EP250828a
GCN Circular 41652
Subject
EP250828a: Early near-infrared follow-up with MIRAGE on the MDM 1.3m telescope
Date
2025-09-02T04:39:41Z (2 months ago)
From
kde1@mit.edu
Via
Web form
K. De (Columbia/CCA), S. Ibrahim (Columbia), T. Suen (Columbia), M. Sitaram (Columbia), D. Schiminovich (Columbia), G. Mo (Caltech/Carnegie), K. Shin (Caltech)
We observed the field of the X-ray transient EP250828a (Liu et al., GCN 41574; Aryan et al., GCN 41579; Taguchi et al., GCN 41582; Yao et al., GCN 41583; Fu et al., GCN 41584; Zheng et al., GCN 41586; Lipunov et al., GCN 41588; Bochenek et al., GCN 41590; Aryan et al., GCN 41591; Ni et al., GCN 41592) during commissioning operations of the MDM InfraRed Astronomy inGaas Explorer (MIRAGE) instrument on the MDM 1.3m telescope. Observations were carried out starting at UTC 2025-08-28 10:59:08, starting ~ 3.4 hours after the time of the X-ray detection.
We obtained a series of dithered exposures in Y-band amounting to a total exposure time of 850 s. Images were aligned, stacked and calibrated to the PS1 catalog. Performing forced photometry at the position of the X-ray and optical counterpart reported in GCN 41582, 41583, 41591, 41592, we do not detect a point source to a 5 sigma limiting magnitude of > 19.1 AB mag. Analysis will be improved as further commissioning data is processed.
MIRAGE is a new YJH-band near-infrared imager for the MDM 1.3m telescope. We thank the MDM Observatory staff for supporting the commissioning observations.
GCN Circular 41592
Subject
EP250828a: refined analysis of the EP-WXT and EP-FXT observations
Date
2025-08-29T13:13:17Z (2 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
K. R. Ni (CCNU), H. L. Peng (NNU), H. Y. Liu, Y. J. Song and H. Sun (NAO,CAS) on behalf of the Einstein Probe (EP) team:
We report on the refined analysis of EP-WXT and FXT observations of the fast X-ray transient EP250828a (Liu et al., GCN 41574), with the optical couterparts observed by Seimei (Taguchi et al., GCN 41582), SVOM/VT (Yao et al., GCN 41583), NOT (Fu et al., GCN 41584), Kinder (Aryan et al., GCN 41591). The WXT data shows that the event started at T0=2025-08-28T07:27:16 (UTC) and lasted for around 1500s. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 7.75 x 10^20 cm^-2 and a photon index of 1.48 (-0.59/+0.63). The derived average unabsorbed 0.5-4 keV flux is 1.25(-0.39/+0.50) x 10^(-10) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
The EP-FXT observed the field of EP250828a about 1.4 hrs after T0. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 327.8932, DEC = 20.5429 (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 7.75 x 10^20 cm^-2 and a photon index of 2.04(-0.41/+0.43). The derived average unabsorbed 0.5-10 keV flux is 4.43 (-1.29/+1.90) x 10^(-13)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 41591
Subject
EP250828a: Optical counterpart detection with Kinder observations
Date
2025-08-29T10:12:24Z (2 months ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
Web form
A. Aryan, C.-S. Lin (both NCU), Y. J. Yang (NYUAD), A. K. H. Kong (NTHU), T.-W. Chen (NCU), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), Y.-H. Lee, A. Sankar. K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, C.-H. Lai, W.-J. Hou, H.-C. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), S. Yang, Z. N. Wang, L. L. Fan, G. H. Sun (all HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz and M. Huber (both IfA, Hawaii) report:
Followed by our r-band observations using the 1m LOT (Aryan et al., GCN 41579), we continued to observe the field of the fast X-ray transient EP250828a (Liu et al., GCN 41574) in i-band using the 40cm SLT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86).
The first SLT epoch of observations in i-band started at 14:17 UTC on the 28th of August 2025 (MJD 60915.595), 6.74 hr after the EP-WXT trigger.
We utilized the astroalign (Beroiz et al. 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al. 2022, ApJ, 935, 167) packages to align and stack the individual frames. We utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction with the Pan-STARRS1 (Chambers et al., 2016 arXiv:1612.05560) image using the 'sfft' (Hu et al., 2022, ApJ, 936,157) algorithm. In the stacked as well as in the difference image, we weakly detected the optical counterpart candidate proposed by Taguchi et al., (GCN 41582). The candidate counterpart was also confirmed by Yao et al. (GCN 41583) and Fu et al. (GCN 41584).
In our i-band difference image, the optical counterpart was detected with an SNR of 2.5. The details of the observations and the measured photometry (in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
SLT | i | 60915.595 | 6.74 | 300 * 24 | 22.46 +/- 0.27 | 1".48 | 1.01
The presented magnitude is calibrated using the field stars from the Pan-STARRS1 catalog and is not corrected for the expected Galactic foreground extinction of A_i = 0.21 mag in the direction of the transient (Schlafly & Finkbeiner 2011).
GCN Circular 41590
Subject
EP250828a: Liverpool Telescope upper limits
Date
2025-08-29T09:28:38Z (2 months ago)
From
A. Bochenek at Liverpool John Moores University <a.m.bochenek@2023.ljmu.ac.uk>
Via
Web form
A. Bochenek and D. A. Perley (LJMU) report:
We observed the field of the fast X-ray transient EP250828a (Liu et al., GCN 41574) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 6x100s exposures in the SDSS r’ and i’ filters starting at 2025-08-29 22:33:53 UT, approximately 15 hours after the trigger.
We do not detect the reported optical counterpart (Taguchi et al., GCN 41582; Yao et al., GCN 41583) or any new objects within the EP/FXT error region of the transient (Liu et al., GCN 41574). The 3-sigma limiting magnitudes on the stacked images are r > 23.7 mag and i > 23.6 mag. This is consistent with the detection of an optical counterpart candidate by NOT at r ~ 24.4 at 15.2 hours post trigger (Fu et al., GCN 41584).
GCN Circular 41588
Subject
EP250828a: Global MASTER-Net observations report
Date
2025-08-29T08:54:21Z (2 months 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 EP250828a ( EP Team et al., GCN 41574) errorbox 75567 sec after notice time and 1 days 1111 sec after trigger time at 2025-08-29 07:51:04 UT, with upper limit up to 18.8 mag. The observations began at zenith distance = 76 deg. The sun altitude is -39.8 deg.
The galactic latitude b = -26 deg., longitude l = 76 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2978651
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
87541 | 2025-08-29 07:51:04 | MASTER-OAFA | (21h 51m 57.17s , +20d 28m 10.2s) | C | 60 | 18.8 |
87618 | 2025-08-29 07:52:21 | MASTER-OAFA | (21h 52m 02.14s , +20d 27m 10.6s) | C | 60 | 18.6 |
87694 | 2025-08-29 07:53:37 | MASTER-OAFA | (21h 51m 56.69s , +20d 26m 10.1s) | C | 60 | 18.6 |
87771 | 2025-08-29 07:54:54 | MASTER-OAFA | (21h 52m 01.95s , +20d 26m 11.4s) | C | 60 | 18.6 |
87831 | 2025-08-29 07:54:54 | MASTER-OAFA | (21h 52m 02.01s , +20d 26m 11.0s) | C | 180 | 18.5 | Coadd
87844 | 2025-08-29 07:56:07 | MASTER-OAFA | (21h 50m 57.78s , +20d 27m 39.7s) | C | 60 | 18.5 |
87918 | 2025-08-29 07:57:20 | MASTER-OAFA | (21h 50m 57.71s , +20d 27m 39.6s) | C | 60 | 18.5 |
87991 | 2025-08-29 07:58:34 | MASTER-OAFA | (21h 50m 57.62s , +20d 27m 39.5s) | C | 60 | 18.5 |
88064 | 2025-08-29 07:59:46 | MASTER-OAFA | (21h 50m 57.50s , +20d 27m 39.7s) | C | 60 | 18.4 |
88137 | 2025-08-29 08:00:59 | MASTER-OAFA | (21h 50m 57.38s , +20d 27m 40.1s) | C | 60 | 18.3 |
88723 | 2025-08-29 08:10:46 | MASTER-OAFA | (21h 52m 01.43s , +20d 26m 02.4s) | C | 60 | 17.4 |
88801 | 2025-08-29 08:12:04 | MASTER-OAFA | (21h 51m 59.20s , +20d 27m 52.2s) | C | 60 | 17.3 |
88879 | 2025-08-29 08:13:22 | MASTER-OAFA | (21h 51m 59.14s , +20d 26m 02.3s) | C | 60 | 17.3 |
88939 | 2025-08-29 08:13:22 | MASTER-OAFA | (21h 51m 59.16s , +20d 26m 02.1s) | C | 180 | 17.4 | Coadd
88954 | 2025-08-29 08:14:36 | MASTER-OAFA | (21h 52m 02.29s , +20d 27m 53.3s) | C | 60 | 17.0 |
89030 | 2025-08-29 08:15:53 | MASTER-OAFA | (21h 51m 56.83s , +20d 26m 54.8s) | C | 60 | 16.6 |
89107 | 2025-08-29 08:17:09 | MASTER-OAFA | (21h 51m 55.72s , +20d 27m 52.7s) | C | 60 | 16.0 |
89182 | 2025-08-29 08:18:25 | MASTER-OAFA | (21h 52m 01.38s , +20d 26m 50.8s) | C | 60 | 15.7 |
89256 | 2025-08-29 08:19:38 | MASTER-OAFA | (21h 51m 55.58s , +20d 25m 48.1s) | C | 60 | 15.1 |
89331 | 2025-08-29 08:20:53 | MASTER-OAFA | (21h 52m 01.76s , +20d 26m 13.7s) | C | 60 | 13.9 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 41586
Subject
EP250828a: KAIT optical upper limit
Date
2025-08-29T06:16:36Z (2 months 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 EP250828a (Liu et al.,
GCN 41574) starting at 09:30, Aug 28 UT, about 1.97 hours after
the burst. A set of clear (roughly R) filter images were obtained.
In our coadd image, we did not detect the reported afterglow
(Taguchi et al., GCN 41582; Yao et al., GCN 41583; Fu et al., GCN
41584) down to a limiting mag of 20.5 mag (Vega) at a mid-time of
2.67 hours after the burst.
GCN Circular 41584
Subject
EP250828a: NOT optical counterpart confirmation
Date
2025-08-29T04:00:42Z (2 months ago)
From
syfu@nao.cas.cn
Via
Web form
S.Y. Fu (HUST), J. An, L.B. He, S.Q. Jiang, X. Liu, Z.P. Zhu, D. Xu (NAOC), J.P.U. Fynbo (DAWN/NBI), A. Henderson (NOT) report:
We observed the field of EP250828a (Liu et al., GCN 41574) using the Nordic Optical Telescope (NOT) located in La Palma (Canary Islands, Spain). Observations were carried out in the r and z filters.
The previously reported counterpart (candidate) within the EP/FXT error circle by Seimei (Taguchi et al., GCN 41582) and SVOM/VT (Yao et al., GCN 41583) is detected in our stacked r-band image and has decayed to r ~ 24.4 (AB) at a median time of 0.634 day post-trigger, calibrated with PanSTARRS-DR2 and not corrected for Galactic extinction. We thus confirm it is the counterpart of EP2508028a.
GCN Circular 41583
Subject
EP250828a: SVOM/VT optical candidate
Date
2025-08-29T03:36:21Z (2 months ago)
From
zhyao@bao.ac.cn
Via
Web form
Z. H. Yao, L. P. Xin, Y. L. Qiu, H. L. Li, C. Wu, 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 (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team.
SVOM/VT performed a Target of Opportunity observation of EP250828a detected by EP/WXT (Liu et al. GCN 41574). SVOM/VT began observing the field at 2025-08-28T11:45:13.5 UTC, 4.21 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
An uncatalogued source compared to Legacy survey is found using VT Xband data, within the error box of EP/FXT (Liu et al. GCN 41574) at R.A., Dec 327.89185, 20.54341 degrees:
RA (J2000) = 21:51:34.04
Dec (J2000) = 20:32:36.27
with an uncertainty of 0.5 arcsec.
The optical candidate is detected in both VT_B and VT_R band, and the magnitudes are:
Mid-Time (T - T0) | exposure time | band | mag (AB) | mag err
--------------------|-----------------|------|----------|--------
4.33 hour | 13*70 sec | VT_B | 23.5 | 0.3
4.33 hour | 12*70 sec | VT_R | 22.27 | 0.08
Our photometry was 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 41582
Subject
EP250828a: Seimei/TriCCS optical counterpart detection
Date
2025-08-29T02:57:38Z (2 months ago)
From
Kenta Taguchi <kentagch@kusastro.kyoto-u.ac.jp>
Via
Web form
Kenta Taguchi (Kyoto U.), Masaomi Tanaka, Seiji Toshikage (Tohoku U.) report on behalf of a larger collaboration:
We observed the field of EP250828a detected by the Einstein Probe (Liu et al., GCN 41574, Aryan et al., GCN 41579) using the Tricolor CMOS Camera and Spectrograph (TriCCS) on the 3.8-m Seimei Telescope at the Okayama Observatory of Kyoto University.
Our observation started at 13:20:39 UT on 2025-08-28 (MJD = 60915.5560).
For each band, we took 37 exposures of 30 seconds (i.e., the total exposure time is 1110 s = 18.5 min).
From our preliminary analysis, the 5 sigma limiting magnitudes are 22.3, 22.1, and 22.5 AB mag for g, r, and i-band, respectively.
Only in the i-band image, we detected an uncatalogued source of 22.3 +/- 0.2 mag at 21:51:34.02 +20:32:36.3 = 327.8917, 20.5434 (J2000), which is within the FXT error circle (16.5 arcsec from the center).
GCN Circular 41579
Subject
EP250828a: Optical upper limit with Kinder observations
Date
2025-08-28T16:27:20Z (2 months ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
Web form
A. Aryan, C.-S. Lin (both NCU), Y. J. Yang (NYUAD), A. K. H. Kong (NTHU), T.-W. Chen (NCU), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), Y.-H. Lee, A. Sankar. K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, C.-H. Lai, W.-J. Hou, H.-C. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), S. Yang, Z. N. Wang, L. L. Fan, G. H. Sun (all HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz and M. Huber (both IfA, Hawaii) report:
We observed the field of the fast X-ray transient EP250828a (Liu et al., GCN 41574) using the 1m LOT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86). The first LOT epoch of observations started at 13:06 UTC on the 28th of August 2025 (MJD 60915.546), 5.56 hr after the EP WXT trigger.
We utilized the astroalign (Beroiz et al. 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al. 2022, ApJ, 935, 167) packages to align and stack the individual frames. We utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction with the DESI Legacy Survey (Dey et al. 2019, AJ 157, 168) DR10 image using the 'hotpants' (Becker A., 2015, ascl.soft. ascl:1504.004) algorithm. In the stacked as well as in the difference image, we do not find any evidence of a new and uncataloged optical counterpart candidate within the 20 arcseconds error circle of EP-FXT localization.
Moreover, we further utilized AutoPhOT to perform the PSF photometry. The details of the observations and the measured 3-sigma upper limit (in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
LOT | r | 60915.546 | 5.56 | 300 * 12 | >22.3 | 1".69 | 1.08
The absence of any associated optical counterpart down to 22.3 mag with the optical observations starting within the first 6 hours of its X-ray discovery aligns well with the 'Dark' nature of FXTs as proposed by Aryan et al. 2025, arXiv:2504.21096.
The presented magnitude is calibrated using the field stars from the PanStarrs1 catalog and is not corrected for the expected Galactic foreground extinction of A_r = 0.28 mag in the direction of the transient (Schlafly & Finkbeiner 2011).
GCN Circular 41574
Subject
EP250828a: Einstein Probe detection of an X-ray transient
Date
2025-08-28T10:51:02Z (2 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
H. Y. Liu (NAO, CAS), K. R. Ni (CCNU) , H. L. Peng (NJNU), Y. J. Song and H. Sun (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 EP250828a. The transient triggered EP-WXT (ID: 01709200900) at 2025-08-28T07:32:33 (UTC). The WXT position of the source is R.A. = 327.905 deg, DEC = 20.567 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 at 2025-08-28T08:48:40 (UTC) . Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 327.8966 deg, DEC = 20.5435 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).