EP250711a
GCN Circular 41212
Subject
EP250711a: CrAO and Assy optical observations
Date
2025-07-31T14:09:22Z (a month ago)
From
Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>
Via
Web form
N. Pankov (HSE, IKI), V. Rumyantsev (CrAO), V. Kim (FAI), A. Pozanenko (IKI), A. Volnova (IKI) report on behalf of IKI-GRB-FuN:
We observed the field of a fast X-ray transient EP250711a detected by EP (Wang et. al, GCN 41078) using the 2.6-meter Shajn telescope (ZTSh) of the Crimean Astrophysical Observatory (CrAO) and the 1.5-meter AZT-20 telescope of the Assy-Turgen Astrophysical Observatory (Assy). The observations at Assy started on 2025-07-21 at 19:53:57 UT, i.e. ~10.14 days since trigger. A series of 101x60 s images in the r'-band was obtained. The observations at CrAO started on 2025-07-24 at 20:28:51 UT, i.e. ~13.17 days since trigger. A series of 61x120 s images was obtained in the R-band. In the co-add image taken by ZTSh we tentatively detect a source at the position of the optical source initially detected by NOT (Fu et. al, GCN 41071) that was also observed by COLIBRÍ (Becerra et. al, GCN 41073; Becerra, GCN 41084). The preliminary photometry is provided below:
Date UT start t-T0 Exp. Filter OT S/N UL Telescope
(mid, days) (n*s) (3sigma)
2025-07-21 19:53:57 10.14828 101x60 r' n/d n/d 22.5 Assy/AZT-20
2025-07-24 20:28:51 13.17980 61x120 R 23.5 2.0 23.1 CrAO/ZTSh
To calibrate both r' and R bands we used common stars of the USNO-B1.0 (R2) and the PS1 DR2 (rmag) catalogs. No correction was made for the Galactic extinction. We note that our result is fainter in comparison with the last COLIBRÍ observation (Becerra, GCN 41084) taken between 1.4192 and 1.725 days since trigger. We may suggest the source might be the host galaxy.
Ref. stars
USNO-B1.0 / PS1
RA Dec R2 rmag
292.1601 4.6919 16.86 17.44
292.1451 4.7120 17.17 17.76
292.1579 4.6937 17.98 18.76
GCN Circular 41084
Subject
EP250711a: COLIBRÍ continuous optical observations
Date
2025-07-13T21:34:30Z (2 months ago)
From
Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Via
Web form
Rosa L. Becerra (U Roma), Alan M. Watson (UNAM), Nikos Mandarakas (LAM),
Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Fredd Alvarez (UNAM), Camila Angulo (UNAM), 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):
We continued the follow-up campaign of EP250711a (Wang et al., GCN Circ. 41070) using the DDRAGO wide-field imager on the COLIBRÍ telescope. Observations were conducted on 2025-07-13 from 04:57 to 10:32 UTC (35.8 to 41.4 hours after the trigger), obtaining 61 minutes of exposure in each of the g, r and i 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. We measure preliminary magnitudes of:
g = 23.02 +/- 0.24
r = 22.78 +/- 0.22
i = 21.64 +/- 0.11
Compared with the value reported in Becerra et al. (GCN Circ. 41073), we estimate a temporal decay index of ~0.9.
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 41077
Subject
EP250711a: Global MASTER-Net observations report
Date
2025-07-12T22:18:20Z (2 months 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 EP250711a ( EP Team et al., GCN 41070) errorbox 51485 sec after notice time and 1 days 18055 sec after trigger time at 2025-07-12 22:11:51 UT, with upper limit up to 16.4 mag. The observations began at zenith distance = 40 deg. The sun altitude is -23.9 deg.
The galactic latitude b = -7 deg., longitude l = 42 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2933283
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
104485 | 2025-07-12 22:11:51 | MASTER-Kislovodsk | (19h 27m 47.40s , +04d 52m 25.4s) | C | 60 | 16.4 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 41074
Subject
EP250711a: COLIBRÍ refined photometry and optical confirmation of the candidate (duplicate submission)
Date
2025-07-12T13:03:56Z (2 months ago)
Edited On
2025-07-14T13:14:14Z (2 months ago)
From
Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov>
Via
Web form
This Circular is a duplicate of GCN Circ 41073.
GCN Circular 41073
Subject
EP250711a: COLIBRÍ refined photometry and optical confirmation of the candidate
Date
2025-07-12T13:02:51Z (2 months ago)
From
Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Via
Web form
Rosa L. Becerra (U Roma), Jean-Luc Atteia (IRAP), Kin Ocelotl López (UNAM), Stéphane Basa (UAR Pytheas), Alan M. Watson (UNAM), William H. Lee (UNAM), Fredd Alvarez (UNAM), Camila Angulo (UNAM), 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), 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 continued the follow-up campaign of EP250711a (Wang et al., GCN Circ. 41070) using the DDRAGO wide-field imager on the COLIBRÍ telescope. Observations were conducted on 2025-07-12 from 03:40 to 05:46 UTC (10.5 to 12.6 hours after the trigger), obtaining 96 minutes of exposure in the i band. Additional observations were carried out from 2025-07-12 09:46 to 11:21 UTC (16.6 to 18.2 hours after the trigger), with a total of 64 minutes of exposure in the r band.
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.
Due to the significantly larger number of images in the stacks than in López et al. (GCN Circ. 41069), it is now possible to confirm the candidate reported by Fu et al. (GCN Circ. 41071). We measure preliminary magnitudes of:
r = 22.02 +/- 0.17
i = 21.13 +/- 0.11
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 41071
Subject
EP250711a: NOT optical counterpart candidate
Date
2025-07-12T08:45:33Z (2 months ago)
From
Dong Xu at NAOC/CAS <dxu@nao.cas.cn>
Via
Web form
S.Y. Fu (HUST), S.Q. Jiang, J. An, Z.P. Zhu, X. Liu, L.B. He, D. Xu (NAOC), J.P.U. Fynbo (DAWN/NBI), V. Vuolteenaho (NOT) report on behalf of a large collaboration:
We observed the field of EP250711a detected by EP/WXT (Wang et al., GCN 41070), using the ALFOSC camera mounted on the Nordic Optical Telescope (NOT). We obtained 5 x 200 s exposures in the SDSS z-band, starting at 03:02:59 UT on 2025-07-12, i.e., 9.87 hrs after the EP trigger.
An uncatalogued optical transient (OT) is detected within the EP/FXT error circle (Wang et al., GCN 41070) at coordinates
R.A.(J2000) = 19:28:37.94
Dec.(J2000) = +4:42:06.80
with an uncertainty of ~ 0.5 arcsec. The source has z ~ 20.2 mag, calibrated with nearby PanSTARRS stars and not corrected for Galactic extinction. We think the source is likely the optical counterpart of EP250711a.
We also note that there is a source north-west to the OT, being offset only by ~ 0.9 arcsec in the PanSTARRS, which has z ~ 21.5 mag and may be physically connected to EP250711a.
GCN Circular 41070
Subject
EP250711a: EP-WXT detection and FXT follow-up observation of a fast X-ray transient
Date
2025-07-12T07:53:13Z (2 months ago)
Edited On
2025-07-12T17:31:52Z (2 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Y. Wang (PMO, CAS), D.Y. Li, W.X. Wang, W. Yuan (NAO, CAS) report on behalf of the Einstein Probe team:
We report on the detection of a fast X-ray transient by EP-WXT, designated EP250711a (trigger ID: 08500000365). The source was detected on ground from the telemetry data, causing a significant delay from the occuring time of the event. The onboard alert system was not triggered due to the low Galactic lattitude of the source, and no automated follow-up was performed with EP-FXT due to the observation was an non-interruptible one.The WXT position of the source is R.A. = 292.153 deg, DEC = 4.707 deg (J2000) with an uncertainty of 2.2 arcmin in radius (90% C.L. statistical and systematic). The observation begun at 2025-07-11T17:10:56 (UTC), and as suggested from the EP-WXT light curve, the transient was already present at the start time of the observation. The light curve exihbits a double-peak profile, and lasted for about 1300 seconds. The average 0.5-4 keV spectrum can be fitted with an absorbed powerlaw model, with the absorption fixed at the Galactic value of 3.78 x 10^22 cm^-2, and a photon index of 2.5 (+/-0.5). The derived average unabsorbed 0.5-4 keV flux is 2.1 (+/-0.4) x 10^-10 erg/s/cm2. The peak flux is about 8 x 10^(-10) erg/s/cm2. The uncertainties are at the 90% confidence level for the above parameters.
We performed a follow-up target of opportunity observation with EP-FXT, starting at 2025-07-12T04:02:18Z, about 11 hours after the WXT detection. Within the WXT error circle, an uncataloggued X-ray source was detected at R.A. = 292.1614 , Dec.= 4.7035 with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). Further analysis will be updated after receiving the telemetry data. An i band upper limit of 21.3 magnitude was given by a COLIBRÍ observation performed about 10.5 hours after the EP-WXT trigger (GCN 41069).
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).