EP250821a
GCN Circular 42089
Subject
EP250821a: iTelescope T-59 possible detection of the optical counterpart
Date
2025-10-03T12:59:37Z (11 days ago)
From
Takanori Sakamoto at AGU <tsakamoto@phys.aoyama.ac.jp>
Via
Web form
H. Ban, T. Katou, Y. Nagata, K. Hotta, Y. Kawakubo, M. Serino, T. Sakamoto (AGU)
We observed the field of EP250821a detected by the Einstein Probe (EP)
(Hu et al., GCN Circ. 41459) with the iTelescope.Net
(http://www.itelescope.net) T59 (Planewave 20" CDK) telescope located at
the Siding Spring Observatory, Australia. We took nine images of 120-second
exposures in the Red filter starting from August 21 at 10:25:34 (UT),
about 3 hours after the trigger, and stopped at 12:48:14 (UT). Please note
that there were no observations between 10:33 and 11:30, and
between 11:39 and 12:41 due to our scheduling error.
We have a weak detection of the optical counterpart
(Dichiara et al., GCN Circ. 41469; Li et al., GCN Circ. 41470;
Yao et al., GCN Circ. 41485) in the stacked image of the first
three exposures from 10:25:34 to 10:32:47 UT. The estimated magnitude
is ~18.5. No optical counterpart was detected in the following six images
collected between 11:30:00 and 12:48:14 (UT). The 5-sigma upper limit
of the stacked image of the last six images is 17.5 mag.
GCN Circular 41597
Subject
EP250821a: SOAR observations
Date
2025-08-29T19:29:04Z (a month ago)
From
James Freeburn at University of North Carolina at Chapel Hill <jamesfreeburn54@gmail.com>
Via
Web form
J. Freeburn (UNC), G. Srinivasaragavan (UMD), I. Andreoni (UNC), Anna Ho (Cornell)
We observed the location of the X-ray transient, EP250821a (Hu et al., GCN 41459) with the Goodman High Throughput Spectrograph mounted on the SOAR telescope in imaging mode (PI: Andreoni). We took two 300s exposures each in g, r and i-band between 2025-08-29T01:15:29 and 2025-08-29T01:43:27 UTC corresponding to ~8 days after the initial trigger.
With image subtraction using Legacy Survey images (Dey et al., 2019), we do not detect the optical counterpart associated with EP250821a (Dichiara et al., GCN 41469; Li et al., GCN 41470; Yao et al., GCN 41485; Oates et al., GCN 41491) and place the following 5-sigma AB magnitude upper limits:
g > 22.9
r > 22.9
i > 22.8
GCN Circular 41549
Subject
EP250821a: Gemini GMOS Redshift Confirmation
Date
2025-08-26T15:49:43Z (2 months ago)
From
Brendan O'Connor at Carnegie Mellon University <boconno2@andrew.cmu.edu>
Via
Web form
Brendan O'Connor (CMU), Xander Hall (CMU), Antonella Palmese (CMU) report on behalf of a larger collaboration:
We observed the location of EP250821a (Hu et al. GCN 41459; Liang et al., GCN 41467) with the Gemini GMOS-S spectrograph starting on 2025-08-26 at 01:53:43 UT. Our spectra cover wavelengths 575 to 1060 nm and consist of 4x1200 s exposures using the R400 grating. In a preliminary reduction, at the location of the optical counterpart (Dichiara et al., GCN 41469; Li et al. GCN 41470; Yao et al. GCN 41485), we clearly detect a bright trace. We detect multiple emission lines from the underlying host galaxy, including [OII], [OIII], Hbeta, [NII], and Halpha, at a consistent redshift of z=0.577. This confirms the previous spectroscopic redshift obtained by the VLT (An et al., GCN 41490).
We thank the staff of the Gemini Observatory, in particular Karleyne Silva, Cinthya Rodriguez, Aleksandar Cikota, Emanuele Farina, for assistance with scheduling and obtaining these observations.
GCN Circular 41530
Subject
EP250821a: MeerKAT radio counterpart
Date
2025-08-24T16:50:44Z (2 months ago)
From
Francesco Carotenuto at INAF/OAR <francesco.carotenuto@inaf.it>
Via
Web form
Authors: F. Carotenuto (INAF-OAR), J. Bright (Oxford), P. G. Jonker (Radboud), on behalf of a larger collaboration
We observed the field of EP250821a detected by the Einstein Probe (GCN 41459, 41470, 41490) with the MeerKAT radio telescope from 2025-08-23 21:44 UTC to 2025-08-23 22:31 UTC (with a total time on source of 44 minutes) using the S-band receivers at a central frequency of 3.06 GHz. Analysing the SARAO Science Data Processor output continuum image, we detect a source at the position of EP250821a with a flux density of around 40 uJy/beam, with a typical image noise of around 6 uJy/beam. Pending verification on the host galaxy emission, we tentatively ascribe it to EP250821a.
Further observations are planned.
We thank the SARAO staff for rapidly scheduling these observations. The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation. This work has made use of the "MPIfR S-band receiver system" designed, constructed and maintained by funding of the MPI für Radioastronomie and the Max-Planck-Society.
GCN Circular 41496
Subject
EP250821a: Swift/XRT counterpart fading
Date
2025-08-22T14:41:37Z (2 months ago)
From
Simone Dichiara at Pennsylvania State University <sbd5667@psu.edu>
Via
Web form
S. Dichiara (PSU), S. R. Oates (Lancaster U.), J.A. Kennea (PSU), K.L. Page (U. Leicester) report on behalf of the Swift-XRT and Swift-UVOT team:
Swift-XRT has performed further follow-up observations of the EP-WXT transient EP250821a (Hu et al., GCN 41459), collecting a total of 3.0 ks of Photon Counting (PC) mode data between T0+61.0 ks and T0+72.9 ks.
The source detected in XRT in the first 1.9 ks of data previously reported as “Source 1” (Dichiara et al., GCN 41469), consistent with the optical counterpart (Li et al., GCN 41470, Yao et al., GCN 41485, An et al., GCN 41490), has faded more than 3 sigma in the latest X-ray observation.
The count rate derived from the follow-up observation is 2.2 (+/-0.3) x 10^-2 cts/s.
The light curve can be modelled with a power-law decay with an index of alpha=1.03 +/- 0.07.
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.0 +/- 0.2. The best-fitting absorption column is 1.4 (+0.7, -0.6) x 10^21 cm^-2 , in excess of the Galactic value of 7.26 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.51 x 10^-11 (4.48 x 10^-11) erg cm^-2 count^-1. The observed (unabsorbed) flux obtained from the follow-up observations from T0+61.0 ks to T0+72.9 ks is thus 7.7 x 10^-13 (9.9 x 10^-12) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at https://www.swift.ac.uk/EP/EP_FIELD00059/
This circular is an official product of the Swift-XRT team.
GCN Circular 41491
Subject
EP250821a: Swift/UVOT further observations.
Date
2025-08-22T09:44:34Z (2 months ago)
From
Samantha Oates at University of Birmingham <samantha.oates@alumni.ucl.ac.uk>
Via
email
S. R. Oates (Lancaster U.) and S. Dichiara (PSU) report on
behalf of the Swift-XRT and Swift-UVOT team:
Swift/UVOT has performed further follow-up observations of the Einstein
Probe/WXT-detected source EP250821a (Hu et al., GCN 41459). The UVOT optical
counterpart reported in (Dichiara et al., GCN 41469) has faded, consistent
with the behaviour reported by Li et al. (GCN 41470) and Yao et al. (GCN 41485).
The refined UVOT position is
RA (J2000) 19:19:32.56
Dec (J2000) -43:22:52.3
with an estimated uncertainty of 0.5 arcsec (radius, 90% confidence). We note that
this position supersedes that reported in Dichiara et al. (GCN 41469), which
is incorrect. We apologise for any confusion.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:
Filter T_start(s) T_stop(s) Exp(s) Mag/3sigUL
v 61584 72904 799 >19.9
u 5186 11029 1897 18.88+\-0.08 (reported in GCN 41469)
u 68377 68575 195 >19.8
uvw1 67966 68372 399 >20.0
uvm2 67555 67961 399 >19.8
uvw2 61180 72857 1168 >20.2
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.07 in the direction of the burst
(Schlegel et al. 1998).
GCN Circular 41490
Subject
EP250821a: VLT X-shooter spectroscopic redshift z = 0.577
Date
2025-08-22T09:01:58Z (2 months ago)
From
nrt3@le.ac.uk
Via
Web form
J. An (NAOC), V. Abril-Melgarejo (LUX-Paris Obs.), R. A. J. Eyles-Ferris (U. Leicester), D. Xu (NAOC), A. L. Thakur (INAF-IAPS), A. J. Levan (Radboud U.), P. G. Jonker (Radboud U.), D. B. Malesani (DAWN/NBI and Radboud), S. R. Oates (U. Lancaster), G. Corcoran (UCD), A. Martin-Carrillo (UCD), V. D’Elia (ASI/SSDC), N. R. Tanvir (U. Leicester) report on behalf of the Stargate collaboration:
We observed the location of the optical counterpart and coincident likely host galaxy (Dichiara et al., GCN 41469; Li et al. GCN 41470; Yao et al. GCN 41485) of the X-ray transient EP250821a detected by Einstein Probe (Hu et al. GCN 41459; Liang et al., GCN 41467), using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph.
Our spectra cover the wavelength range 3000-21000 AA, and consist of 4 exposures of 600 s each. Observations started on 2025-08-22 at approximately 04:10 UT (20.9 hr after the EP trigger). In a preliminary reduction of the spectra, we detect absorption lines of Fe II (fine structure), Mg II, Ca II, Na I, and emission lines of [O II], [O III], H alpha, H beta, H gamma, [N II], [S II], from which we infer a redshift of z = 0.577. We note that our spectroscopic redshift measurement is roughly consistent with the photometric redshift value of 0.64 +/- 0.06 provided for the catalogued object in Legacy Survey that was noted by Li et al. (GCN 41470).
From the X-shooter acquisition imaging we estimate a magnitude of r = 20.9 at 2025-08-22 03:41 UT, approximately 20.4 hr post-trigger. Given the Legacy Survey host magnitude of r = 21.79 +/- 0.02, this suggests a significant transient contribution at this epoch.
We acknowledge excellent support from the ESO observing staff in Paranal, in particular Celia Desgrange, Lorena Faundez, Camila de Sa Freitas. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495).
GCN Circular 41485
Subject
EP250821a: SVOM/VT optical observation
Date
2025-08-22T03:16:13Z (2 months ago)
From
zhyao@bao.ac.cn
Via
Web form
Z. H. Yao, Y. N. Ma, L. P. Xin, H. L. Li, Y. L. Qiu, C. Wu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team.
SVOM performed a Target of Opportunity observation of EP250821a detected by EP/WXT (Hu et al., GCN 41459). SVOM/VT began observing the field at 2025-08-21T11:23:41.5 UTC, 4.087 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
An uncatalogued source is found using VT X-band data, within the error box of EP/FXT (Hu et al., GCN 41459; Liang et al., GCN 41467), Swift/XRT and UVOT (Dichiara et al., GCN 41469) and Las Cumbres (Li et al., GCN 41470) at R.A., Dec 289.88564, -43.38115 degrees:
RA (J2000) = 19:19:32.55
Dec (J2000) = -43:22:52.14
with an uncertainty of 0.5 arcsec.
The source is detected in both VT_R and VT_B and was fading by ~0.5 mag between the first 2 VT observing sequences, the magnitudes are:
T-T0 (h) | Exposure Time (s) | Band | Mag (AB) | Mag err
--------------------|-------------------|------|----------|--------
4.087 | 70 | VT_B | 19.44 | 0.12
4.087 | 70 | VT_R | 18.49 | 0.05
6.363 | 70 | VT_B | 19.93 | 0.05
6.363 | 70 | VT_R | 19.02 | 0.03
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 Centre 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 41470
Subject
EP250821a: Rapidly fading optical source detected in Las Cumbres followup
Date
2025-08-21T16:53:21Z (2 months ago)
From
Wenxiong Li at NAOC <liwenxiong1992@gmail.com>
Via
Web form
Wenxiong Li (NAOC), Iair Arcavi (TAU), Ido Keinan (TAU), David Sand (U of Arizona)
We observed the position of EP250821a (Hu, Liang and Jin, GCN 41459; Liang, Hu and Jin, GCN 41467) with a Las Cumbres 1m telescope at Siding Spring Observatory 1.25 hours after the Einstein Probe WXT trigger and again 6.5 hours after the trigger. We took 2x300s exposures in each visit in the broad optical w band.
We find a rapidly fading source at RA=289.8853 Dec=-43.3813 (slightly offset from the position of the Swift counterpart; Dichara et al. GCN 41469) within the EP/FXT error circle and measure the following preliminary photometry calibrated to the r band:
Epoch 1: MJD 60908.362 Mag 18.11 +- 0.03
Epoch 2: MJD 60908.535 Mag 19.32 +- 0.02
We find a faint red likely extended source at this position in Legacy Survey data (https://www.legacysurvey.org/viewer).
Addition followup is encouraged.
GCN Circular 41469
Subject
EP250821a: Swift XRT and UVOT counterpart detection
Date
2025-08-21T16:30:52Z (2 months ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
S. Dichiara (PSU), S. R. Oates (Lancaster U.), J.A. Kennea (PSU),
K.L. Page (U. Leicester) report on behalf of the Swift-XRT and Swift-UVOT team:
Swift-XRT has performed follow-up observations of the Einstein Probe/WXT-detected
source EP250821a (Hu et al., GCN 41459), collecting 1.6 ks of Photon Counting (PC)
mode data between T0+4.9 ks and T0+10 ks after the trigger. A likely counterpart has
been found. The details of this source are:
Source 1 (SWIFT J191932.6-432250):
==================================
RA (J2000.0): 289.8861 = 19 19 32.66
Dec (J2000.0): -43.3807 = -43 22 50.5
Error: 3.6 (arcsec, radius, 90% confidence).
Detect flag: GOOD
Distance: 2.2 arcmin from the Einstein Probe/WXT position.
Mean rate: 0.263 +/- 0.015 ct s^-1
Mean flux: (9.44 +/- 0.53)e-12 erg cm^-2 s^-1
Peak rate: 0.440 +/- 0.099 ct s^-1
Peak flux: (1.58 +/- 0.35)e-11 erg cm^-2 s^-1
ECF: 3.58e-11 erg cm^-2 ct^-1
assuming NH=1.38e+21 cm^-2, gamma=1.93
determined from a spectral fit.
XMM UL: 5.5e-12 erg cm^-2 s^-1, (0.3-10 keV)
so the source is 2.9-sigma above this 3-sigma upper limit.
The source may be fading, at the 1.1-sigma level.
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_FIELD00059.
Source 1 was also detected in the UVOT u-band image. The preliminary UVOT position
is:
RA (J2000.0): 289.883 = 19 19 31.86
Dec (J2000.0): -43.3816 = -43 22 53.8
Preliminary detection u-band magnitude in 1897 s exposure is 18.88 +/- 0.08, using
the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373)
We note that the source is also listed in the Guide Star Catalog (Lasker et al,
2008) and
it has clearly brightened from its catalogued brightness (Bmag = 22.3).
This circular is an official product of the Swift XRT and UVOT team.
GCN Circular 41467
Subject
EP250821a: Preliminary analysis of the EP-WXT and EP-FXT observations
Date
2025-08-21T16:27:06Z (2 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Y. F. Liang, D. F. Hu (PMO, CAS), C. C. Jin (NAO, CAS) report on behalf of the Einstein Probe team:
The X-ray transient EP250821a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Hu et al., GCN 41459). We report on the further results of WXT and FXT observations. The analysis of the WXT data shows that the event started at T0=2025-08-21T07:17:04 (UTC) and lasted for at least 80 s. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 7.33 x 10^20 cm^-2 and a photon index of 1.2 (-/+0.5). The derived average unabsorbed 0.5-4 keV flux is 1.3(-/+0.4) 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 320 s after T0. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 289.8862, DEC = -43.3808 (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 1.25 x 10^21 cm^-2 and a photon index around 3.5. The derived average unabsorbed 0.5-10 keV flux is about 2.6 x 10^(-10) erg/s/cm^2 during the time interval from 0 to 1000 seconds after the start of the observation.The results from FXT may have been affected by pile-up effect, particularly in the brightest time interval. Therefore, the values reported here should be considered preliminary and are intended to provide a rough estimate of the source intensity.
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 41459
Subject
EP250821a: Einstein Probe detection of an X-ray transient
Date
2025-08-21T08:25:08Z (2 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
D. F. Hu, Y. F. Liang (PMO, CAS), 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 EP250821a. The transient triggered EP-WXT (ID: 01709200464) at 2025-08-21T07:18:29 (UTC). The WXT position of the source is R.A. = 289.856 deg, DEC = -43.410 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. = 289.8828 deg, DEC = -43.3816 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).