GCN 44170: EP260327a: Xinglong optical upper limit
2026-04-01T04:32:23.684Z | rev 0
Yu-Zhang(NAOC), Junjie-Jin(NAOC), Haiyang-Mu(NAOC), Junjun-Jia(NAOC), Jie-Zheng(NAOC), Zhou-Fan(NAOC), Hong-Wu(NAOC) report on behalf of a large collaboration:
Following the detection of EP260327a by EP-WXT (C. Y. Dai, GCN 44126), we observed the field of EP260327a using the 2.16-m telescope at Xinglong Observatory, NAOC. We obtained 6x300s clear-band frames with a median time of 2026-03-27T14:06:25 i.e., 8.2 hr after the EP trigger. No uncatalogued optical transient is detected in the stacked images within the 20 arcsec EP/FXT error circle (C. Y. Dai, GCN 44126), down to 3-sigma limiting magnitudes of clear ~ 20.87 mag, calibrated with Pan-STARRS sources in the field. Also there is no apparent brightening for the catalogued sources within the error circle.
GCN 44136: EP260327a: refined analysis of the EP-WXT and autonomous EP-FXT observations
2026-03-28T07:22:27.374Z | rev 0
Z. H. Yang(IHEP), C. Y. Dai (NJU), Q. C. Zhao(IHEP), W. F. Wen(SZTU) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP260327a was detected by the Wide-field X-ray Telescople (WXT) on board the Einstein Probe (EP) mission (Dai et al, GCN 44126). No reliable optical counterpart has been identified, and only upper limits have been reported (GCNs 44126, 44127, 44129, 44131, 44132, and 44135). The refined analysis of the WXT data shows that the event started at T0=2026-03-27 05:54:41 (UTC), and lasted for approximately 100 seconds. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 7.4×10^20 cm^-2, and a photon index of 1.2 (-0.4/+0.4). The derived average unabsorbed 0.5-4 keV flux is 1.9 (-0.4/+0.5) × 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 2026-03-27 05:56:22 (UTC, T0+109 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. = 215.3694, DEC = 42.1379 (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 7.4×10^20 cm^-2, and a photon index of 3.4 (-0.3/+0.3). The derived average unabsorbed 0.5-10 keV flux is 7.3 (-1.3/+1.3) × 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.
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 44132: EP260327a: WINTER J-band upper limit
2026-03-27T17:20:48.734Z | rev 0
Benjamin Schneider (LAM), Geoffrey Mo (Caltech/Carnegie), Viraj Karambelkar (Columbia), Tomas Ahumada (NOIRLab), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of the EP260327a (Dai et al., GCN Circ. 44126) in the near-infrared with the Palomar 1-m telescope, equipped with the 1-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024). Observations started on 2026-03-27 at 06:33:36 UT (38.92 min after the trigger) and consisted of 30 exposures of 120 s in the J-band.
The images were processed using the WINTER data reduction pipeline implemented with mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565) and analysed with STDWeb/STDPipe (Karpov 2025). The photometric calibration was performed using nearby stars from the 2MASS catalog. The magnitude 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 (Dai et al., GCN Circ. 44126) and derive the following 5-sigma upper limit:
J > 19.6
We note that van Hoof et al., (GCN Circ. 44129) and Guelfand et al., (GCN Circ. 44131) report deep, early-time optical non-detections.
WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.
GCN 44131: EP260327a: COLIBRÍ optical upper limits
2026-03-27T17:16:15.183Z | rev 0
Marion Guelfand (CPPM), Enrique Moreno Méndez (UNAM), Jean-Grégoire Ducoin (CPPM), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), Benjamin Schneider (LAM), William H. Lee (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Antonio de Ugarte Postigo (LAM) report:
We imaged the field of the X-ray transient EP260327a (Dai et al., GCN Circ. 44126) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-03-27 06:03:18 to 08:10:24 UTC (from 8.6 to 135 minutes after the trigger) and obtained 5760 seconds of simultaneous exposure in the r/z filters.
The data were reduced and coadded with the COLIBRÍ ASU pipeline and 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 the stacked image, and after subtracting template images from the Subaru template images from Hyper Suprime-Cam Strategic Program (Aihara et al. 2021, doi:10.1093/pasj/psab122), we do not detect any new source at the FXT refined position (Dai et al., GCN Circ. 44126) down to the following 5-sigma limits:
r > 24.4,
z > 23.3.
We do not find any optical transient at the position of the Gemini candidate (Rob Eyles-Ferris et al GCN Circ. 44129), although our observations started approximately three hours earlier.
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 44129: EP260327a: Gemini-North optical upper limits
2026-03-27T13:44:41.142Z | rev 0
A. van Hoof (Radboud), G. Corcoran (UCD), J. Quirola-Vasquez (Radboud), R. A. J. Eyles-Ferris (Leicester), A. J. Levan (Radboud), A. Martin-Carrillo (UCD), F. E. Bauer (SSI and UTA), J. Chácon (PUC) and P. G. Jonker (Radboud) report on behalf of a larger collaboration:
We observed the field of the fast X-ray transient EP260327a (Dai et al., GCN 44126) using the GMOS-N instrument mounted on the Gemini-North telescope. A series of 4x120 s exposures were taken in each of the r’ and z’ filters, starting at 2026-03-27 08:55:05 UT (3.00 hr after the trigger).
We performed subtractions on our stacked images using template images from Hyper Suprime-Cam Strategic Program (Aihara et al. 2021, doi:10.1093/pasj/psab122) and compared the stacks and templates manually. Within the EP-FXT localisation (Dai et al., GCN 44126), we do not identify any new sources in our stacked images or subtractions in either band. For our stacked images, we calculate 3-sigma limits of r > 24.72 and z > 24.62 calibrated to nearby stars from PanSTARRS DR1 and not corrected for Galactic extinction.
We do note the presence of a residual within the EP-FXT localisation in some of our subtractions at a position RA 215.3703, Dec 42.1406. This position is coincident with a DESI Legacy Survey object with a photometric redshift of 0.481 +/- 0.023. Given the magnitude of this object and depth of our images, we believe this to be a subtraction artefact rather than a real source.
We thank the Gemini-North staff, particularly Denise Hung, for their rapid execution of these observations.
GCN 44127: EP260327a: Global MASTER-Net observations report
2026-03-27T08:54:22.110Z | rev 0
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 EP260327a ( EP Team et al., GCN 44126) errorbox 510 sec after trigger time at 2026-03-27 06:03:11 UT, with upper limit up to 20.3 mag. The observations began at zenith distance = 74 deg. The sun altitude is -55.4 deg.
The galactic latitude b = 66 deg., longitude l = 77 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3214974
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
561 | 2026-03-27 06:03:11 | MASTER-OAFA | (14h 22m 35.11s , +42d 24m 43.5s) | C | 100 | 19.7 |
696 | 2026-03-27 06:03:11 | MASTER-OAFA | (14h 22m 35.11s , +42d 24m 43.5s) | C | 370 | 20.3 | Coadd
678 | 2026-03-27 06:04:58 | MASTER-OAFA | (14h 22m 35.03s , +42d 24m 50.5s) | C | 120 | 19.8 |
819 | 2026-03-27 06:07:04 | MASTER-OAFA | (14h 22m 34.92s , +42d 24m 58.1s) | C | 150 | 19.8 |
930 | 2026-03-27 06:09:40 | MASTER-OAFA | (14h 22m 34.83s , +42d 25m 03.4s) | C | 60 | 19.5 |
990 | 2026-03-27 06:09:40 | MASTER-OAFA | (14h 22m 34.83s , +42d 25m 03.4s) | C | 180 | 20.2 | Coadd
995 | 2026-03-27 06:10:46 | MASTER-OAFA | (14h 22m 34.77s , +42d 25m 06.2s) | C | 60 | 19.5 |
1060 | 2026-03-27 06:11:51 | MASTER-OAFA | (14h 22m 34.69s , +42d 25m 09.4s) | C | 60 | 19.5 |
1126 | 2026-03-27 06:12:57 | MASTER-OAFA | (14h 22m 34.62s , +42d 25m 12.2s) | C | 60 | 19.5 |
1186 | 2026-03-27 06:12:57 | MASTER-OAFA | (14h 22m 34.62s , +42d 25m 12.2s) | C | 180 | 20.2 | Coadd
1192 | 2026-03-27 06:14:02 | MASTER-OAFA | (14h 22m 34.55s , +42d 25m 15.1s) | C | 60 | 19.5 |
1257 | 2026-03-27 06:15:08 | MASTER-OAFA | (14h 22m 34.47s , +42d 25m 18.0s) | C | 60 | 19.5 |
1323 | 2026-03-27 06:16:14 | MASTER-OAFA | (14h 22m 34.40s , +42d 25m 21.0s) | C | 60 | 19.5 |
1383 | 2026-03-27 06:16:14 | MASTER-OAFA | (14h 22m 34.40s , +42d 25m 21.0s) | C | 180 | 20.0 | Coadd
1389 | 2026-03-27 06:17:20 | MASTER-OAFA | (14h 22m 34.35s , +42d 25m 24.2s) | C | 60 | 19.5 |
1455 | 2026-03-27 06:18:25 | MASTER-OAFA | (14h 22m 34.26s , +42d 25m 27.1s) | C | 60 | 19.5 |
1521 | 2026-03-27 06:19:31 | MASTER-OAFA | (14h 22m 34.17s , +42d 25m 30.0s) | C | 60 | 19.5 |
1581 | 2026-03-27 06:19:31 | MASTER-OAFA | (14h 22m 34.17s , +42d 25m 30.0s) | C | 180 | 20.2 | Coadd
1586 | 2026-03-27 06:20:36 | MASTER-OAFA | (14h 22m 34.09s , +42d 25m 32.9s) | C | 60 | 19.5 |
1651 | 2026-03-27 06:21:42 | MASTER-OAFA | (14h 22m 34.03s , +42d 25m 35.6s) | C | 60 | 19.4 |
1717 | 2026-03-27 06:22:47 | MASTER-OAFA | (14h 22m 33.95s , +42d 25m 38.2s) | C | 60 | 19.5 |
1777 | 2026-03-27 06:22:47 | MASTER-OAFA | (14h 22m 33.95s , +42d 25m 38.2s) | C | 180 | 20.2 | Coadd
1782 | 2026-03-27 06:23:53 | MASTER-OAFA | (14h 22m 33.88s , +42d 25m 41.1s) | C | 60 | 19.5 |
1848 | 2026-03-27 06:24:59 | MASTER-OAFA | (14h 22m 33.80s , +42d 25m 43.6s) | C | 60 | 19.5 |
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 44126: EP260327a: Einstein Probe detection of an X-ray transient
2026-03-27T08:28:31.849Z | rev 0
C. Y. Dai (NJU), Z. H. Yang, Q. C. Zhao (IHEP), W. F. Wen (SZTU), H. W. Pan (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 EP260327a. The transient triggered EP-WXT (ID: 01709259176) at 2026-03-27T05:54:41 (UTC). The WXT position of the source is R.A. = 215.371 deg, DEC = 42.130 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. = 215.3746 deg, DEC = 42.1396 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).