EP250526a
GCN Circular 40550
Subject
EP250526a: Einstein Probe detection of a fast X-ray transient
Date
2025-05-27T03:02:34Z (3 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
D. Y. Li (NAO, CAS), B.-T. Wang (YNAO, CAS), W. F. Wen (SZTU), J. H. Wu (GZHU), Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team:
We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250526a. The transient triggered EP-WXT (ID: 01709176979) at 2025-05-26 19:28:23 (UTC). The WXT position of the source is R.A. = 217.049 deg, DEC = -34.355 deg (J2000) with an uncertainty of 2.9 arcmin in radius (90% C.L. statistical and systematic).
A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 217.0509 deg, DEC = -34.3293 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic).
Further information will be updated when the telemetry data is received.
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).
GCN Circular 40551
Subject
EP250526a: Global MASTER-Net observations report
Date
2025-05-27T07:36:46Z (2 days ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko,
G.Antipov, A.Sankovich, Yu.Tselik, 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.Sosnovskij (CrAO),
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 EP250526a ( EP Team et al., GCN 40550) errorbox 15422 sec after notice time and 42778 sec after trigger time at 2025-05-27 07:21:21 UT, with upper limit up to 18.6 mag. The observations began at zenith distance = 57 deg. The sun altitude is -51.0 deg.
The galactic latitude b = 24 deg., longitude l = 325 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2883070
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
42809 | 2025-05-27 07:21:21 | MASTER-OAFA | (14h 28m 05.95s , -34d 00m 45.3s) | C | 60 | 18.6 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 40552
Subject
EP250526a: COLIBRÍ optical upper limits
Date
2025-05-27T08:15:01Z (2 days ago)
From
Benjamin Schneider at MIT <bschn@mit.edu>
Via
Web form
Camila Angulo (UNAM), Benjamin Schneider (LAM), Rosa L. Becerra (U Roma), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Fredd Alvarez (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (AUS), Sarah Antier (OCA), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM) and Antonio de Ugarte Postigo (LAM):
We imaged the field of the EP250526a (Li et al., GCN Circ. 40550) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-05-27 03:30 to 5:35 UTC (from 8.0 to 10.1 hours after the trigger and 58 minutes after the notice) and obtained 32 minutes of exposure in g, r and i filters (each one).
The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the SkyMapper catalog, is in the AB system, and is not corrected for Galactic extinction.
In the stacked images, we do not detect any new source at the FXT position (Li et al., GCN Circ. 40550) down to the following 3-sigma limit:
g > 23.6
r > 23.4
i > 23.0
We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.
COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.
GCN Circular 40554
Subject
EP250526a: refined analysis of the EP-WXT and EP-FXT observations
Date
2025-05-27T12:44:31Z (2 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Jiahua Wu (GZHU), D. Y. Li (NAO, CAS), B.-T. Wang (YNAO, CAS), W. F. Wen (SZTU) and Y. Liu (NAO, CAS) report on behalf of the Einstein Probe team:
The X-ray transient EP250526a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Li et al., GCN 40550), and followed up by several optical telescopes (Lipunov et al, GCN 40551, Angulo et al., GCN 40552), while no optical counterpart has been detected yet. Refined analysis of the WXT data shows that the event started at T0=2025-05-26T19:24:55 (UTC) and lasted for about 175 s (T90). The peak flux (0.5-4 keV) is estimated to be 4 x 10^-9 erg/s/cm^2 (T0+39 s). The averaged 0.5-4 keV spectrum of T90 can be fitted with an absorbed power-law with a Galactic hydrogen column density of 7 x 10^20 cm^-2 and a photon index of 1.31 (-/+0.42). The derived average unabsorbed 0.5-4 keV flux is 1.75 (-0.37, +0.51) x 10^-9 erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2025-05-26T19:31:10, about 375 seconds after T0, with an exposure time of 2270 seconds. The averaged 0.5-10 keV spectrum can be fitted with an absorbed power-law with a photon index of 2.59 (-0.43, +0.44). The derived unabsorbed 0.5-10 keV flux is 7.0 (-1.4, +2.0) x 10^-13 erg/s/cm^2. Another FXT observation was conducted at 2025-05-27T04:18:56, about 9 hours after T0. The X-ray spectrum can be fitted with an absorbed power-law with a photon index of 2.37 (-0.93, +0.91), and the derived unabsorbed 0.5-10 keV flux is 1.2 (-0.5, +1.2) x 10^-13 erg/s/cm^2. The absorption was fixed at the Galactic value during the spectral fitting. The uncertainties are at the 90% confidence level for the above parameters.
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).
The contact TA of this source is Jiahua Wu, please contact him via the email jhwu@e.gzhu.edu.cn if needed.
GCN Circular 40562
Subject
EP250526a: SVOM/VT optical upper limit
Date
2025-05-28T12:59:03Z (a day ago)
From
Liping Xin at NAOC, SVOM <xlp@nao.cas.cn>
Via
Web form
L. P. Xin, Y. L. Qiu, H. L. Li, C. Wu, Z. H. Yao, Y. N. Ma, X. H. Han, J. Wang, D. Y. Li (NAO, CAS), B.-T. Wang (YNAO, CAS), W. F. Wen (SZTU), J. H. Wu (GZHU), Y. Liu (NAO, CAS) report on behalf of the SVOM and EP mission team
SVOM/VT conducted ToO follow-up observations of the EP250526a (Li et al., GCN 40550, Wu et al., GCN 40554). The observation started on 2025-05-27T04:04:19 UT (i.e. 7.66 hour after trigger time) in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channel simultaneously.
No uncatalogued source was detected, compared to the Legacy survey, within errorbox of EP-FXT (Li et al., GCN 40550), down to 3 sigma upper limit of VT_R~23.0 mag (AB) in 16*70 sec stacked images at the mid time of 7.81 hour.
The upper limit is consistent with the reports(Lipunov et al., GCN 40551, Angulo et al., GCN 40552).
The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.