalf of a larger collaboration:

We obtained photometric observations of the field of the fast X-ray transient EP250402a discovered by Einstein Probe (Li et al., GCN 40017
Loading...
 
 , GCN 40038
Loading...
 
 ) using the Planaetry Defense 1.3m telescope localized in Cerro Tololo Inter-American Observatory, Chile. We obtained 30 images of 120 s each in the r-band filter beginning at 2025-04-03 07:06:41 UT and with a middle time at T0+0.797 days after the EP trigger.

Within the EP-FXT error circle, we detect no plausible optical counterpart down to a 3sigma limit of r > 21.8 AB mag, calibrated against SkyMapper nearby reference stars

We acknowledge the excellent support from the PD 1.3m  staff, in particular Tyler Linder.

A. Aryan, Y.-H. Lee, T.-W. Chen, W.-J. Hou (all NCU), S. Brennan (OKC), 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.-Y. Hsiao, C.-S. Lin, H.-C. Lin, C.-H. Lai, J.-K. Guo (all NCU), S. Yang, L. L. Fan, Z. N. Wang, 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 EP250402a (Li et al., GCN 40017
Loading...
 
 , GCN 40038
Loading...
 
 ) using the  40cm SLT telescope at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al. 2024, arXiv:2406.09270
Loading...
 
 ). The first SLT epoch of observations in the r-band started at 14:18 UT on the 3rd of April 2025 (MJD = 60768.538), ~24.43 hrs after the EP 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 detect any uncataloged optical counterpart candidate within the EP-WXT/EP-FXT localization error circle of radius 2.5 arcminutes/10 arcseconds. The field of FXT is only covered by SkyMapper. 

We further employed the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, 62) to perform PSF photometry on our stacked frames. The details of the observations and measured three sigma upper limits (in the AB system) were as follows:  

Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass   
SLT | r | 60768.538 | 24.43 | 300 * 24 | >20.5 | 1".83 | 3.11   

The presented upper limit was calibrated using the field stars from the SkyMapper catalog and was not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_r = 0.38 mag, respectively, in the direction of the transient (Schlafly & Finkbeiner 2011).

R.-Z. Li (YNAO, CAS), H. Q. Cheng (NAO, CAS), Q. C. Zhao, Z. H. Yang (IHEP, CAS), Y. Liu (NAO, CAS) report on behalf of the Einstein Probe team:

The X-ray transient EP250402a was detected by EP-WXT (Li et al., GCN 40017
Loading...
 
 ). Two follow-up observations were conducted by EP-FXT.

The first follow-up observation was performed at 2025-04-03T07:25:06 (UTC), about 18.93 hours after the trigger, with an exposure time of 1 ks. On-ground analysis of the EP-FXT data identified an uncatalogued X-ray source at the coordinates (J2000):
R.A., Dec. = 172.1798, -47.2954 deg,
with an uncertainty of about 10 arcsec (radius, 90% confidence level, including both statistical and systematic errors). This source lies within the EP-WXT error circle (Li et al., GCN 40017
Loading...
 
 ). The 0.5-10 keV spectrum can be fitted with an absorbed power-law model, with the column density nH fixed at the Galactic value of 1.43e21 cm^-2, and a photon index of 2.9 (+1.2, -1.0). The unabsorbed 0.5-10 keV flux is 1.9 (+1.2, -0.7) e-13 erg/s/cm^2.

The second follow-up observation was performed at 2025-04-03T17:11:45 (UTC), about 28.71 hours after the trigger, with an exposure time of 4.4 ks. The 0.5-10 keV spectrum can also be fitted with an absorbed power-law model, with nH fixed at the Galactic value of 1.43e21 cm^-2, and a photon index fixed at 2.9. The unabsorbed 0.5-10 keV flux is 9.1 (+5.0, -3.5) e-14 erg/s/cm^2.

All uncertainties quoted above are at the 90% confidence level.

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).

R.-Z. Li (YNAO, CAS), H. Q. Cheng (NAO, CAS), Q. C. Zhao, Z. H. Yang (IHEP, CAS), Y. Liu (NAO, CAS) report on behalf of the Einstein Probe team:

We report on the detection of a fast X-ray transient EP250402a (trigger ID: 01709133860) at 2025-04-02T12:29:06.850 (UTC) by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The position of the source is R.A., DEC. = 172.168, -47.290 deg (J2000) with an uncertainty of 2.5 arcmin (radius, 90% C.L. statistical and systematic). The transient event lasted for about 80 seconds, with a peak 0.5-4 keV flux of 5.7 x 10^-9 erg/s/cm^2. 

The 0.5-4.0 keV spectrum can be fitted with an absorbed powerlaw model, with the column density nH fixed at the Galactic value of 1.43 x 10^21 cm^-2 and a photon index of 1.3 (+0.7/-0.7). The unabsorbed 0.5-4.0 keV flux is 1.2 (+0.6, -0.4) x10^-9 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

Follow-up observations by EP-FXT are prepared.

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).