EP250911a
GCN Circular 42164
Subject
EP250911a: Koshka Zeiss-1000 Optical Upper Limit
Date
2025-10-08T08:38:27Z (a year ago)
From
Alina Volnova at IKI RAS <alinusss@gmail.com>
Via
Web form
A. Volnova (IKI), A. Novichonok (Petrozavodsk State University, KIAM), I. Nikolenko (INASAN), N. Pankov (HSE, IKI), A. Pozanenko (IKI) report on behalf of IKI-GRB-FuN:
We observed the localization region of the Fast X-ray Transient EP250911a (Liang et al., GCN 41788; Liang et al., GCN 41809) using the 1-meter Zeiss-1000 telescope of the Simeiz (Koshka) Observatory with the R-band. Observations were performed on Sep. 13, 14, and 16 in the presence of the full Moon. In the stacked frame of the three epochs we did not detect the optical afterglow (Quirola-Vasquez et al., GCN 41790; Angulo et al., GCN 41793; Lai et al., GCN 41801; Evans et al., GCN 41802; Moskvitin & Spiridonova, GCN 41805; Ma et al., GCN 41807). The photometry based on nearby PS1 stars (Lupton's transformation) is the following:
Date UT start t-T0 Exp. Filter OT Err. UL
(mid, days) (n*s) (3sigma)
2025-09-13 22:44:37 2.60543 45*120 R n/d n/d 21.0
2025-09-14 19:41:15 3.52607 60*120 R n/d n/d 21.7
2025-09-16 20:58:57 5.54397 60*120 R n/d n/d 21.9
2025-09-13+14+16 22:44:37 4.08066 165*120 R n/d n/d 22.6
GCN Circular 41954
Subject
EP250911a: e-MERLIN radio observations
Date
2025-09-23T12:59:20Z (a year ago)
From
Giulia Gianfagna at INAF-IAPS <giulia.gianfagna@inaf.it>
Via
Web form
G. Gianfagna, G. Bruni, L. Piro, A. L. Thakur (INAF-IAPS) report:
We observed the Fast X-ray Transient EP250911a (Liang et al., GCN 41788) with the e-MERLIN radio telescope under the project CY20206 (PI: Gianfagna) at 5 GHz starting on Sep. 17, 16:00 UT (~6 days after the EP trigger) for a total of 11 hours.
1331+3030 was used for flux scale calibration, and 2344+2952 for complex gain. The beam size was 63x37 milli-arcsec. Data was reduced with the e-MERLIN pipeline and imaged with CASA. The image RMS was 18 uJy/beam.
We did not find statistically significant emission at a level above 5-sigma at the position of the optical counterpart reported by Quirola-Vasquez et al. (GCN 41790), Angulo et al (GCN 41793), Lai et al. (GCN 41801), Moskvitin et al. (GCN 41805), Ma et al. (GCN 41807), Malesani et al. (GCN 41811). Thus, we estimate a 5-sigma upper limit of ~ 90 uJy at 5 GHz. Further e-Merlin observations are planned.
We thank the e-Merlin staff for the prompt scheduling of these observations. e-MERLIN is a National Facility operated by the University of Manchester at Jodrell Bank Observatory on behalf of STFC, part of UK Research and Innovation.
GCN Circular 41821
Subject
EP250911a: Global MASTER-Net observations report
Date
2025-09-13T00:18:39Z (a year 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-Tavrida robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Lomonosov MSU, SAI Crimea astronomical station) was pointed to the EP250911a ( EP Team et al., GCN 41788) errorbox 27051 sec after notice time and 30571 sec after trigger time at 2025-09-11 17:27:59 UT, with upper limit up to 13.6 mag. Observations started at twilight. The observations began at zenith distance = 57 deg. The sun altitude is -15.9 deg.
MASTER-Kislovodsk robotic telescope located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the EP250911a errorbox 1 days 51178 sec after notice time and 1 days 54698 sec after trigger time at 2025-09-13 00:10:06 UT, with upper limit up to 17.5 mag. The observations began at zenith distance = 34 deg. The sun altitude is -27.5 deg.
The galactic latitude b = -32 deg., longitude l = 108 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2989778
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
13450 | 2025-09-11 12:42:07 | MASTER- | (23h 49m 56.46s , +28d 43m 10.6s) | C | 60 | 16.4 | Coadd
30602 | 2025-09-11 17:27:59 | MASTER-Tavrida | (23h 49m 27.05s , +29d 17m 08.7s) | C | 60 | 13.6 |
141129 | 2025-09-13 00:10:06 | MASTER-Kislovodsk | (23h 49m 03.03s , +29d 23m 20.1s) | C | 60 | 17.1 |
141292 | 2025-09-13 00:12:50 | MASTER-Kislovodsk | (23h 49m 01.01s , +29d 23m 04.4s) | C | 60 | 17.5 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 41815
Subject
EP250911a: Upper limit from Fermi-GBM Observations
Date
2025-09-12T14:23:51Z (a year 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 transients EP250911a (Liang et al., GCN 41788 and GCN 41809) detected by EP-WXT, and found to be at z=3.84 (Malesani et al., GCN 41811). There was no Fermi-GBM onboard trigger around the refined EP trigger time T0=2025-09-11T08:57:55 (UTC).
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 transients, both temporally and spatially, is identified, as confirmed also by visual inspection of the data.
Assuming a “normal” spectral template (Band function with Epeak = 230 keV, alpha = -1.0, beta = -2.3), whose alpha value is consistent with the power law index reported by EP (Liang et al., GCN 41809), and a timescale of 8.192 s, we derive a flux upper limit of 5.3e-08 erg/cm2/s in the energy band 10-1000 keV.
[1] Goldstein et al. 2019 arXiv:1903.12597
GCN Circular 41811
Subject
EP250911a: GTC/OSIRIS+ spectroscopic redshift z = 3.84
Date
2025-09-12T09:59:04Z (a year ago)
Edited On
2025-09-12T13:30:07Z (a year ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
D. B. Malesani (DAWN/NBI and Radboud), A. J. Levan (Radboud and Warwick), G. Corcoran (UCD), R. A. J. Eyles-Ferris (Leicester), P. G. Jonker (Radboud), A. Martin-Carrillo (UCD), A. L. Cabrera Lavers (GTC), D. Reverte-Payá (GTC), A. Pérez-Romero (GTC) report for a larger collaboration:
We observed the afterglow (Quirola-Vasquez et al., GCN 41790; Angulo et al., GCN 41793; Lai et al., GCN 41801; Evans et al., GCN 41802; Moskvitin & Spiridonova, GCN 41805; Ma et al., GCN 41807) of EP250911a (Liang et al. GCN 41788; Liang et al., GCN 41809) using the Gran Telescopio Canairas (GTC) equipped with the OSIRIS+ instrument.
In a sequence of images taken in the r and z filters (mean times 13.97 and 14.12 hr after the trigger time, respectively), we measure magnitudes r = 22.0 +/- 0.1, z = 21.4 +/- 0.1 (all AB, calibrated against nearby Pan-STARRS objects, and not corrected for Galactic extinction).
A sequence of 4 spectra with 1200 s exposure time each was obtained using the R1000B grism, covering the wavelength range 3700-7700 AA (mean time 15.4 hr after trigger). Inspection of the combined spectrum shows a weak continuum. A broad trough is observed around 5890 AA, which is interpreted as a DLA feature at a redshift z ~ 3.84. This is confirmed by the detection of a few metal absorption lines at a consistent redshift, among which we identify S II / Si II 1260, O I / Si II 1303, C II 1334, Si IV 1394, C IV 1548,1550. We also detect a continuum break around 4400 AA, which corresponds to the Lyman limit at this redshift.
Note that, despite some temporal and spatial proximity, this redshift is inconsistent with the distance (~1.1 Gpc) derived for the LVK GW source S250911ac (LVK collaboration, GCNs 41786, GCN 41795), excluding an association between the two events.
GCN Circular 41809
Subject
EP250911a: refined analysis of the EP-WXT and EP-FXT observations
Date
2025-09-12T05:02:03Z (a year ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Y. F. Liang (PMO), Q. Y. Wu, W. D. Zhang(NAO, CAS) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP250911a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Liang et al., GCN 41788), and followed by Swift/XRT (Evans et al., GCN 41802) and several optical telescopes (Quirola-Vasquez et al., GCN 41790 , Angulo et al., GCN 41793, Lai et al., GCN 41801, Moskvitin et al., GCN 41805, Ma et al., GCN 41807). The refined analysis of the WXT data shows that the event started at T0=2025-09-11T08:57:55 (UTC) and lasted for 34 s with the peak flux of 9.3 x 10^(-9) erg/s/cm^2, before the observation was interrupted by the autonomous follow-up observation. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 4.79 x 10^20 cm^-2 and a photon index of 0.77 (-/+0.95). The derived average unabsorbed 0.5-4 keV flux is 3.4 (-1.5/+3.1) x 10^(-9) erg/s/cm^2.
The autonomous observation by the Follow-up X-ray Telescope (FXT) was performed at 2025-09-11T09:03:21 (UTC), about six minutes after T0. The exposure time of this observation is 3856 s. The on-ground analysis shows that an uncatalogued source was detected at R.A., Dec. = 357.5194, 29.2999 deg (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic equivalent hydrogen column density of 4.79 x 10^20 cm^-2 and a photon index of 2.02 (-/+0.22). The derived average unabsorbed 0.5-10 keV flux is 7.5 (-1.1/+1.4) 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 41807
Subject
EP250911a: SVOM/VT optical observation
Date
2025-09-12T03:27:06Z (a year ago)
From
Yinuo Ma <mayn@bao.ac.cn>
Via
Web form
Y. N. Ma, L. P. Xin, Z. H. Yao, 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 EP250911a detected by EP/WXT (Liang et al., GCN 41788). SVOM/VT began observing the field at 2025-09-11T15:33:30 UTC, 6.584 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
With X-band data available, the optical counterpart (Quirola-Vasquez et al., GCN 41790, Angulo et al., GCN 41793; Lai et al., GCN 41801; Moskvitin & Spiridonova, GCN 41805) at a location consistent with Swift XRT position (Evans et al., GCN 41802) was clearly detected in both VT_B and VT_R bands. The magnitudes are:
mid time (h) | exposure time (s) | band | mag (AB) | mag err
-------------|-------------------|------|----------|--------
6.945 | 27*100 | VT_B | 22.06 | 0.08
6.945 | 27*100 | VT_R | 20.87 | 0.05
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 41805
Subject
EP250911a: SAO RAS optical observations
Date
2025-09-11T23:47:13Z (a year ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin, O. Spiridonova (SAO RAS)
report on behalf of GRB follow-up collaboration.
We observed the field of EP250911a (Liang et al., GCN 41788,
Evans et al., GCN 41802) with the 1-m SAO RAS telescope
Zeiss-1000 equipped with CCD-photometer. We obtained 9 x 300 sec.
images in Rc band on September 11, 21:38:05--22:34:14 UT
(t_mid - T0 = 13.12819 hours = 0.547008 days).
The OT (Quirola-Vasquez et al., GCN 41790, Angulo et al., GCN 41793;
Lai et al., GCN 41801) is clearly detected in the stacked image
with the brightness of R = 21.90 +/- 0.11 (R_lim = 23.0 mag).
This preliminary photometry is based on nearby SDSS stars
(magnitudes were converted with the Lupton 2005 equations)
and not corrected for Galactic extinction.
GCN Circular 41802
Subject
EP250911a: Swift-XRT observations
Date
2025-09-11T17:36:52Z (a year 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 EP250911a (Liang et al., GCN Circ 41788), collecting 1.7 ks of Photon
Counting (PC) mode data between T0+5.5 ks and T0+11 ks after the trigger. We have
detected 1 source consistent with the EP/FXT position (Liang et al., GCN Circ
41788). This has been automatically classified as an uncatalogued X-ray source.
Uncatalogued X-ray sources
--------------------------
Source 1 (SWIFT J235004.5+291757):
==================================
RA (J2000.0): 357.5191 = 23 50 04.58
Dec (J2000.0): +29.2992 = +29 17 57.1
Error: 4.6 (arcsec, radius, 90% confidence).
Detect flag: GOOD
Distance: 62 arcsec from the Einstein Probe/WXT position.
Mean rate: 0.0239 +/- 0.0047 ct s^-1
Mean flux: (8.3 +/- 1.6)e-13 erg cm^-2 s^-1
Peak rate: 0.0239 +/- 0.0047 ct s^-1
Peak flux: (8.3 +/- 1.6)e-13 erg cm^-2 s^-1
ECF: 3.47e-11 erg cm^-2 ct^-1
assuming NH=5.84e+20 cm^-2, gamma=1.92
determined from a spectral fit.
XMM UL: 7.6e-12 erg cm^-2 s^-1, (0.3-10 keV)
so the source is not above this 3-sigma upper limit.
There is no evidence for fading.
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.
We note that source 1 also matches the position of the optical counterpart to
EP250911a reported by Quirola-Vasquez et al. (GCN Circ. 41790) and Angulo et al.
(GCN Circ. 41793).
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_FIELD00073.
This circular is an official product of the Swift-XRT team.
GCN Circular 41801
Subject
EP250911a: Optical detection with Kinder observations
Date
2025-09-11T17:32:52Z (a year ago)
From
Janet Chen at National Central University <janetstars@gmail.com>
Via
Web form
C.-H. Lai, A. Aryan, T.-W. Chen, C.-S. Lin (all NCU), Y.-C. Shiau (FEMH), J. Gillanders, S. J. Smartt (both Oxford), Y. J. Yang (NYUAD), A. K. H. Kong (NTHU), Y.-H. Lee, A. Sankar. K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, 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 EP250911a (Liang et al., GCN 41788) using the 1m LOT and 40cm SLT 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 in r-band started at 13:17 UTC on the 11th of September 2025 (MJD 60929.554), 4.31 hr after the EP WXT trigger. Moreover, the first SLT epoch of observations in i-band started at 13:18 UTC on the 11th of September 2025 (MJD 60929.554), 4.33 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. In the stacked frames of r, i and z bands from LOT and i-band from SLT, we clearly detected the optical counterpart candidate proposed by Quirola-Vasquez et al. (GCN 41790) and confirmed by Angulo et al. (GCN 41793).
Moreover, we further utilized AutoPhOT to perform the PSF photometry. The details of the observations and the measured magnitude (in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
LOT | r | 60929.554 | 4.31 | 300 * 6 | 21.30 +/- 0.05 | 0".74 | 1.29
SLT | i | 60929.554 | 4.33 | 300 * 24 | 20.44 +/- 0.11| 1".3 | 1.14
LOT | i | 60929.601 | 5.46 | 300 * 6 | 20.71 +/- 0.07 | 0".73 | 1.10
LOT | z | 60929.642 | 6.43 | 300 * 6 | 20.50 +/- 0.09 | 0".97| 1.02
The presented magnitudes are calibrated using the field stars from the SDSS catalog and are not corrected for the expected Galactic foreground reddening of E(B-V) = 0.05 mag in the direction of the transient (Schlafly & Finkbeiner 2011).
GCN Circular 41793
Subject
EP250911a: COLIBRÍ optical observations
Date
2025-09-11T12:10:52Z (a year ago)
From
Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Via
Web form
Camila Angulo (UNAM), Sarah Antier (OCA), Rosa L. Becerra (U Roma), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Jean-Luc Atteia (IRAP), Fredd Alvarez (UNAM), Dalya Akl (NYUAD), Nathaniel R. Butler (ASU), Damien Dornic (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):
We imaged the field of the EP250711a (Liang et al., GCN Circ. 41788) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-11-09 09:29 to 10:33 UTC (from 0.52 to 1.58 hours after the trigger, and 13 minutes after the notice) and obtained 45 minutes of exposure in the r and z filters.
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 our both filters, we detected the optical counterpart reported by LCO (Quirola-Vasquez et al., GCN Circ. 41790), at preliminary magnitudes of:
r = 20.57 +/- 0.02
z = 19.87 +/- 0.03
Further observations and analysis are ongoing.
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 41790
Subject
EP250911a: LCO optical counterpart discovery
Date
2025-09-11T10:23:38Z (a year ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
J. A. Quirola-Vasquez (Radboud), R. A. J. Eyles-Ferris (Leicester), D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), A. Martin-Carrillo (UCD), L. Cotter (UCD), P. G. Jonker (Radboud), F. E. Bauer (SSI and UTA), and J. Chacón (PUC) report on behalf of a larger collaboration:
We observed the location of EP250911a (Liang et al., GCN 41788) with the Sinistro instrument mounted on the 1-m telescopes at McDonald Observatory, available via the LCO network. 6 x 200 s exposures were obtained in the SDSS r filter at a start time of 2025-09-11 UT 09:35:35 (~37.1 min post trigger).
Within the WXT and FXT error circles (Liang et al., GCN 41788) we locate a new source in our stacked r-band image, not visible in the archival Legacy Survey images. Its coordinates (0.5" error) are:
RA(J2000): 23:50:04.83
DEC(J2000): +29:17:59.6
Calibrated to nearby stars from the Pan-STARRS catalog, we measure a magnitude r = 20.04 +/- 0.07 (AB), not corrected for Galactic extinction.
Given the non-detection in the deeper archival images and the consistency with the FXT error circle, we suggest that this source is the optical counterpart of EP250911a.
GCN Circular 41788
Subject
EP250911a: Einstein Probe detection of an X-ray transient
Date
2025-09-11T09:56:36Z (a year ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Y. F. Liang (PMO), Q. Y. Wu, W. D. Zhang(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 EP250911a. The transient triggered EP-WXT (ID: 01709240741) at 2025-09-11T08:58:28 (UTC). The WXT position of the source is R.A. = 357.532 deg, DEC = 29.286 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. = 357.5204 deg, DEC = 29.3018 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).