EP250308a

Z.P. Zhu, X. Liu, Jie An, S.Q. Jiang, D. Xu (NAOC), S.Y. Fu (HUST, NAOC), D. B. Malesani (DAWN/NBI and Radboud U.), P. Jonker (Radboud U.), A. H. Fuente (NOT) report on behalf of a large collaboration:

We observed the field of EP250308a detected by EP/WXT (Trigger ID: 01709132392), using the ALFOSC instruments mounted on the Nordic Optical Telescope (NOT), with 3 x 300 s in the SDSS-r band. 

Preliminary analysis shows that no new source is detected within the EP/WXT error circle, down to the 5-sigma upper limits of r > ~ 22.7 (AB) at ~ 0.41 day after the EP trigger, calibrated with nearby Pan-STARRS stars and without Galactic extinction correction.

Y.-H. I. Yin (NJU), Q. Y. Wu (NAO, CAS), J. H. Wu (GZHU), H. W. Pan (NAO, CAS) report on behalf of the Einstein Probe team:
 
We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250308a. The transient triggered EP-WXT (ID: 01709132392) at 2025-03-08T17:03:57 (UTC). The WXT position of the source is R.A. = 210.581 deg, DEC = 18.515 deg (J2000) with an uncertainty of 2.9 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 Galactic hydrogen column density of 2.37 x 10^20 cm^-2 and a photon index of 2.36 (-0.59/+0.61). The derived average unabsorbed 0.5-4 keV flux is 3.62 (-0.82/+1.18) x 10^(-10) erg/s/cm^2.

The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 165 seconds after the trigger with an exposure time of 5.5 ks. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 210.5875, DEC = 18.4967 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent positionally with the WXT transient. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 2.37 x 10^20 cm^-2 and a photon index of 2.59 (-0.07/+0.08). The derived average unabsorbed 0.5-10 keV flux is 9.71 (-0.42/+0.45) x 10^(-13) 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 transient position. 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).

Malte Busmann (LMU), Brendan O’Connor (Carnegie Mellon U.), Daniel Gruen (LMU) and Antonella Palmese (Carnegie Mellon U.) report:

We observed the 90% of EP-WXT trigger 01709132392, corresponding to EP250308a, with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i and J band simultaneously for 19 x 180 s starting at 2025-03-09T03:27:28 UT (0.43 days after the trigger). We performed difference imaging in the i-band with templates from Legacy Survey and detect no new or changing source.

The 3 sigma mean depth of our observations are:

r > 24.0 AB mag
i > 23.8 AB mag

The magnitudes are calibrated against the PS1 catalog and not corrected for Galactic extinction.

We thank Michael Schmidt from the Wendelstein Observatory staff for obtaining these observations.

W. X. Li, S. J. Xue (NAOC), M. Andrews, J. Farah, D. A. Howell, M. Newsome, E. Padilla Gonzalez, C. McCully, and G. Terreran (Las Cumbres Observatory), on behalf of a larger collaboration, report:
Following the detection of the fast X-ray transient EP250308a by the Einstein Probe (trigger ID: 01709132392; Yin et al., GCN 39625
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 ), we initiated observations of its location starting on 2025 March 9th at 05:07 UT (~12 hours after the EP/WXT trigger) in the i band. These observations were conducted using the 1-meter telescope at the Las Cumbres Observatory node located at Cerro Tololo Inter-American Observatory in Chile.
No new optical source was detected in the co-added images within the EP/FXT error box down to ~21.5 mag.
These observations were taken as part of the Global Supernova Project.

V.Lipunov, E.Gorbovskoy, N.Tiurina,  P.Balanutsa, , D.Vlasenko, I.Panchenko,
A.Kuznetsov,  G.Antipov,  A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, Ya.Kechin,
V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile,  F. Podesta, R.Podesta, E. Gonzalez  (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) 

MASTER-OAFA robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP250308a ( EP Team et al., GCN 39625
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 ) errorbox  194 sec after notice time and 44884 sec after trigger time at 2025-03-09 05:32:01 UT, with upper limit up to  20.4 mag. The observations began at zenith distance = 57 deg. The sun  altitude  is -52.8 deg. 

The galactic latitude b = 71 deg., longitude l =  9 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2805476

We obtain a following upper limits.  

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   44915 | 2025-03-09 05:32:01 |         MASTER-OAFA | (14h 02m 29.31s , +18d 46m 58.8s) |   C |    60 | 20.4 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

Shiyan Zhong, Jinghua Zhang, Guowang Du, Xin Chang, Bin Yan, Xingzhu Zou, Brajesh Kumar, Yuan Fang, Helong Guo, Edoardo P. Lagioia, Yuanpei Yang, Xiangkun Liu, Xiaowei Liu (all SWIFAR, YNU) report on behalf of the Mephisto Team:

The field of EP250308a (Yin et al., GCN 39625
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 ) was observed with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. Two exposures of 180s in the MEPHISTO u band, and six exposures of 50s in the MEPHISTO g band were simultaneously (ug) obtained starting from 2025-03-09T20:07:03 (~27h after the trigger). In our stacked u and g band images, we did not detect any source at the position reported by Yin et. al., GCN 39625
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 . The preliminary 3-sigma upper limits are below, which is consistent with previously reported GCN (Zhu et al., GCN 39624
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 ; Busmann et al., GCN 39626
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 ; Lipunov et al. GNC 39628).

Start_Time(UT)      | Band | Exp(s) |  LimMag(AB)
--------------------|------|--------|------------
2025-03-09T20:07:03 | u    | 180*2  |  >22.20
2025-03-09T20:07:03 | g    |  50*6  |  >22.38
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Mephisto (Multi-channel Photometric Survey Telescope) is a 1.6-m wide-field multi-channel telescope, the first of its type in the world, capable of imaging the same field of view in three optical bands simultaneously. It provides real-time, high-quality colors of stellar objects. The on-site telescope assemblage and commissioning were carried out in September 2022. The first light in all three channels was achieved on 2023 December 21.
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