EP250806a
GCN Circular 41273
Subject
EP250806a: Upper limit from Fermi-GBM Observations
Date
2025-08-07T20:05:54Z (18 days ago)
From
mariaedvige.ravasio@ru.nl
Via
Web form
M. E. Ravasio and P.G. Jonker (Radboud Univ.) on behalf of the Einstein Probe Team
and
E. Burns (LSU) on behalf of the Fermi-GBM Team, report:
Fermi-GBM had full spatial coverage of the transient EP250806a detected by EP-WXT (Yang et al., GCN 41246). There was no Fermi-GBM onboard trigger around the refined EP starting time T0=2025-08-06T09:17:30 UTC (Liang et al., GCN 41256).
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 transient, 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 3.7e-08 erg/cm2/s in the energy band 10-1000 keV.
[1] Goldstein et al. 2019 arXiv:1903.12597
GCN Circular 41266
Subject
EP250806a:SVOM/VT refined analysis
Date
2025-08-07T14:41:19Z (18 days ago)
From
Liping Xin at NAOC, SVOM <xlp@nao.cas.cn>
Via
Web form
L. P. Xin, H. L. Li, Y. N. Ma, Y. L. Qiu, C. Wu, Z. H. Yao, 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.
After the refined analysis, the candidate reported (Xin et al., GCN 41254) was a cataloged source in Legacy survey, not an uncatalogued source, as reported by Becerra et al., (GCN 41255).
No any candidates were found in VT images within EP/FXT errorbox (Yang et al., GCN 41246; Liang et al., GCN 41256), the 3 sigma limit is about VT_B~23.6 mag and VT_R~23.1 mag in AB mag at 8.19 hours after the EP/WXT trigger.
The results are consistent with the reports (Fortin et al. GCN 41248; Becerra et al., GCN 41255; Lipunov et al., GCN 41258; Kumar et al., GCN 41263).
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 41263
Subject
EP250806a: Liverpool telescope optical upper limit
Date
2025-08-07T11:08:34Z (18 days ago)
From
Amit Kumar at Royal Holloway - UoL/ U of Warwick, UK <amitkundu515@gmail.com>
Via
Web form
A. Kumar, J. R. Maund (RHUL), N. C. Sun (UCAS), W. X. Li, Y. N. Wang (NAOC), and K. Wiersema (Herts) report:
We observed the field of EP/WXT triggered X-ray transient EP250806a (Yang et al., GCN 41246; Liang et al., GCN 41256) with the IO:O Imager at the 2m Liverpool telescope. We observed 360s x 2 frames in the g-band starting at 2025-08-07 UT 01:03:11 (~15.7 hours post-trigger). Preliminary photometry on the stacked image was performed and calibrated against the SDSS catalogue stars.
We do not detect any source at the location of the SVOM/VT optical candidate of EP250806a (Xin et al., GCN 41254; Becerra et al., GCN 41255) down to a limiting g-band magnitude of >22.5 (AB).
Reported magnitude is not extinction-corrected.
GCN Circular 41259
Subject
EP#01709198415/EP250806a: Swift-XRT observations
Date
2025-08-07T09:00:17Z (18 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
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), 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 EP250806a (EP trigger 01709198415, GCN #41246), collecting 1.5 ks of Photon
Counting (PC) mode data between T0+11 ks and T0+13 ks after the trigger. No X-ray
sources have been detected in the WXT error region.
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_FIELD00057.
This circular is an official product of the Swift-XRT team.
GCN Circular 41258
Subject
EP250806a: Global MASTER-Net observations report
Date
2025-08-07T08:54:20Z (18 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-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 EP250806a ( EP Team et al., GCN 41246) errorbox 28505 sec after notice time and 32341 sec after trigger time at 2025-08-06 18:17:21 UT, with upper limit up to 17.5 mag. The observations began at zenith distance = 72 deg. The sun altitude is -28.1 deg.
The galactic latitude b = -57 deg., longitude l = 25 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2962585
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
32371 | 2025-08-06 18:17:21 | MASTER-SAAO | (22h 16m 28.41s , -27d 00m 15.2s) | C | 60 | 17.4 |
32371 | 2025-08-06 18:17:21 | MASTER-SAAO | (22h 18m 28.98s , -27d 15m 51.3s) | C | 60 | 17.5 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 41256
Subject
EP250806a: refined analysis of the EP-WXT and EP-FXT observations
Date
2025-08-07T04:42:31Z (18 days ago)
Edited On
2025-08-07T13:18:05Z (18 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
R. D. Liang, H. N. Yang (NAO, CAS), Y. J. Zhang (THU), W. D. Zhang (NAO, CAS) report on behalf of the Einstein Probe team:
The X-ray transient EP250806a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Yang et al., GCN 41246). Refined analysis of the WXT data shows that the event started at T0=2025-08-06 09:17:30 (UTC) and lasted for about 50 s. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 1.47 x 10^20 cm^-2 and a photon index of 2.3 (-/+0.6). The derived average unabsorbed 0.5-4 keV flux is 4.1(-/+0.5) x 10^(-11) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 150 s after T0. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 335.1028, DEC = -26.8738 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent positionally with the WXT transient. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 1.47 x 10^20 cm^-2 and a photon index of 3.24 (-/+0.25). The derived average unabsorbed 0.5-10 keV flux is 8.42 (-/+0.47) x 10^(-13) erg/s/cm^2 during the time interval from 0 to 1000 seconds after the start of the observation. 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 41255
Subject
EP250806a: Earlier Detection of the VT Candidate by COLIBRÍ and LS
Date
2025-08-07T03:11:09Z (18 days ago)
From
Alan Watson at UNAM <alan@astro.unam.mx>
Via
Web form
Rosa L. Becerra (U Roma), Alan M. Watson (UNAM), Fredd Sánchez Álvarez (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Camila Angulo (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), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report:
In Fortin et al. (GCN Circ. 41248), we reported no optical counterpart of the EP X-ray transient EP250806a (Yang et al., GCN Circ. 41246) in a 32-minute exposure in i taken with COLIBRÍ. We here discuss our full 60-minute exposure in i taken from 2025-08-06 10:27 to 11:48 UTC (from 1.1 to 2.5 hours after the trigger).
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.
At the position of the VT optical candidate reported by Xin et al. (GCN Circ. 41254), we detect a source with
i = 22.47 +/- 0.11
This magnitude is similar to the magnitude of VT_R = 22.6 +/- 0.2 about 8 hours after the trigger reported by Xin et al.
Furthermore, the source is visible in the Legacy Survey DR10 (Dey et al. 2019) catalog with
i = 22.0 +/- 0.07
In both our image and the LS image, the source appears to be resolved.
Given the detection in the LS image, the lack of fading between our observations and the VT observations, and the resolved appearance, we suggest that this is not the optical counterpart of the EP transient.
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 41254
Subject
EP250806a: SVOM /VT optical candidate
Date
2025-08-07T01:45:11Z (18 days ago)
From
Liping Xin at NAOC, SVOM <xlp@nao.cas.cn>
Via
Web form
L. P. Xin, H. L. Li, Y. N. Ma, Y. L. Qiu, C. Wu, Z. H. Yao, 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), H. N. Yang, R. D. Liang (NAOC), Y. J. Zhang (THU), W. D. Zhang (NAOC) report on behalf of the SVOM and EP teams.
SVOM performed a Target of Opportunity observation of EP250806a detected by EP/WXT (Yang et al., GCN 41246). SVOM/VT began observing the field at 2025-08-06T17:09:08 UTC, 7.9 hours after the EP trigger time, 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 errorbox of EP/FXT (Yang et al., GCN 41246), compared to Legacy survey. The optical source is at R.A., Dec 335.103182, -26.877607 degrees:
RA (J2000) = 22:20:24.76
Dec (J2000) = -26:52:39.38
with an uncertainty of 0.5 arcsec.
The source is detected in both VT_B and VT_R images. The magnitudes are derived as below:
DeltaT(Mid time) | exposure time (s) | band | mag (AB) | mag err
-------------------- |-------------------|------|----------|--------
8.19 hours | 49*50 | VT_B | 23.3 | 0.2
8.19 hours | 49*50 | VT_R | 22.6 | 0.2
Our photometry is 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 41248
Subject
EP250806a: COLIBRÍ optical upper limit
Date
2025-08-06T14:02:40Z (19 days ago)
From
F. Fortin at IRAP <ffortin.sci.edu@gmail.com>
Via
Web form
Francis Fortin (IRAP), Camila Angulo (UNAM), Rosa L. Becerra (U Roma), 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), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM):
We imaged the field of the EP X-ray transient EP250806a (Yang et al., GCN Circ. 41246) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-08-06T10:27:49 to 11:10:24 UTC (from 1.1 to 1.9 hours after the trigger) and obtained 32 minutes of exposure in the i-band 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 FXT source position (Yang et al., GCN Circ. 41246) down to the following 3-sigma limit:
i > 23.38
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 41246
Subject
EP250806a: Einstein Probe detection of an X-ray transient
Date
2025-08-06T10:21:46Z (19 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
H.N. Yang, R. D. Liang (NAOC), Y. J. Zhang (THU), W. D. Zhang (NAOC) 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 EP250806a. The transient triggered EP-WXT (ID: 01709198415) at 2025-08-06T09:18:20 (UTC). The WXT position of the source is R.A. = 335.100 deg, DEC = -26.886 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. = 335.1046 deg, DEC = -26.8759 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).