- GCN Circular #40429
T. Y. Lian (NAO, CAS), A. Li (BNU), Y. L. Wang (NAO, CAS; ICE, CSIC-IEEC), C. C. Jin (NAO, 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 EP250511a. The transient triggered EP-WXT (ID: 01709175648) at 2025-05-11T07:59:59 (UTC). The WXT position of the source is R.A. = 202.096 deg, DEC = -2.725 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. = 202.1113 deg, DEC = -2.7023 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 #40430
I. Perez-Garcia, E. J. Fernandez-Garcia, M. D. Caballero-Garcia, S.-Y. Wu, R. Sanchez-Ramirez, S. Guziy and A. J. Castro-Tirado (IAA-CSIC, Granada), I. M. Carrasco (SMA), M. Gritsevich (Univ. of Helsinki), C. Perez del Pulgar (Univ. of Malaga), Y.-D. Hu (GXU), S. Jeong (ADD, Daejeon), G. Garcia-Segura and D. Hiriart (IA-UNAM, Ensenada), D. R. Xiong (Yunnan Observatories of CAS, Kunming), and W. H. Lee (UNAM, Mexico DF), on behalf of a larger collaboration, report:
Following the detection of EP250511a by EP (Lian et al., GCNC [40429](https://gcn.nasa.gov/circulars/40429)), the 0.6m BOOTES-5/JGU robotic telescope at San Pedro Martir Observatory (Mexico) automatically responded to this high-energy event starting on May 11, 09:22:17 UT (i.e. 1.3 hours after detection and 1 min after notification). No optical transient is detected down to 18.2 mag (clear filter).
We would like to thank the staff at San Pedro Martir Observatory for their excellent support.
- GCN Circular #40431
Benjamin Schneider (LAM), Kin Ocelotl López (UNAM), Rosa L. Becerra (U Roma), Camila Angulo (UNAM), 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), Noémie Globus (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 EP250511a (Lian et al., GCN Circ. 40429) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-05-11 09:25:48 to 10:08:02 UTC (from 1.43 to 2.13 hours after the trigger, and 2 minutes after the notice) and obtained 32 minutes of exposure in the i filter.
The data were reduced and coadded with the COLIBRÍ pipeline and analyzed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS catalog, is in the AB system, and is not corrected for Galactic extinction.
We marginally detect a new source consistent with the FXT position (Lian et al., GCN Circ. 40429) and not visible in PanSTARRS at:
RA(J2000) = 13:28:26.68 = 202.1111 degrees
Dec(J2000) = -2:42:11.1 = -2.7030 degrees
with an uncertainty of 0.5 arcsec.
The preliminary magnitude derived for that source is:
i ~ 21.9
We note the presence of a possible host galaxy in deeper Legacy Survey images, located 0.5 arcsec from the optical counterpart candidate, with magnitudes of g = 23.61 +/- 0.04 and r = 23.67 +/- 0.05.
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 #40432
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-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) was pointed to the EP250511a ( EP Team et al., GCN 40429) errorbox 28887 sec after notice time and 44100 sec after trigger time at 2025-05-11 20:14:59 UT, with upper limit up to 16.9 mag. The observations began at zenith distance = 30 deg. The sun altitude is -55.6 deg.
The galactic latitude b = 58 deg., longitude l = 322 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2867812
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
44115 | 2025-05-11 20:14:59 | MASTER-SAAO | (13h 27m 09.60s , -02d 50m 52.7s) | C | 30 | 16.8 |
44220 | 2025-05-11 20:14:59 | MASTER-SAAO | (13h 27m 09.60s , -02d 50m 52.5s) | C | 240 | 16.8 | Coadd
44165 | 2025-05-11 20:15:48 | MASTER-SAAO | (13h 27m 07.01s , -02d 49m 12.4s) | C | 30 | 16.9 |
44259 | 2025-05-11 20:16:38 | MASTER-SAAO | (13h 27m 08.05s , -02d 50m 48.6s) | C | 120 | 16.7 |
44399 | 2025-05-11 20:18:57 | MASTER-SAAO | (13h 27m 12.36s , -02d 49m 05.6s) | C | 120 | 16.8 |
44538 | 2025-05-11 20:21:16 | MASTER-SAAO | (13h 27m 08.49s , -02d 49m 44.1s) | C | 120 | 16.6 |
44707 | 2025-05-11 20:23:36 | MASTER-SAAO | (13h 27m 08.91s , -02d 48m 40.9s) | C | 180 | 15.3 |
44907 | 2025-05-11 20:26:55 | MASTER-SAAO | (13h 27m 18.75s , -02d 49m 37.8s) | C | 180 | 16.1 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
- GCN Circular #40433
H. L. Li, L. P. Xin, Y. L. Qiu, J. Wang, C. Wu, Z. H. Yao, Y. N. Ma, X. H. Han, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei, T. Y. Lian (NAOC), A. Li (BNU), Y. L. Wang (NAOC; ICE, CSIC-IEEC), C. C. Jin (NAOC) report on behalf of the SVOM and Einstein Probe (EP) teams:
SVOM/VT observed the X-ray transient EP250511a (Lian et al., GCN 40429) in ToO mode in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. The observations started at 2025-05-11T15:46:49 UT, about 7.78 hours after the trigger.
The candidate reported by Bchneider et al. (GCN 40431) within the error box of EP/FXT (Lian et al., GCN 40429) was clearly detected by VT in both channels. The brightness of the candidate was derived as below :
Mid-time Band mag(AB) exposure
8.16 hour VT_B 22.41 +/- 0.13 36*70 sec
8.14 hour VT_R 22.29 +/- 0.13 36*70 sec
9.77 hour VT_B 22.65 +/- 0.15 37*70 sec
9.78 hour VT_R 22.52 +/- 0.16 37*70 sec
Our photometry was estimated in AB magnitude and not corrected for Galactic extinction.
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 #40434
A. Li (BNU), Y. L. Wang (NAO, CAS), T. Y. Lian (NAO, CAS), C. C. Jin (NAO, CAS) behalf of the Einstein Probe (EP) team:
The X-ray transient EP250511a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Lian et al., GCN 40429), and followed up by several telescopes (Perez-Garcia et al, GCN 40430; Schneider et al, GCN 40431; Lipunov et al, GCN 40432; Li et al, GCN 40433). Refined analysis of the WXT data shows that the event started at T0=2025-05-11T07:59:59 (UTC) and lasted for about 50s. The peak flux is about 3.8 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 fixed Galactic hydrogen column density of 2.7 x 10^20 cm^-2 and a photon index of 2.17 (+/-0.71). The derived average unabsorbed 0.5-4 keV flux is 2.3 (-0.8/+0.6) x 10^(-10) 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 2025-05-11T08:03:06 (UTC). Within the WXT error circle, on-ground analysis of the FXT data found an uncatalogued source at R.A. = 202.1113, DEC = -2.7023 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The averaged 0.5-10 keV FXT spectrum can be fitted by an absorbed power law model with a fixed Galactic hydrogen column density of 2.7 x 10^20 cm^-2 and a photon index of 2.98 (-0.59/+0.62). The derived average unabsorbed 0.5-10 keV flux is 1.7 (-0.7/+0.9) x 10^(-13) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
The contact TA of EP250511a is A. Li. Please contact him via email anli@mail.bnu.edu.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 #40435
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 EP250511a detected by EP/WXT (Lian et al., GCN 40429) 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 11 at 22:53:26 UT (i.e. 14.9 hr after the burst), and lasted for about 1 hour.
From preliminary inspection, we do not detect any possible counterpart consistent with the candidate optical afterglow (Schneider et al., GCN 40431; Li et al., GCN 40433) down to the following 3sigma limits:
r > 19.4 (AB; calibrated against the Pan-STARRS catalogue),
at a mid-time of 15.3 hr after the trigger;
H > 16.2 (Vega; calibrated against the 2MASS catalogue),
at a mid-time of 15.0 hr after the trigger.
- GCN Circular #40442
Shiyan Zhong, Jianhui Lian, Xufeng Zhu, Yicheng Jin, Dan Zhu, Guowang Du, Xingzhu Zou, Brajesh Kumar, Yuan Fang, Helong Guo, Yu Pan, Xinlei Chen, Jianghua Zhang, Edoardo P. Lagioia, Yuanpei Yang, Xiangkun Liu, Xiaowei Liu (all SWIFAR, YNU) report on behalf of the Mephisto Team:
The field of EP250511a (Lian et al., GCN 40429) was observed with the 1.6-meter Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University, located at Lijiang Observatory. The simultaneous observations in Mephisto u/g/i bands were started from 2025-05-11T13:58:03 (~6 h after the trigger). The field was also simultaneously observed in v/r/z bands, starting from 2025-05-11T14:07:46. In our stacked u/g/i images and the single frame v/z images (r-band image was saturated due to high background of moon light), we did not detect any source at the position reported by Schneider et. al. (GCN 40431) and Li et al. (GCN 40433). The preliminary 3-sigma upper limits (listed below) are consistent with Perez-Garcia et al. (GCN 40430), Lipunov et al. (GCN 40432), and Brivio et al. (GCN 40435).
Start_Time (UT) | Band | Exp(s) | LimMag(AB)
--------------------|------|--------|-------------
2025-05-11T13:58:03 | u | 180*2 | > 19.965
2025-05-11T14:07:46 | v | 180*1 | > 19.152
2025-03-09T20:07:03 | g | 60+180 | > 19.876
2025-05-11T13:58:02 | i | 180*2 | > 19.810
2025-05-11T14:07:45 | z | 180*1 | > 18.032
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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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- GCN Circular #40445
Malte Busmann (LMU), Brendan O’Connor (Carnegie Mellon U.), Daniel Gruen (LMU), and Antonella Palmese (Carnegie Mellon U.) report:
We observed the localization of EP250511a (Lian et al., GCN 40429; Li et al., GCN 40434) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i and J band simultaneously for 10 x 180 s starting at 2025-05-11T21:30:13 UT (0.56 days after the trigger). We do not detect the counterpart (Schneider et al., GCN 40431; Li et al., GCN 40433) placing 3 sigma upper limits at
r > 22.4 mag
i > 22.0 mag
J > 21.8 mag.
These upper limits are consistent with the mentioned detections and previous upper limits (Perez-Garcia et al., GCN 40430; Lipunov et al., GCN 40432; Brivio et al., GCN 40435; Zhong et al., GCN 40442).
The r and i band magnitudes are calibrated against the PS1 catalog and the J band is calibrated with the 2MASS Catalog. All magnitudes are provided in the AB system and are not corrected for Galactic extinction.
We thank Michael Schmidt from the Wendelstein Observatory staff for obtaining these observations.
- GCN Circular #40449
J. Sanchez-Sierras (Radboud), R. A. J. Eyles-Ferris (Leicester), A. Martin-Carrillo (UCD), A. J. Levan (Radboud and Warwick), G. Corcoran (UCD), P. G. Jonker (Radboud), F. E. Bauer (SSI and UTA), D. B. Malesani (DAWN/NBI and Radboud), J. A. Quirola-Vasquez (Radboud), report on behalf of a larger collaboration:
We observed the field of the fast X-ray transient EP250511a (Lian et al., GCN 40429) using the Gemini-South telescope. Observations started on 2025 May 12 at 01:19:15 UT (17.32 hr after the EP trigger), and consisted of 5x60 s exposures in each of the SDSS i and z bands.
At the location of the optical counterpart reported by Schneider et al. (GCN 40431) and Li et al. (GCN 40433) an extended source is clearly visible, with a magnitude of i = 23.04 +/- 0.06 (AB system, calibrated against nearby Pan-STARRS objects). This magnitude is brighter than the archival value reported in the Legacy Survey for the underlying candidate host galaxy (Schneider et al., GCN 40431). Image subtraction using PyZOGY (Zackay et al. 2016, doi:10.3847/0004-637X/830/1/27), using the Legacy Survey as template, reveals indeed a residual source with magnitude, i = 23.64 +/- 0.26 (AB system).
We thank excellent support from the observing staff at Gemini, in particular Cinthya Rodrigez and Joan Font.
- GCN Circular #40451
N. Pankov (HSE, IKI), A. Novichonok (Petrozavodsk State University, KIAM), I. Nikolenko (INASAN), A. Pozanenko (IKI) report on behalf of IKI-GRB-FuN:
We performed optical observations of the field of EP250511a (Lian et. al, GCN 40429; Li et. al, GCN 40434) using the 1-meter Zeiss-1000 telescope of the Simeiz (Koshka) Observatory. The R-band observations began on 2025-05-11 19:37:13 UT, i.e. ~0.51 days since trigger at a low altitude (~40 deg) and in the presence of the full Moon. The optical counterpart (Schneider et. al, GCN 40431; Li et. al, GCN 40433; Sanchez-Sierras et. al, GCN 40449) is not detected in the stacked image of an exposure of 86*60 sec. The preliminary upper limit is given below:
Date UT start t-T0 Exp. Filter OT Err. UL
(mid, days) (n*s) (3sigma)
2025-05-11 19:37:13 0.51405 86*60 R n/d n/d 18.9
The photometry was calibrated using nearby stars from USNO-B1.0 and has not been corrected for the Galactic extinction. Our result is consistent with non-detections reported by (Lipunov et. al, GCN 40432; Brivio et. al, GCN 40435; Zhong et. al, GCN 40442; Busmann et. al, GCN 40445).
Ref. stars
USNO-B1.0
RA Dec R2
202.1106 -2.6632 15.94
202.0790 -2.7119 15.27
202.0868 -2.7163 15.00
Previous IAU Circulars 