EP250905a
GCN Circular 41742
Subject
EP250905a: Liverpool Telescope i-band upper limits
Date
2025-09-06T21:23:03Z (7 days 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 EP250905a (Liu et al., GCN 41574) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 6x100s exposures in the SDSS i’ filter starting at 2025-09-05 22:06:03 UT, approximately 10.9 hours after the trigger.
We performed image subtraction of the stacked images against Pan-STARRS using PSF matching with the help of PSFEx. We do not detect the reported optical counterpart (He et al., GCN 41732, as well as GCN 41734) nor any new objects within the EP/FXT error region of the transient (Cheng et al., GCN 41725; Wang et al., GCN 41737). The 3-sigma limiting magnitude on the stacked images is i > 22.6 mag.
GCN Circular 41738
Subject
EP250905a: Liverpool Telescope observations
Date
2025-09-06T13:20:01Z (7 days ago)
From
Rob Eyles-Ferris at U of Leicester <raje1@leicester.ac.uk>
Via
Web form
R. A. J. Eyles-Ferris and N. Habeeb (U of Leicester) report on behalf of a larger collaboration:
We observed the field of EP250905a with the 2m Liverpool Telescope using the IO:O instrument. We obtained 6x150s exposures in each of the SDSS r’ and SDSS g’ filters starting at 2025-09-05 22:20:05 UT, approximately 11.1 hours after the X-ray detection.
We performed image subtraction on the stacked images using reference images from Pan-STARRS and also compared the stacked and reference images manually. We detect no new sources in either the subtractions or stacked images. At the position of the optical source proposed as the counterpart by He et al. (GCN 41732) and GCN 41734, we use forced photometry to measure r = 22.6 +/- 0.4. We therefore identify no significant variability.
We derive 3-sigma upper limits of r’ > 22.5 and g’ > 21.5 with photometry calibrated to Pan-STARRS and not corrected for Galactic extinction.
GCN Circular 41737
Subject
EP250905a: refined analysis of the EP-WXT and EP-FXT observations
Date
2025-09-06T11:24:15Z (7 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Y. Wang (PMO), Y.H.Cheng (SWIFAR,YNU), R. D. Liang, W. D. Zhang (NAOC) on behalf of the Einstein Probe (EP) team:
The X-ray transient EP250905a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (GCN 41725), with several multi-wavelength follow-up observations (GCN 41727, GCN 41732, GCN 41734). The refined WXT data shows that the flare started at about 2025-09-05 11:07:08
UTC (T0). WXT detected the source from T0 to before the slew with a peak flux of 1.4 x 10^(-9) erg/s/cm^2. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 7.0 x 10^20 cm^-2 and a photon index of 1.7 (+/-0.4). The derived average unabsorbed 0.5-4 keV flux is around 5.7 (-1.3/+1.6) x 10^(-10) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 405 s after T0. On-ground analysis of the FXT data found an fading source at R.A. = 4.4504 deg, DEC = 37.4697 deg (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 7.0 x 10^20 cm^-2 and a photon index of 2.3(+/-0.4). The derived average unabsorbed 0.5-10 keV flux is around 4.5(-1.0/+1.5) 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 41734
Subject
EP250905a: FTW one optical and multiple NIR counterpart candidates
Date
2025-09-06T02:50:51Z (8 days ago)
Edited On
2025-09-08T14:02:53Z (5 days ago)
From
xjh@andrew.cmu.edu
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Xander J Hall at Carnegie Mellon University <xjh@andrew.cmu.edu>
Via
Web form
Xander J. Hall (Carnegie Mellon U.), Malte Busmann (LMU), Brendan O’Connor (Carnegie Mellon U.), Daniel Gruen (LMU), Antonella Palmese (Carnegie Mellon U.) report:
We observed the localization of the x-ray flare EP250905a (GCN 41725), with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i, and J bands starting at 2025-09-05T20:12:28 UTC (0.38 days after the X-ray flare) for 52 x 180s.
In the difference imaging with templates from PS1, we detect no new sources down to r ~ 23.2 and i ~ 23.1.
However, our image depth is far deeper (r ~ 24.5, i ~ 24.3, J ~ 23.2). By eye, we compare to PS1 and 2MASS catalogs and present 5 potential new sources within the 20” error circle:
| id | ra | dec | mag_J | mag_J_err | mag_r | mag_r_err |
|---|---|---|---|---|---|---|
| EP250905a_2 | 00:17:47.9940 | +37:28:19.306 | 21.3 | 0.1 | 22.14 | 0.04 |
EP250905a_2 was reported by He et al. (GCN 41732), however in our difference imaging we see no source. In our science image, we note a distinct point source there below the galaxy. We report a noticeably brighter r ~ 22.14 than reported by He et al. (GCN 41732), but there is significant bleed from the nearby galaxy. There also appears to be a faint source at that position in the PS1 template.
We note that these following four sources are not detected above our three sigma limit in r band photometry, but have clear sources in J.
| id | ra | dec | mag_J | mag_J_err |
|---|---|---|---|---|
| EP250905a_1 | 00:17:47.4836 | +37:28:25.975 | 22.1 | 0.1 |
| EP250905a_3 | 00:17:46.8641 | +37:28:16.316 | 21.40 | 0.06 |
| EP250905a_4 | 00:17:47.4834 | +37:28:07.901 | 21.9 | 0.1 |
| EP250905a_5 | 00:17:47.4630 | +37:28:05.478 | 21.9 | 0.1 |
We encourage continued follow-up to determine the variable nature of these sources.
The magnitudes are reported in AB and calibrated against the PS1 and 2MASS catalog and not corrected for Galactic extinction.
We thank Michael Schmidt from the Wendelstein Observatory staff for obtaining these observations.
GCN Circular 41732
Subject
EP250905a: JinShan likely optical counterpart detection
Date
2025-09-06T01:19:57Z (8 days ago)
Edited On
2025-09-08T14:02:39Z (5 days ago)
From
Dong Xu at NAOC/CAS <dxu@nao.cas.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Dong Xu at NAOC/CAS <dxu@nao.cas.cn>
Via
Web form
L.B. He, X. Liu, J. An, D. Xu, S.Q. Jiang, Z.P. Zhu (NAOC), S.Y. Fu (HUST), J.Z. Liu (XAO) report on behalf of a large collaboration:
We observed the field of EP250905a detected by Einstein Probe (Cheng et al., GCN 41725), using the 100C telescope of the JinShan project, located at Altay, Xinjiang, China. We obtained 40 x 180 s Sloan i-band frames, starting from 2025-09-05 13:56:23 UT, i.e., 2.74 hrs since the EP trigger.
A new source is detected positionally in a known GLADE galaxy (GLADE+ #2980788) via image subtraction, which is within the EP-FXT 20-arcsec error circle , at coordinates
R.A. (J2000) = 0:17:48.01
Dec. (J2000) = +37:28:19.44
with an uncertainty of ~ 0.6 arcsec. The source has i ~ 22.9 mag, calibrated with PanSTARRS.
We think the source is very likely the counterpart of EP250905a. In this case, the galaxy is the host with a redshfit of z ~ 0.28 from GLADE.
Further observations are planned and encouraged.
GCN Circular 41727
Subject
EP250905a: Global MASTER-Net observations report
Date
2025-09-05T18:09:28Z (8 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-Kislovodsk robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the EP250905a ( EP Team et al., GCN 41725) errorbox 18309 sec after notice time and 24012 sec after trigger time at 2025-09-05 17:52:01 UT, with upper limit up to 17.2 mag. The observations began at zenith distance = 51 deg. The sun altitude is -23.2 deg.
The galactic latitude b = -25 deg., longitude l = 116 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2985447
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
24043 | 2025-09-05 17:52:01 | MASTER-Kislovodsk | (00h 16m 34.13s , +37d 41m 06.2s) | C | 60 | 17.2 |
24633 | 2025-09-05 18:01:52 | MASTER-Kislovodsk | (00h 16m 32.36s , +37d 38m 50.7s) | C | 60 | 17.2 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 41725
Subject
EP250905a: Einstein Probe detection of an X-ray transient
Date
2025-09-05T12:44:26Z (8 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Y.H. Cheng (SWIFAR,YNU), Y. Wang (PMO), R. D. Liang, 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 EP250905a. The transient triggered EP-WXT (ID: 01709201261) at 2025-09-05T11:11:49 (UTC). The WXT position of the source is R.A. = 4.441 deg, DEC = 37.493 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. = 4.4517 deg, DEC = 37.4728 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 are 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).