EP250908b
GCN Circular 41766
Subject
EP250908b/EP#01709240653: Swift/UVOT Detection
Date
2025-09-09T20:26:09Z (3 days ago)
From
Alice Breeveld at MSSL-UCL <a.breeveld@ucl.ac.uk>
Via
email
A.A. Breeveld (UCL-MSSL) and M. Ferro (INAF-OAB) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of the EP/WXT detected burst EP250908b (Yin et al., GCN Circ. 41752) 1103 s after the trigger. The afterglow reported by Ma et al. (GCN Circ. 41758) and also detected as source 1 in the Swift/XRT (Evans et al., GCN Circ. 41765), is detected in the initial U-band exposure.
The preliminary magnitude using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the initial exposure is:
Filter T_start(s) T_stop(s) Exp(s) Mag
u 1103 2836 1706 20.0 +/- 0.14
The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.048 in the direction of the burst (Schlegel et al. 1998).
GCN Circular 41759
Subject
EP250908b: SVOM/VT optical candidate (Coordinate supplement for GCN 41758)
Date
2025-09-09T03:54:18Z (4 days ago)
From
Yinuo Ma <mayn@bao.ac.cn>
Via
Web form
Y. N. Ma, Z. H. Yao, L. P. Xin, Y. L. Qiu, C. Wu, H. L. Li, 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 performed a Target of Opportunity observation of EP250908b detected by EP/WXT (Yin et al., GCN 41752). SVOM/VT began observing the field at 2025-09-08T19:29:58 UTC, 1.061 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
With X-band data availible, an uncatalogued fading optical candidate (Ma et al., GCN 41758) within the error box of EP/FXT, compared to Legacy survey, was clearly detected in both VT_B and VT_R at R.A., Dec 297.476836, -57.4930538 degrees:
RA (J2000) = 19:49:54.44
Dec (J2000) = -57:29:34.99
with an uncertainty of 0.5 arcsec.
The magnitudes are:
mid time (h) | exposure time (s) | band | mag (AB) | mag err
-------------|-------------------|------|----------|--------
1.491 | 57*50 | VT_B | 21.13 | 0.06
3.103 | 32*50 | VT_B | 22.5 | 0.2
1.484 | 60*50 | VT_R | 20.66 | 0.06
3.103 | 32*50 | VT_R | 21.4 | 0.1
Our photometry was not corrected for Galactic extinction.
We proposed that this is the optical candidate for the trigger.
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 41758
Subject
EP250908b: SVOM/VT optical candidate
Date
2025-09-09T03:20:04Z (4 days ago)
From
Yinuo Ma <mayn@bao.ac.cn>
Via
Web form
Y. N. Ma, Z. H. Yao, L. P. Xin, Y. L. Qiu, C. Wu, H. L. Li, 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 performed a Target of Opportunity observation of EP250908b detected by EP/WXT (Yin et al., GCN 41752). SVOM/VT began observing the field at 2025-09-08T19:29:58 UTC, 1.061 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
With X-band data availible, an uncatalogued fading optical candidate within the error box of EP/FXT, compared to Legacy survey, was clearly detected in both VT_B and VT_R. The magnitudes are:
mid time (h) | exposure time (s) | band | mag (AB) | mag err
-------------|-------------------|------|----------|--------
1.491 | 57*50 | VT_B | 21.13 | 0.06
3.103 | 32*50 | VT_B | 22.5 | 0.2
1.484 | 60*50 | VT_R | 20.66 | 0.06
3.103 | 32*50 | VT_R | 21.4 | 0.1
Our photometry was not corrected for Galactic extinction.
We proposed that this is the optical candidate for the trigger.
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 41752
Subject
EP250908b: EP-WXT detection of a fast X-ray transient and EP-FXT observation
Date
2025-09-09T02:42:35Z (4 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Y.-H. I. Yin (NJU), M. H. Zhang, M. J. Liu, X. X. Sun, T. Y. Liu, H. Y. Ren, H. Sun (NAO, CAS), on behalf of the Einstein Probe (EP) team:
We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250908b. The transient triggered EP-WXT (ID: 01709240653) at 2025-09-08T18:26:19 (UTC). The WXT position of the source is R.A. = 297.534 deg, DEC = -57.505 deg (J2000) with an uncertainty of 2.5 arcmin in radius (90% C.L. statistical and systematic). The WXT data shows that the event started at T0=2025-09-08T18:26:08 (UTC) and lasted for about 13 s before the interruption of the autonomous follow-up. The WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a hydrogen column density of 1.59 x 10^21 cm^-2 and a photon index of 1.14 (-0.45/+0.45). The derived peak unabsorbed 0.5-4 keV flux is 8.14 (-1.60/+2.00) x 10^-9 erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 164s after T0. On-ground analysis of the FXT data found a fading uncatalogued source at R.A. = 297.4784, DEC = -57.4938 (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 hydrogen column density of 1.59 (-0.76/+0.76) x 10^21 cm^-2 and a photon index of 2.02 (-0.23/+0.23). The derived average unabsorbed 0.5-10 keV flux is 1.10 (-0.09/+0.09) 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).