EP260409a
GCN Circular 44296
Subject
EP260409a: Early NIR limits from MIRAGE on the MDM 1.3m telescope
Date
2026-04-14T17:44:51Z (2 months ago)
From
Viraj Karambelkar at Indian Inst of Tech,Bombay <karambelkarvraj21197@gmail.com>
Via
Web form
V. Karambelkar, K. De (Columbia University), G. Mo (Carnegie/Caltech), S. Ibrahim, D. Schiminovich (Columbia University)
We observed the field of the fast X-ray transient EP260409a (Jiang et al. 2026, GCN 44240) using the MDM InfraRed Astronomy inGaas Explorer (MIRAGE) instrument on the MDM 1.3m telescope in the near-infrared J and Hs bands. Observations were carried out starting at UTC 2026-04-09T03:10:30 in the J-band and 2026-04-09T03:40:58 in the H-band, approximately three hours after the EP trigger. The exposures lasted for a total of 16 minutes in the J-band and 32 minutes in the H-band.
We do not detect the counterpart reported by Quirola-Vasquez et al., GCN 44264 to 5-sigma limits of
J >~ 19.5 mag (AB)
Hs >~ 19.8 mag (AB)
MIRAGE is a new YJHs-band near-infrared imager for the MDM 1.3m telescope. We thank the MDM Observatory staff for supporting the observations.
GCN Circular 44264
Subject
EP260409a: Gemini/FLAMINGOS2 J-band counterpart
Date
2026-04-11T09:24:03Z (2 months ago)
From
P.G. Jonker at Radboud University <p.jonker@astro.ru.nl>
Via
Web form
J. Quirola-Vasquez (Radboud), J. Sanchez-Sierras (Radboud), A. J. Levan (Radboud & Warwick), P. G. Jonker (Radboud), A. Martin-Carrillo (UCD), D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), R. A. J. Eyles-Ferris (Leicester), F. E. Bauer (SSI and UTA), J. Chácon (PUC), B. Lemaux (Gemini) report on behalf of a larger collaboration:
We observed the field of the fast X-ray transient EP260409a (Jiang et al. 2026, GCN 44242) using the FLAMINGOS2 instrument mounted on the Gemini-South telescope. Two J-band observations were obtained, one on 2026-04-09 06:52:40 (~0.27 d after the EP trigger, T0, 8x40 s exposures) and one on 2026-04-11 02:13:36 UT (i.e., ~2.08 d after T0, 16x40 s exposures).
In our first epoch stacked images, we detect a candidate counterpart at coordinates (J2000):
RA: 11:31:37.985
DEC: -9:20:43.00
Photometry yields J = 21.06 +/- 0.10 (Vega) calibrated against nearby stars from the 2MASS catalog (not corrected for Galactic extinction). During the second epoch, the target is no longer detected, with a 3sigma limit of J>22.65 (Vega), implying it faded significantly.
No source is detected at this location in the Legacy Survey in any band, nor in archival J-, H-, and K-band images from the VISTA Hemisphere Survey.
Given the lack of optical detection (Globus et al., GCN 44243; Li et al., GCN 44244) and from our Las Cumbres observations we report a 3 sigma limit of r>23.5 and z>21.5 AB mag (obtained at ~1.37 and 3.26 hours, respectively, after T0 with an exposure time of 6x300 s), this candidate counterpart is significantly red, implying EP260409a could be a high-redshift or high extinction Fast X-ray Transient.
We thank the Gemini-South staff, for their rapid execution of these observations.
GCN Circular 44251
Subject
EP260409a: refined analysis of the EP-WXT and autonomous EP-FXT observations
Date
2026-04-10T06:06:36Z (2 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Z. X. Li, G. L. Huang, J. Y. Cao (IHEP, CAS), S. Q. Jiang, W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP260409a was detected by the Wide-field X-ray Telescople (WXT) on board the Einstein Probe (EP) mission (Jiang et al., GCN 44240). The refined analysis of the WXT data shows that the event started at T0=2026-04-09T00:16:43 (UTC). WXT coverage of the transient lasted for approximately 100 seconds after trigger, and was interrupted due to 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 2.74 × 10^(20) cm^-2, and a photon index of 0.6 +/- 0.7. The derived average unabsorbed 0.5-4 keV flux is 3.2 (-1.2, +1.7) × 10^(-10) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP autonomously observed this source a few minutes after the on board trigger. The FXT observation started at 2026-04-09T00:22:08 (UTC, T0+325 s). The effective exposure time of the observation is around 2 ks. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 172.9096, DEC = -9.3457 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The average FXT 0.5-10 keV spectra can be fitted with an absorbed power-law with a fixed Galactic hydrogen column density of 2.74 × 10^(20) cm^-2, and a photon index of 1.9 +/- 0.4. The derived average unabsorbed 0.5-10 keV flux is 6.4 (-1.8, +2.8) × 10^(-13) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
Further FXT follow-up observations have been arranged.
The optical follow-up observations were performed by Lipunov et al. (GCN 44241), Noémie Globus et al. (GCN 44243), Li et al. (GCN 44244).
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 44244
Subject
EP260409a: SVOM/VT optical upper limits
Date
2026-04-09T09:55:42Z (2 months ago)
Edited On
2026-04-09T13:47:42Z (2 months ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, C. Wu, L. P. Xin, Y. L. Qiu, Y. N. Ma, Z. H. Yao, J. R. Xu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.
SVOM/VT performed ToO observations of EP260409a triggered by Einstein Probe (Jiang et al., GCN 44240). The observation started at 2026-04-09T01:35:09 UTC, approximately 1.28 hours post trigger, in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
No uncatalogued sources were detected within the error box of EP-WXT (Jiang et al., GCN 44240), compared to the Legacy Survey. The following measurements are in the AB magnitude and are not corrected for Galactic extinction:
Mid time | Band | Exposure Time | 3 sigma upper limit
3.04 hr VT_B 106*50 sec > 23.7 mag
3.04 hr VT_R 101*50 sec > 23.6 mag
Our non-detection is consistent with the result from COLIBRÍ (Globus et al., GCN 44243).
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 44243
Subject
EP260409a: COLIBRÍ optical upper limits
Date
2026-04-09T04:04:15Z (2 months ago)
From
globus@astro.unam.mx
Via
Web form
Noémie Globus (UNAM), Marion Guelfand (CPPM), Camila Angulo (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report:
We imaged the field of the EP260409a (Jiang et al., GCN Circ. 44240) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-04-09 02:56 to 03:41 (from 2.6 to 3.4 hours after the trigger) and obtained 34 min of simultaneous exposure in the r and z filters.
The data were reduced and coadded with the ASU pipeline. 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 down to the following 5-sigma limit:
r > 23.14,
z > 21.84.
We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.
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 44241
Subject
EP260409a: Global MASTER-Net observations report
Date
2026-04-09T02:00:29Z (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-OAFA robotic telescope [1] located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP260409a ( EP Team et al., GCN 44240) errorbox 2305 sec after trigger time at 2026-04-09 00:56:46 UT, with upper limit up to 19.7 mag. The observations began at zenith distance = 36 deg. The sun altitude is -33.7 deg.
The galactic latitude b = 49 deg., longitude l = 273 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3241458
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
2335 | 2026-04-09 00:56:46 | MASTER-OAFA | (11h 31m 41.72s , -09d 16m 57.1s) | C | 60 | 19.5 |
2401 | 2026-04-09 00:57:52 | MASTER-OAFA | (11h 31m 41.64s , -09d 16m 56.1s) | C | 60 | 19.7 |
2467 | 2026-04-09 00:58:58 | MASTER-OAFA | (11h 31m 41.57s , -09d 16m 55.4s) | C | 60 | 19.6 |
2533 | 2026-04-09 01:00:04 | MASTER-OAFA | (11h 31m 41.51s , -09d 16m 54.5s) | C | 60 | 19.7 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
[1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp.
http : // www.pereplet.ru/lipunov/625.html
GCN Circular 44240
Subject
EP260409a: Einstein Probe detection of an X-ray transient
Date
2026-04-09T01:55:16Z (2 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
S. Q. Jiang (NAO, CAS), G. L. Huang, Z. X. Li, J. Y. Cao (IHEP, CAS), 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 EP260409a. The transient triggered EP-WXT (ID: 01709259735) at 2026-04-09T00:18:21 (UTC). The WXT position of the source is R.A. = 172.895 deg, DEC = -9.348 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. = 172.9080 deg, DEC = -9.3442 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).