Gamma-ray Burst 250827B = EP250827a

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN Circular #41553
Y. L. Hua (PMO,CAS), K. R. Ni (CCNU) , M. J. Liu and 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 EP250827a. The transient triggered EP-WXT (ID: 01709200787) at 2025-08-27T07:40:43 (UTC). The WXT position of the source is R.A. = 3.488 deg, DEC = -56.463 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. = 3.46915 deg, DEC = -56.4998 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 #41554
A. J. Levan (Radboud and Warwick), J. A. Quirola-Vasquez (Radboud), 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 location of EP250827a (Hua et al. GCN 41553) with Gemini-South and the GMOS instrument. Observations began at 08:14 UT approximately 34 minutes after the EP trigger time. Within the WXT and FXT error circles we locate a new source in our first r-band exposure with a location of
```
RA(J2000): 00:13:52.32
DEC(J2000) -56:29:52.9
```
The error is about 0.3" in each coordinate. The source is not visible in the Legacy Survey and has a magnitude of r = 18.70 +- 0.02 (calibrated against nearby stars from the SkyMapper catalog and not corrected for Galactic extinction). This is very likely the optical counterpart of EP250827a.
We thank the staff of Gemini for their outstanding work in rapidly obtaining these observations.
GCN Circular #41555
Wenxiong Li (NAOC), Ido Keinan (TAU), Iair Arcavi (TAU), David Sand (U of Arizona)
We observed the position of EP250827a (Hua et al. GCN 41553) with a Las Cumbres 1m telescope at CTIO starting on 2025-08-27 07:51:31 UT (less than 5 minutes after the alert and less than 11 minutes after the event). We obtained 2x300s exposures in the broad optical w band.
We find a source at RA=3.4679 Dec=56.4979, not present in archival Legacy Survey (https://www.legacysurvey.org/viewer) images and measure a magnitude of 18.71 +- 0.06, calibrated to the r band (consistent with the Gemini detection; Levan et al. GCN 41554).
Addition followup is encouraged.

GCN Circular #41556
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 EP250827a ( EP Team et al., GCN 41553) errorbox 371 sec after trigger time at 2025-08-27 07:46:54 UT, with upper limit up to 21.0 mag. The observations began at zenith distance = 29 deg. The sun altitude is -41.2 deg.
The galactic latitude b = -60 deg., longitude l = 313 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2978010
We obtain a following upper limits.

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

     407 | 2025-08-27 07:46:54 |         MASTER-OAFA | (00h 13m 54.06s , -56d 38m 35.7s) |   C |    70 | 20.5 |        
     492 | 2025-08-27 07:48:10 |         MASTER-OAFA | (00h 13m 54.34s , -56d 38m 33.9s) |   C |    90 | 20.7 |        
     599 | 2025-08-27 07:49:46 |         MASTER-OAFA | (00h 13m 54.64s , -56d 38m 32.6s) |   C |   110 | 20.8 |        
     680 | 2025-08-27 07:51:43 |         MASTER-OAFA | (00h 13m 54.87s , -56d 38m 31.2s) |   C |    40 | 20.2 |        
     726 | 2025-08-27 07:52:29 |         MASTER-OAFA | (00h 13m 55.02s , -56d 38m 30.5s) |   C |    40 | 20.2 |        
     772 | 2025-08-27 07:53:14 |         MASTER-OAFA | (00h 13m 55.19s , -56d 38m 29.9s) |   C |    40 | 20.3 |        
     817 | 2025-08-27 07:54:00 |         MASTER-OAFA | (00h 13m 55.31s , -56d 38m 29.1s) |   C |    40 | 20.2 |        
     863 | 2025-08-27 07:54:46 |         MASTER-OAFA | (00h 13m 55.44s , -56d 38m 28.5s) |   C |    40 | 20.2 |        
     909 | 2025-08-27 07:55:31 |         MASTER-OAFA | (00h 13m 55.59s , -56d 38m 27.9s) |   C |    40 | 20.3 |        
     955 | 2025-08-27 07:56:18 |         MASTER-OAFA | (00h 13m 55.72s , -56d 38m 27.0s) |   C |    40 | 20.2 |        
     995 | 2025-08-27 07:56:18 |         MASTER-OAFA | (00h 13m 55.72s , -56d 38m 27.0s) |   C |   120 | 20.9 |  Coadd 
    1001 | 2025-08-27 07:57:03 |         MASTER-OAFA | (00h 13m 55.84s , -56d 38m 26.2s) |   C |    40 | 20.2 |        
    1046 | 2025-08-27 07:57:49 |         MASTER-OAFA | (00h 13m 55.97s , -56d 38m 25.7s) |   C |    40 | 20.2 |        
    1093 | 2025-08-27 07:58:35 |         MASTER-OAFA | (00h 13m 56.15s , -56d 38m 25.0s) |   C |    40 | 20.2 |        
    1133 | 2025-08-27 07:58:35 |         MASTER-OAFA | (00h 13m 56.15s , -56d 38m 25.0s) |   C |   120 | 21.0 |  Coadd 
    1138 | 2025-08-27 07:59:21 |         MASTER-OAFA | (00h 13m 56.27s , -56d 38m 24.5s) |   C |    40 | 20.2 |        
    1184 | 2025-08-27 08:00:06 |         MASTER-OAFA | (00h 13m 56.41s , -56d 38m 23.7s) |   C |    40 | 20.3 |        
    1230 | 2025-08-27 08:00:52 |         MASTER-OAFA | (00h 13m 56.58s , -56d 38m 23.2s) |   C |    40 | 20.2 |        
    1270 | 2025-08-27 08:00:52 |         MASTER-OAFA | (00h 13m 56.57s , -56d 38m 23.2s) |   C |   120 | 21.0 |  Coadd 
    1276 | 2025-08-27 08:01:38 |         MASTER-OAFA | (00h 13m 56.69s , -56d 38m 22.7s) |   C |    40 | 20.2 |        
    1321 | 2025-08-27 08:02:24 |         MASTER-OAFA | (00h 13m 56.85s , -56d 38m 21.8s) |   C |    40 | 20.2 |        
    1367 | 2025-08-27 08:03:10 |         MASTER-OAFA | (00h 13m 56.98s , -56d 38m 21.1s) |   C |    40 | 20.2 |        
    1413 | 2025-08-27 08:03:55 |         MASTER-OAFA | (00h 13m 57.14s , -56d 38m 20.5s) |   C |    40 | 20.2 |        
    1458 | 2025-08-27 08:04:40 |         MASTER-OAFA | (00h 13m 57.27s , -56d 38m 19.9s) |   C |    40 | 20.2 |        
    1504 | 2025-08-27 08:05:26 |         MASTER-OAFA | (00h 13m 57.42s , -56d 38m 19.4s) |   C |    40 | 20.2 |        
    1544 | 2025-08-27 08:05:26 |         MASTER-OAFA | (00h 13m 57.42s , -56d 38m 19.4s) |   C |   120 | 21.0 |  Coadd 
    1549 | 2025-08-27 08:06:12 |         MASTER-OAFA | (00h 13m 57.57s , -56d 38m 18.6s) |   C |    40 | 20.2 |        
    1595 | 2025-08-27 08:06:58 |         MASTER-OAFA | (00h 13m 57.70s , -56d 38m 18.1s) |   C |    40 | 20.2 |        
    1642 | 2025-08-27 08:07:44 |         MASTER-OAFA | (00h 13m 57.88s , -56d 38m 17.5s) |   C |    40 | 20.2 |        
    1687 | 2025-08-27 08:08:30 |         MASTER-OAFA | (00h 13m 58.02s , -56d 38m 17.0s) |   C |    40 | 20.1 |        
    1732 | 2025-08-27 08:09:15 |         MASTER-OAFA | (00h 13m 58.16s , -56d 38m 16.4s) |   C |    40 | 20.1 |        
    3986 | 2025-08-27 08:46:38 |         MASTER-OAFA | (00h 14m 05.29s , -56d 37m 56.2s) |   C |    60 | 19.4 |        
    4046 | 2025-08-27 08:46:38 |         MASTER-OAFA | (00h 14m 05.29s , -56d 37m 56.2s) |   C |   180 | 20.0 |  Coadd 
    4061 | 2025-08-27 08:47:53 |         MASTER-OAFA | (00h 14m 08.12s , -56d 36m 11.6s) |   C |    60 | 19.3 |        
    4137 | 2025-08-27 08:49:10 |         MASTER-OAFA | (00h 14m 03.64s , -56d 37m 03.5s) |   C |    60 | 19.3 |        
    4215 | 2025-08-27 08:50:28 |         MASTER-OAFA | (00h 14m 02.43s , -56d 36m 02.5s) |   C |    60 | 19.3 |        
    4275 | 2025-08-27 08:50:28 |         MASTER-OAFA | (00h 14m 02.43s , -56d 36m 02.5s) |   C |   180 | 20.0 |  Coadd 
    4291 | 2025-08-27 08:51:43 |         MASTER-OAFA | (00h 14m 10.29s , -56d 37m 02.2s) |   C |    60 | 19.4 |        
    4369 | 2025-08-27 08:53:01 |         MASTER-OAFA | (00h 14m 02.79s , -56d 38m 01.6s) |   C |    60 | 19.5 |        
    4447 | 2025-08-27 08:54:19 |         MASTER-OAFA | (00h 14m 09.55s , -56d 37m 48.8s) |   C |    60 | 19.4 |        
    4525 | 2025-08-27 08:55:37 |         MASTER-OAFA | (00h 14m 07.17s , -56d 36m 15.0s) |   C |    60 | 19.3 |        
    4599 | 2025-08-27 08:56:51 |         MASTER-OAFA | (00h 14m 07.39s , -56d 37m 46.8s) |   C |    60 | 19.3 |        
    4674 | 2025-08-27 08:58:07 |         MASTER-OAFA | (00h 14m 10.80s , -56d 36m 08.9s) |   C |    60 | 19.4 |        
    4750 | 2025-08-27 08:59:23 |         MASTER-OAFA | (00h 14m 04.33s , -56d 36m 58.4s) |   C |    60 | 19.4 |        
    4827 | 2025-08-27 09:00:39 |         MASTER-OAFA | (00h 14m 05.93s , -56d 35m 57.5s) |   C |    60 | 19.4 |        
    4900 | 2025-08-27 09:02:02 |         MASTER-OAFA | (00h 14m 10.40s , -56d 36m 57.1s) |   C |    40 | 20.2 |        
    4945 | 2025-08-27 09:02:48 |         MASTER-OAFA | (00h 14m 10.58s , -56d 36m 56.9s) |   C |    40 | 20.2 |        
    4991 | 2025-08-27 09:03:33 |         MASTER-OAFA | (00h 14m 10.70s , -56d 36m 56.6s) |   C |    40 | 20.1 |        
    5047 | 2025-08-27 09:04:30 |         MASTER-OAFA | (00h 14m 04.87s , -56d 37m 56.3s) |   C |    40 | 20.1 |        
    5093 | 2025-08-27 09:05:16 |         MASTER-OAFA | (00h 14m 05.01s , -56d 37m 55.8s) |   C |    40 | 20.1 |        
    5138 | 2025-08-27 09:06:01 |         MASTER-OAFA | (00h 14m 05.17s , -56d 37m 55.6s) |   C |    40 | 20.1 |        
    5193 | 2025-08-27 09:06:55 |         MASTER-OAFA | (00h 14m 11.73s , -56d 37m 28.0s) |   C |    40 | 20.1 |        
    5238 | 2025-08-27 09:07:41 |         MASTER-OAFA | (00h 14m 11.89s , -56d 37m 27.8s) |   C |    40 | 20.2 |        
    5284 | 2025-08-27 09:08:26 |         MASTER-OAFA | (00h 14m 12.04s , -56d 37m 27.5s) |   C |    40 | 20.2 |        
    5342 | 2025-08-27 09:09:25 |         MASTER-OAFA | (00h 14m 09.80s , -56d 36m 14.9s) |   C |    40 | 20.2 |        
    5388 | 2025-08-27 09:10:10 |         MASTER-OAFA | (00h 14m 09.96s , -56d 36m 14.4s) |   C |    40 | 20.1 |        
    5433 | 2025-08-27 09:10:56 |         MASTER-OAFA | (00h 14m 10.12s , -56d 36m 14.4s) |   C |    40 | 20.1 |        
    5489 | 2025-08-27 09:11:52 |         MASTER-OAFA | (00h 14m 10.28s , -56d 37m 47.0s) |   C |    40 | 20.1 |        
    5535 | 2025-08-27 09:12:37 |         MASTER-OAFA | (00h 14m 10.41s , -56d 37m 46.7s) |   C |    40 | 20.1 |        
    5580 | 2025-08-27 09:13:23 |         MASTER-OAFA | (00h 14m 10.58s , -56d 37m 46.4s) |   C |    40 | 20.2 |        
    5637 | 2025-08-27 09:14:19 |         MASTER-OAFA | (00h 14m 12.90s , -56d 35m 55.2s) |   C |    40 | 20.1 |

Filter C is a clear (unfiltred) band.
The observation and reduction will continue. The message may be cited.

GCN Circular #41557
A. J. Levan (Radboud and Warwick), J. A. Quirola-Vasquez (Radboud), P. G. Jonker (Radboud), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), R. A. J. Eyles-Ferris (Leicester), A. Martin-Carrillo (UCD), report on behalf of a larger collaboration:
We observed the location of the optical counterpart (Levan et al., GCN 41554) to EP250827a (Hua et al., GCN 41553) with Gemini-South and the GMOS instrument using the B480 grating and a 1 arcsec slit. Four 300 s exposures were obtained. Observations began on 2025 Aug 27 at 08:51 UT, approximately 71 min after the EP trigger time. The data was reduced using DRAGONS.
Several metallic absorption lines (due to e.g. Fe II and Mg II) at a redshift of z = 1.613 are present, suggesting the FXT lies at this redshift or beyond.
We thank the staff of Gemini, especially K. Silva, for their outstanding work in rapidly obtaining these observations.
GCN Circular #41558
V.Lipunov, A.Kuznetsov, I.Panchenko, G.Antipov, P.Balanutsa, K.Zhirkov (Lomonosov MSU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (OAFA), A.Chasovnikov, E.Gorbovskoy, N.Tiurina, Ya.Kechin, D.Vlasenko, V.Senik, M. Shilova, I.Lipunov, V.Topolev (Lomonosov MSU), O.Gress, N.Budnev(ISU), D.Buckley (SAAO), A.Sosnovskij (Crao RAS), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), 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 AstrophysicsObservatory A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)
MASTER-OAFA robotic telescope (Lipunov et al. GCN 41556) started EP GRB 250827A observation at 2025-08-27 07:46:54 UT and detected optical couterpart of this GRB MASTER OT J001352.27-562952.8 with m_OT (unfiltered) = 18.9m (preliminary) at 2025-08-27 07:48:10 https://master.sai.msu.ru/site/master2/event.php?id=2978010 The light curve demonstrates multipeak structure from 18 to 19 during last hour.
This OT coincide with GEMINI-South detection ( GCN 41554 Levan et al.) and Las Cumbres one (GCN 41555 Wenxiong Li et al.)
GCN Circular #41559
M. Gritsevich (Univ. of Helsinki), I. Perez-Garcia, E. Fernandez-Garcia, S.-Y. Wu, M. D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy, and A. J. Castro-Tirado (IAA-CSIC, Granada), C. Perez del Pulgar (Univ. of Malaga), D.-R. Xiong (YNAO, Kumming), Y.-D. Hu (GXU, Guanxi), B.-B. Zhang (Nanjing Univ.), and A. Maury (Space Obs., San Pedro de Atacama), on behalf of a larger collaboration, report:
Following the detection of EP250827a by Einstein Probe (Hua et al., GCNC 41553), the 0.6m BOOTES-7 robotic telescope at Space Observatory (San Pedro de Atacama, Chile) automatically observed the X-ray transient location starting on Aug. 27, 07:47:36 UT (i.e. 413 s after trigger). A series of images in clear filter were gathered for 2 h and the optical counterpart reported by Levan et al. (GCNC 41554), Li et al. (GCNC 41555) and Lipunov et al. (GCNC 41558) was detected at early times in our dataset. We measure a magnitude of 18.67 +/- 0.05 on a co-added image (60s x 5 exposures) at 07:53:11 UT mid time. Further data analysis is ongoing.
We would like to thank the staff at San Pedro de Atacama Space Observatory for their excellent support.
GCN Circular #41560
J. An (NAOC), S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), S.Y. Fu (HUST), L.B. He, X. Liu, Z.P. Zhu, S.Q. Jiang, Z. Fan, W.X. Li, N.C. Sun, Y.N. Wang, D. Xu (NAOC) report on behalf of a large collaboration:
We observed the field of EP250827a (Hua et al., GCN 41553), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Cerro Tololo Inter-American Observatory, Chile. (CTO) Observations started at 08:19:45 UTC on 2025-08-27, i.e., ~ 39 min after the EP/WXT trigger and a series of frames in the r band were obtained.
We clearly detect the optical counterpart(Levan et al., GCN 41554; Li et al., GCN 41555; Lipunov et al., GCN 41558; Gritsevich et al., GCN 41559) in our images. Preliminary photometry shows that the source has r ~ 18.93 +/- 0.03 mag at 51.6 min post-trigger, calibrated with Legacy DR10 catalog and not corrected for Galactic extinction.
GCN Circular #41563
P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi (INAF-IASFPA), A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N. Burrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-OAR), A. D'Ai (INAF-IASFPA), P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), R.A.J. Eyles-Ferris (U. Leicester), M. Ferro (INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of the Einstein Probe/WXT-detected source EP250827a, collecting 1.6 ks of Photon Counting (PC) mode data between T0+3.1 ks and T0+9.2 ks after the trigger. A likely counterpart has been found. The details of this source are:
Source 1 (SWIFT J001351.7-562952):
==================================
```
    RA (J2000.0):   3.4656  =  00 13 51.74
    Dec (J2000.0):  -56.4979  =  -56 29 52.4
    Error:	    3.7 (arcsec, radius, 90% confidence).
    Detect flag:    GOOD
    Distance:	    2.2 arcmin from the Einstein Probe/WXT position.
    Mean rate:	    0.161 +/- 0.011 ct s^-1
    Mean flux:	    (5.48 +/- 0.39)e-12 erg cm^-2 s^-1
    Peak rate:	    0.253 +/- 0.057 ct s^-1
    Peak flux:	    (8.6 +/- 1.9)e-12 erg cm^-2 s^-1
    ECF:	    3.41e-11 erg cm^-2 ct^-1
		      assuming NH=1.45e+20 cm^-2, gamma=1.93
		      determined from a spectral fit.
    XMM UL:	    2.2e-12 erg cm^-2 s^-1, (0.3-10 keV)
		      so the source is 3.3-sigma above this 3-sigma upper limit.
    The source may be fading, at the 1.5-sigma level.
```
This position is consistent with the optical counterpart previously reported (Levan et al. GCN 41554, Li et al. GCN 41555, Lipunov et al. GCN 41558, Gritsevich et al. GCN 41559, An et al. GCN 41560) at a redshift of 1.613 (Levan et al. GCN 41557).
All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with catalogues and upper limits from other missions, we assumed a power-law spectrum with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated.
The results of the XRT-team automatic analysis of the XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/EP/EP_FIELD00062.
This circular is an official product of the Swift-XRT team.
GCN Circular #41564
Ankur Ghosh, Soebur Razzaque (CAPP, University of Johannesburg), Rahul Gupta (NASA GSFC), Alexander Moskvitin, Yulia Sotnikova (SAO RAS), Naveen Dukiya (ARIES) on behalf of a larger collaboration.
We observed the field of the EP 250827A triggered by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Hua et al., GCN 41553) in the r, V filter of the 1-meter Sinistro telescope at the Las Cumbres Observatory Global Telescope (LCOGT) node located at Siding Spring is near Coonabarabran, New South Wales, Australia . The 1-m Sinistro telescope is equipped with a 4K x 4K CCD (FOV: 26 x 26 arcmin, scale: 0.39 arcsec/pixel).
Observations began on August 27, 2025, starting 5.135 hours after the GRB trigger. Further observations are currently ongoing.
We clearly detect the optical transient (OT) reported by GCNs (Levan et al., GCN 41554; Li et al., GCN 41555; Lipunov et al., GCN 41558; Gritsevich et al., GCN 41559, An et al., GCN 41660) in our r band image. A brightening of ≈ 0.5 mag was observed within 1–5 hours after the burst, with An et al,. GCN 41660, reported a magnitude of 18.9 mag at ~1 hour post-burst.
```
|Date|		|UTstart|	|t-T0 (hours)|	|Exp (sec)|	|Filter|	|Magnitude| 
--------------------------------------------------------------------------------------------------      
2025-08-27	12:48:48.96	5.135		1 x 900 	r	        r = 18.44 +/- 0.01
```
The field was calibrated against nearby APASS stars, with magnitudes converted using Lupton (2005) equations, and has not been corrected for Galactic extinction.
GCN Circular #41567
D. O'Neill, B. P. Gompertz, G. Ramsay, A. Kumar, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, B. Godson, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares
Report on behalf of the GOTO collaboration:
We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to the EP250827a (Hua et al. GCN 41553). Targeted observations were taken at 2025-08-27 15:50:54 UT (+8.17h post trigger). The observations consisted of 4x90s exposures in the GOTO L-band (400-700 nm). Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations.
We detect the optical counterpart (Levan et al., GCNs 41554, 41557; Li et al., GCN 41555; Lipunov et al., GCN 41558; Gritsevich et al., GCN 41559, An et al., GCN 41660, Ghosh et al. GCN 41564) at L = 18.63 ± 0.04 AB mag. We find no evidence of the source prior to the GRB trigger time in previous GOTO observations taken at 2025-08-25 12:46:35 UT down to a 5-sigma limit L>19.42 AB mag. Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and not corrected for Galactic extinction.
GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC).
GCN Circular #41568
I. Pérez-Fournon, F. Poidevin (IAC and ULL), D. Cano-Morales, A.E. Hernández-Díaz, I. Correa-Plasencia (ULL), and A. López-Oramas (IAC and ULL)
We observed the field of EP250827a, detected by Einstein Probe (Hua et al. GCN 41553) and Swift-XRT (Evans et al. GCN 41563) at a redshift of z = 1.613 (Levan et al. GCN 41557) with one of the three 1-m telescopes of the Las Cumbres Observatory Global Telescope network (LCO) equipped with Sinistro cameras located at the LCO node at the Cerro Tololo Inter-American Observatory (CTIO), Chile. The observation, of 300 seconds in the SDSS-r filter, started at 2025-08-27 10:04:04 UT, about 2.39 hours after the EP trigger. We detect clearly a source within the EP-FXT and Swift-XRT error circles at a position consistent with the detections reported by Levan et al. (GCN 41554) and Li et al. (GCN 41555) and with other optical detections reported by Lipunov et al., GCN 41558; Gritsevich et al., GCN 41559; An et al., GCN 41560, Ghosh et al., GCN 41564; and O'Neill GCN 41567). We measure a magnitude of r = 18.65 +/- 0.04, calibrated with the Legacy Surveys DR10 catalog and not corrected for Galactic extinction. Using a similar r-band filter, we confirm the optical rebrightening detected also by Ghosh et al. (GCN 41564) between the TRT observation (r = 18.93 +/- 0.03, 0.86 hours after the EP trigger) and our LCO observation (r = 18.65 +/- 0.04, 2.39 hours after the EP trigger).
This work makes use of observations from the Las Cumbres Observatory global telescope network program IAC2025B-008 (SGLF).
GCN Circular #41570
Niccolò Passaleva (U Rome), Rosa L. Becerra (U Rome) , Massine El Kabir (U Rome), Yu-Han Yang (U Rome), Eleonora Troja (U Rome) report on behalf of the ERC BHianca team:
We observed the optical counterpart of EP250827a (Hua et al., GCN 41553) reported by Levan et al. (GCN 41554) and Wenxiong et al. (GCN 41555) with the X-Shooter spectrograph on the ESO VLT UT3 (Melipal). Observations began at T+1.67 hours and obtained a total of 4x600s spectra at an average airmass of about 1.45.
At the position of the optical counterpart we detect a bright continuum. We identify absorption features (e.g. Fe II, Mg I, Mg II, C lV, Si IV, Si II, Al II, Al III) at a common redshift of z ~ 1.61 confirming the result reported by Levan et al. (GCN 41557).
We thank the staff at the VLT, for the rapid execution of these observations.
GCN Circular #41573
K. R. Ni (CCNU), Y. L. Hua (PMO,CAS), M. J. Liu and C. C. Jin (NAO,CAS) on behalf of the Einstein Probe (EP) team:
The X-ray transient EP250827a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Hua et al., GCN 41553). We report on the refined analysis of WXT and FXT observations. The WXT data shows that the event started at T0=2025-08-27T07:39:01 (UTC) and lasted for at least 90 s. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 1.19 x 10^20 cm^-2 and a photon index of 0.73 (-0.47/+0.50). The derived average unabsorbed 0.5-4 keV flux is 1.71(-0.34/+0.42) x 10^(-9) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 102 s after T0. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 3.46915, DEC = -56.4998 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), within the WXT error circle. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 1.19 x 10^20 cm^-2 and a photon index of 2.0(-0.10/+0.10). The derived average unabsorbed 0.5-10 keV flux is 4.38 (-0.38/+0.39) x 10^(-12)erg/s/cm^2. 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).
GCN Circular #41576
R. A. J. Eyles-Ferris (Leicester), J. A. Quirola-Vasquez (Radboud), A. J. Levan (Radboud and Warwick), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), F. E. Bauer (SSI and UTA), and J. Chacón (PUC) report on behalf of a larger collaboration:
We observed the location of the optical counterpart (Levan et al., GCN 41554; Li et al., GCN 41555; Levan et al., GCN 41557; Lipunov et al., GCN 41558; Gritsevich et al., GCN 41559; An et al., GCN 41560; Ghosh et al., GCN 41564; O’Neill et al., GCN 41567; Pérez-Fournon et al., GCN 41568; Passaleva et al., GCN 41570) to EP250827a (Hua et al., GCN 41553; Ni et al., GCN 41573) with the Sinistro instruments mounted on 1-m telescopes at the LCO SAAO and Cerro Tololo Inter-American Observatory nodes. 5x300 s exposures were obtained in each of the SDSS u, g and r filters with start times from 2025 Aug 27 at 22:38:04 to 2025 Aug 28 04:36:48 UT.
The counterpart is clearly detected in both the g and r filters and is marginally detected in the u filter image. At 15.0 hr after the EP trigger, we measure r = 19.99 +/- 0.05, ~1.5 mags fainter than observed by Ghosh et al. (GCN 41564). Assuming an unbroken power-law decay between the two epochs, we derive a temporal index alpha ~ 1.4 for the decay.
GCN Circular #41580
A. Santos (CBPF), C. R. Bom (CBPF), C. D. Kilpatrick (Northwestern), L. Santana-Silva (CBPF), P. Darc (CBPF), Gabriel Teixeira (CBPF), C. Mendes de Oliveira (IAG-USP) report on behalf of the STEP collaboration:
We conducted optical follow up with the T80S 0.8-m robotic telescope as part of the S-PLUS Transient Extension Program (Santos et al., 2024, MNRAS, 529, 59) targeting the fast X-ray transient EP250827a by the Einstein Probe instrument (GCN 41533). The T80S observations started on Aug 28 06:34:09 UT (~23 hours after the trigger). We obtained imaging with exposure time 2x120s r-band and 2x150s in i-band with the T80S camera centered at the position of the optical afterglow detection reported in Levan et al., (GCN 41554); Li et al., GCN (41555); Levan et al., (GCN 41557); Lipunov et al., (GCN 41558); Gritsevich et al., (GCN 41559); An et al., (GCN 41560); Ghosh et al., GCN (41564); O’Neill et al., (GCN 41567); Pérez-Fournon et al., (GCN 41568); Passaleva et al., (GCN 41570) to EP250827a Hua et al., (GCN 41553); Ni et al., (GCN 41573) and Eyles-Ferris et al. (GCN 41576).
Subtracting DECam template images from the T80S stacked frames using photpipe (Rest et al., 2005, ApJ, 634, 1103), we detect the afterglow at the previously reported position.
```
MEAN-MJD FILTER Magnitude Uncertainty Limit (3sigma)
60915.2823 r 21.32 0.12 22.86
60915.2778 i 20.98 0.16 22.39
```
The counterpart is detected both in r and i bands. Given the time of imaging at 23.0 hr after the EP trigger and the measured magnitude, we argued the optical counterparting is fading after the rebrightening reported by Ghosh et al. (GCN 41564), confirming the fading reported by Eyles-Ferris (GCN 41576).
We acknowledge the T80S technical team with the provided support during the night.
GCN Circular #41585
M. H. Siegel (PSU) reports on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of EP250827a 3.2 ks after the EP trigger (Hua et al., GCN Circ. 41553). An uncatalogued source consistent with the XRT position (Evans et al., GCN Circ. 41563) and the optical transient (Levan et al., GCN Circ. 41554; Li et al., GCN Circ. 41555; Lipunov et al., GCN Circ. 41558; Gritsevich et al., GCN Circ. 41559; An et al., GCN Circ. 41560; Ghosh et al., GCN Circ. 41564; O'Neill et al., GCN Circ. 41567) is detected in the UVOT exposures.
The preliminary UVOT position is:
```
    RA  (J2000) =  00:13:52.24 =   3.46767 (deg.)
    Dec (J2000) = -56:29:52.2  = -56.49784 (deg.) 
```
with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence).
Preliminary detections and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are:
```
Filter         T_start(s)   T_stop(s)      Exp(s)           Mag

u                 3192         9257         1566      18.29+/-0.04
u                54232        60071          939      19.69+/-0.19
u               121367       127685          754            >20.36
uvw1             53286        59775         1903      19.86+/-0.13
uvw1            120301       127006         2040            >21.21
```
The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.011 in the direction of the burst (Schlegel et al. 1998)
GCN Circular #41596
I. Pérez-Fournon, F. Poidevin (IAC and ULL), D. Cano-Morales, A.E. Hernández-Díaz, I. Correa-Plasencia (ULL), A. López-Oramas, and D.S. Aguado (IAC and ULL)
We report on very early ATLAS detections of the optical afterglow of EP250827a, a fast X-ray transient discovered by Einstein Probe WXT (ID: 01709200787) on 2025-08-27 07:40:43 UTC (Hua et al., GCN 41553; and Ni et al., GCN 41573), with UV-optical detections reported in Levan et al. (GCN 41554), Li et al. (GCN 41555), Levan et al. (GCN 41557), Lipunov et al. (GCN 41558), Gritsevich et al. (GCN 41559), An et al. (GCN 41560), Ghosh et al. (GCN 41564), O’Neill et al. (GCN 41567), Pérez-Fournon et al. (GCN 41568), Passaleva et al. (GCN 41570), Eyles-Ferris et al. (GCN 41576), Santos et al. (GCN 41580), and Siegel et al. (GCN 41585), spectroscopic redshift of z = 1.613-1.61 (Levan et al., GCN 41557; and Passaleva et al., GCN 41570) and Swift-XRT follow-up observations (Evans et al., GCN 41563).
Using the ATLAS forced-photometry server (Shingles et al. 2021, Transient Name Server AstroNote 2021-7), we found that there were four ATLAS observations that covered the EP250827a position on 2025-08-27, two of them before the EP trigger and two after it. As expected, the source was not detected in the ATLAS pre-trigger observations but it is well detected in the two post-trigger observations, the first of them at a start time of only about 43 sec (about 16 sec in the source rest frame) after the EP trigger. There is a flux rise between the two observations. The source is well detected in both the original (reduced) and difference (reduced - reference) ATLAS images. For the forced photometry measurements, we used the coordinates of the optical counterpart given by Levan et al. (GCN 41554) from the analysis of Gemini-South GMOS observations.
In the following, we give the ATLAS limiting magnitudes and forced-photometry magnitudes (using difference images), not corrected for Galactic extinction.
```
MJD           | 5-sigma limit  | filter | ATLAS observation |
60914.306552  |      20.14     | orange | 04a60914o0247o    |
60914.313502  |      20.18     | orange | 04a60914o0252o    |


MJD          | mag    | error | filter | ATLAS observation | 
60914.320437 | 18.707 | 0.070 | orange | 04a60914o0257o    |
60914.327372 | 18.568 | 0.065 | orange | 04a60914o0262o    |
```
The reported ATLAS observations MJD times are the start times of the individual 30-sec exposures.
The first detection is likely the earliest one after the trigger of an optical afterglow of EP-discovered fast X-ray transients.
GCN Circular #41708
M. E. Ravasio and P.G. Jonker (Radboud Univ.) on behalf of the Einstein Probe Team and E. Burns (LSU), C. M. Hui (NASA MSFC) and R. Hamburg (USRA) on behalf of the Fermi-GBM Team, report:
Fermi-GBM had full spatial coverage of the transients EP250827a (Hua et al., GCN 41553, Ni et al., GCN 41573) and EP250827b (Schroeder et al., GCN 41635) detected by EP-WXT. There was no Fermi-GBM onboard trigger around both EP trigger times (T0=2025-08-27T07:39:01 UTC for EP250827a, and T0=2025-08-27T06:22:27 UTC for EP250827b).
The GBM targeted search [1], the most sensitive, coherent search for GRB-like signals, was run in the time interval [T0-50; T0+500] s, seeking signals between 64 ms and 32.768 s in duration. No signal consistent with the EP transients, both temporally and spatially, is identified, as confirmed also by visual inspection of the data.
Assuming a “soft” spectral template (Band function with Epeak = 70 keV, alpha = -1.9, beta = -3.7), and a duration of 8.192 s, we derive a flux upper limit of 3.5e-08 erg/cm2/s for EP250827a, and a flux upper limit of 3.0e-08 erg/cm2/s for EP250827b, in the energy band 10-1000 keV. [1] Goldstein et al. 2019 arXiv:1903.12597

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Jochen Greiner, last update: 04-Sep-2025 [Disclaimer]