EP250508a
GCN Circular 40390
Subject
EP250508a: Einstein Probe detection of an X-ray transient
Date
2025-05-08T06:37:42Z (21 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
R.D. Liang, T. Zhao, H. Q. Cheng, W. D. Zhang (NAOC, CAS) on behalf of the Einstein Probe (EP) 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 EP250508a. The transient triggered EP-WXT (ID: 01709175339) at 2025-05-08T06:03:47 (UTC). The WXT position of the source is R.A. = 178.266 deg, DEC = -19.900 deg (J2000) with an uncertainty of 3 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. = 178.2540 deg, DEC = -19.9225 deg (J2000) with an uncertainty of 20 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 40391
Subject
EP250508a: COLIBRÍ Optical Limit
Date
2025-05-08T07:01:07Z (21 days ago)
Edited On
2025-05-08T13:18:55Z (21 days ago)
From
Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Via
email
Antonio de Ugarte Postigo (LAM), Noémie Globus (UNAM), Rosa L. Becerra (U Roma), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Fredd Alvarez (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (AUS), Sarah Antier (OCA), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), and Benjamin Schneider (LAM):
We imaged the field of the EP250508a (ID: 01709175339; Liang et al. GCN Circ. 40390) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-05-08 06:15 to 06:34 UTC (from 0.2 to 0.5 hours after the trigger) and obtained 16 minutes of exposure in the i filter.
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 PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
In the stacked image, we do not detect any new source at the WXT source position (Liang et al. GCN Circ. 40390) down to the following 3-sigma limit:
i > 21.7 mag
Further observations and analysis are ongoing.
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 40393
Subject
EP250508a: Global MASTER-Net observations report
Date
2025-05-08T07:27:33Z (21 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 EP250508a ( EP Team et al., GCN 40390) errorbox 2079 sec after notice time and 4289 sec after trigger time at 2025-05-08 07:15:16 UT, with upper limit up to 17.4 mag. The observations began at zenith distance = 78 deg. The sun altitude is -50.3 deg.
The galactic latitude b = 41 deg., longitude l = 286 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2865106
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
4319 | 2025-05-08 07:15:16 | MASTER-OAFA | (11h 53m 09.22s , -19d 33m 11.7s) | C | 60 | 17.4 |
4394 | 2025-05-08 07:16:31 | MASTER-OAFA | (11h 53m 07.05s , -19d 31m 34.0s) | C | 60 | 17.4 |
4470 | 2025-05-08 07:17:47 | MASTER-OAFA | (11h 53m 07.04s , -19d 33m 09.7s) | C | 60 | 17.3 |
4546 | 2025-05-08 07:19:02 | MASTER-OAFA | (11h 53m 09.55s , -19d 31m 44.2s) | C | 60 | 17.2 |
4623 | 2025-05-08 07:20:19 | MASTER-OAFA | (11h 53m 03.85s , -19d 32m 19.3s) | C | 60 | 17.1 |
4698 | 2025-05-08 07:21:35 | MASTER-OAFA | (11h 53m 04.97s , -19d 31m 19.3s) | C | 60 | 17.1 |
4774 | 2025-05-08 07:22:50 | MASTER-OAFA | (11h 53m 10.78s , -19d 32m 20.0s) | C | 60 | 17.1 |
4851 | 2025-05-08 07:24:07 | MASTER-OAFA | (11h 53m 03.30s , -19d 33m 20.6s) | C | 60 | 17.0 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 40395
Subject
EP250508a: REM optical/NIR observations
Date
2025-05-08T10:22:35Z (21 days ago)
From
Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>
Via
Web form
R. Brivio, M. Ferro, P. D'Avanzo, S. Covino, D. Fugazza (INAF-OAB) report on behalf of the REM team:
We observed the field of GRB EP250508a detected by EP/WXT (Liang et al., GCN 40390) with the REM 60 cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the g, r, i, z, J, H, and K bands, started on 2025 May 08 at 06:12:51 UT (i.e. about 9 minutes after the burst), and lasted for about 2 hours.
From preliminary inspection, we do not detect any possible counterpart inside the EP/FXT error circle down to the following 3sigma limits:
r > 19.2 (AB; calibrated against the Pan-STARRS catalogue),
at a mid-time of 51 minutes after the trigger;
H > 16.0 (Vega; calibrated against the 2MASS catalogue),
at a mid-time of 57 minutes after the trigger.
GCN Circular 40399
Subject
EP250508a: refined analysis of the EP-WXT and EP-FXT observations
Date
2025-05-08T16:08:00Z (21 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
T.Zhao, R. D. Liang, H. Q. Cheng, W. D. Zhang (NAOC, CAS) on behalf of the Einstein Probe (EP) team:
The X-ray transient EP250508a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Liang et al., GCN 40390), and followed up by several telescopes (Antonio et al, GCN 40391; Lipunov et al, GCN 40393; Brivio et al, GCN 40395). Refined analysis of the WXT data shows that the event started at T0=2025-05-08T06:02:28.850 (UTC) and lasted for about 60 s before the observation was interrupted by the autonomous follow-up observation. The peak flux is about 3.6 x 10^(-9) erg/s/cm^2. The averaged 0.5-4 keV spectrum can be fitted by an absorbed power law model with a Galactic hydrogen column density of 3.56 x 10^20 cm^-2 and a photon index of 0.91 (+/-0.54). The derived average unabsorbed 0.5-4 keV flux is 1.4 (-0.4/+0.5) x 10^(-9) erg/s/cm^2. The uncertainties are quoted at the 90% confidence level for the above parameters.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 3 min after T0, starting at 2025-05-08T06:05:21 (UTC). Within the WXT error circle, on-ground analysis of the FXT data found an uncatalogued source at R.A. = 178.2555, DEC = -19.9244 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). During the observation a decline in the source flux lasting for a few hundred seconds is observed from the beginning. The averaged 0.5-10 keV FXT spectrum can be fitted by an absorbed power law model with a Galactic hydrogen column density of 3.56 x 10^20 cm^-2 and a photon index of 2.30 (-0.45/+0.45). The derived average unabsorbed 0.5-10 keV flux is 2.0 (-0.5/+0.8) x 10^(-13) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
EP-FXT will continue monitoring this source. More follow-up observations are encouraged. The contact TA of EP250508a is R. D. Liang. Please contact him via email liangrd@bao.ac.cn if needed.
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 40401
Subject
EP250508a: KAIT optical upper limit
Date
2025-05-08T18:38:55Z (21 days ago)
From
Weikang Zheng at UC Berkeley <weikang@berkeley.edu>
Via
email
WeiKang Zheng (UCB), Xuhui Han (NAOC), Pinpin Zhang (NAOC) and
Alexei V. Filippenko (UCB) report on behalf of the KAIT GRB team:
The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at
Lick Observatory, observed the field of EP250508a (Liang et al.,
GCN 40390) starting at 06:17, May 08 UT, about 14 minutes after
the burst. A set of clear (roughly R) filter images were obtained.
Preliminary analysis do not reveal any new optical counterpart
candidate within the X-ray afterglow error circles (Liang et al.,
GCN 40390; Zhao et al., GCN 40399). Due to the bright moonlight,
our image quality are degraded and we estimate the limiting
magnitude of our single image to be ~17.0 mag, shallower than the
upper limit reported by other groups (Ugarte Postigo et al., GCN
40391; Lipunov et al., GCN 40393; Brivio et al., GCN 40395).
GCN Circular 40402
Subject
EP250508a: SVOM/VT optical candidate
Date
2025-05-09T05:40:30Z (21 days 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, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei, T. Zhao, R. D. Liang, H. Q. Cheng, W. D. Zhang (NAOC, CAS), Y. D. Hu (GXU), L. Zhang (IHEP), X. L. Chen(YNU) report on behalf of the SVOM and EP Teams:
SVOM/VT observed the X-ray transient EP250508a (Liang et al., GCN 40390，Zhao et al., GCN 40399) in ToO mode in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. The observation started at 2025-05-08T11:38:19 UT, about 5.6 hours after the trigger.
An uncatalogued faint candidate is detected in both channels within FXT error circle, compared to Legacy Survey, with the coordinate:
RA= 11:53:01.05 (178.25438 deg)
DEC=-19:55:24.2 (-19.92339 deg)
Error = 0.5 arcseconds
J2000
The brightness of the candidate is derived as below :
Mid-time band mag(AB) exposure
6.74 hour VT_R 23.0+/-0.2 71*70 sec
6.75 hour VT_B 23.6+/-0.3 71*70 sec
Our photometry was estimated in AB magnitude and not corrected for Galatic extinction.
Give the faintness of the candidate, we cannot determine whether it is fading. More follow-ups are encouraged to confirm the nature of the transient.
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.
GCN Circular 40403
Subject
EP250508a: follow-up observation with EP-FXT
Date
2025-05-09T08:07:31Z (20 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
R. D. Liang, T. Zhao, H. Q. Cheng, W. D. Zhang (NAOC, CAS) on behalf of the Einstein Probe (EP) team:
Following the detection of the fast X-ray transient EP250508a (Liang et al., GCN 40390, Zhao et al. GCN 40399) and its multi-wavelength follow-up observations (Postigo et al., GCN 40391; Lipunov et al., GCN 40393; Brivio et al., GCN 40395; Zheng GCN 40401; Xin et al. GCN 40402), we performed an observation with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe mission.
The observations began at 2025-05-08 13:41:03 (UTC), about 7.6 hours after the EP-WXT detection. The exposure time is 7250 seconds. Preliminary analysis shows that the source was detected in this epoch. The spectrum in the 0.5-10 keV band can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 3.56e20 cm^-2 and a photon index of 2.22(-0.47, +0.47). The unabsorbed 0.5-10 keV flux is 4.53(-1.88, +1.88)e-14 erg/s/cm^2 (90% C.L.), about five times lower than that detected in the autonomous FXT follow-up observation (Zhao et al. GCN 40399), demonstrating a rapid decay trend in the source's X-ray emission.
EP-FXT will continue monitoring the source. More follow-up observations are encouraged. The contact TA of EP250508a is R. D. Liang. Please contact him via email liangrd@bao.ac.cn if needed.
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 40404
Subject
EP250508a: Liverpool Telescope optical upper limits
Date
2025-05-09T09:01:45Z (20 days ago)
From
Rob Eyles-Ferris at U of Leicester <raje1@leicester.ac.uk>
Via
Web form
R. A. J. Eyles-Ferris, P. T. O’Brien and R. L. C. Starling (U of Leicester) report:
We observed the field of the X-ray transient EP250508a (Liang et al., GCN 40390; Zhao et al., GCN 40399; Liang et al., GCN 40403) with the 2m Liverpool Telescope using the IO:O instrument. We obtained 6x150s exposures in the SDSS r’ filter starting at 2025-05-08 21:07:19 UT and 6x150s exposures in the SDSS z’ filter starting at 2025-05-08 21:24:25 UT, approximately 15.1 hours after the X-ray detection.
We performed image subtraction on the stacked images using reference images from Pan-STARRS and also compared the stacked and reference images manually. Consistent with Ugarte Postigo et al. (GCN 40391), Lipunov et al. (GCN 40393), Brivio et al. (GCN 40395) and Zheng et al. (GCN 40401), we identify no new sources within the error region of the EP/FXT source.
At the position of the optical counterpart candidate identified by Xin et al. (GCN 40402), we derive 3-sigma upper limits of r’ > 21.5 and z’ > 21.5 with photometry calibrated to Pan-STARRS and not corrected for Galactic extinction.
GCN Circular 40412
Subject
EP250508a: SVOM/VT optical candidate fading
Date
2025-05-10T01:53:50Z (20 days ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu, Z. H. Yao, Y. N. Ma, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei, T. Zhao, R. D. Liang, H. Q. Cheng, W. D. Zhang (NAOC, CAS), Y. D. Hu (GXU), L. Zhang (IHEP), X. L. Chen(YNU) report on behalf of the SVOM and EP Teams:
SVOM/VT made a second ToO observation for EP250508a (Liang et al., GCN 40390，Zhao et al., GCN 40399) at about 29.44 hour after trigger. The observation was performed in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
The brightness of the candidate reported by Xin et al. (GCN 40402) was VT_R=23.9+/-0.3 mag and VT_B>24.0 with the exposure time of 52*100 seconds.
Our photometry was derived in AB magnitude and not corrected for Galactic extinction.
Considering the fading behavior of the candidate, we suggest that it is the optical counterpart of the transient.
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.
GCN Circular 40428
Subject
EP250508a: Gemini NIR upper limits
Date
2025-05-11T07:27:31Z (18 days ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
email
A. J. Levan (Radboud and Warwick), J. C. Rastinejad (Northwestern), A.
Martin-Carrillo (UCD), J. A. Quirola-Vasquez (Radboud), F. E. Bauer (SSI
and UTA), G. Corcoran (UCD), R. A. J. Eyles-Ferris (Leicester), P. G.
Jonker (Radboud), D. B. Malesani (DAWN/NBI and Radboud), report on
behalf of a larger collaboration:
We observed the field of the fast X-ray transient EP250508a (Liang et
al., GCN 40390; Zhao et al., GCN 40399) using the Gemini-South telescope
equipped with the FLAMINGOS-2 near-infrared imager (program
GS-2025A-FT-106, PI Bauer). Observations were started on 2025 May 9 at
02:08:01 UT (20.07 hr after the EP trigger), and we secured 9 and 4.8
min imaging in the J and Ks bands, respectively. The seeing was modest
at ~1.7" in the J-band image.
No source is detected at the coordinates of the optical afterglow
reported by SVOM/VT (Xin et al., GCN 40402; Li et al., GCN 40412), down
to 5-sigma limiting magnitudes:
J > 22.9 (AB system, 20.28 hr after trigger);
Ks > 22.2 (AB system, 22.58 hr after trigger).
These upper limits were calibrated using nearby stars from the VISTA
Hemisphere Survey (VHS) catalogue.
We thank excellent support from the observing staff at Gemini, in
particular Venu Kalari and Henrique Reggiani.