EP260321a — All Circulars

Yu-Zhang(NAOC), Junjie-Jin(NAOC), Haiyang-Mu(NAOC), Yuguang-Sun(NAOC), Pengliang-Du (NAOC), Jie-Zheng(NAOC), Zhou-Fan(NAOC), Hong-Wu(NAOC) report on behalf of a large collaboration:

We performed optical observations of the field of a fast X-Ray transient EP260321a(Q.J. Huang et al., GCN 44068) in the g, r and i filter using Tsinghua-NAOC 0.8-m telescope (TNT) located at Xinglong, Hebei, China. A series of 300-s exposures were obtained on three epochs (March 25, 26, and 27, 2026), with the first observation at 2026-03-25T14:37:57, approximately 4 days after the EP FXT trigger (2026-03-21T12:30:18). We measure a preliminary magnitude calibrated against nearby stars from the Pan-STARRS catalog.

We summarize our observation results as follows:
Obs. No. | Time (UTC) | Exposure Time (s) | Filter | Apparent mag (AB) | Telescope Name
1 | 2026-03-25T14:37:57 | 300 s | g |17.70 +/- 0.24| Tsinghua-NAOC 0.8-m telescope (TNT)
2 | 2026-03-25T14:53:18 | 300 s | r |17.69 +/- 0.20| Tsinghua-NAOC 0.8-m telescope (TNT)
3 | 2026-03-25T15:08:39 | 300 s | i |17.42 +/- 0.25| Tsinghua-NAOC 0.8-m telescope (TNT)
4 | 2026-03-26T16:00:16 | 300 s | g |17.17 +/- 0.24| Tsinghua-NAOC 0.8-m telescope (TNT)
5 | 2026-03-26T16:15:37 | 300 s | r |17.30 +/- 0.23| Tsinghua-NAOC 0.8-m telescope (TNT)
6 | 2026-03-26T16:40:01 | 300 s | i |17.38 +/- 0.30| Tsinghua-NAOC 0.8-m telescope (TNT)
7 | 2026-03-27T13:25:21 | 300 s | g |16.61 +/- 0.01| Tsinghua-NAOC 0.8-m telescope (TNT)
8 | 2026-03-27T13:40:42 | 300 s | r |16.53 +/- 0.01| Tsinghua-NAOC 0.8-m telescope (TNT)
9 | 2026-03-27T13:56:03 | 300 s | i |16.43 +/- 0.02| Tsinghua-NAOC 0.8-m telescope (TNT)

Lin. B. He, X. Liu (NAOC), S.Y. Fu, A.D. Zhu, L. Lei, H.Z. Wu, W.H. Lei, Y.C. Zou (HUST), J. An, S.Q. Jiang, Z.P. Zhu, D. Xu (NAOC), J.Z. Liu (XAO), Zhong-Nan Dong, Bin Ma (Sun Yat-sen University)

We observed the field of EP260321a (Huang et. al, GCN 44068) using the JinShan 100C telescope, equipped with the INS Mars640 SWIR Camera, located at Altay, Xinjiang, China. Observation started at 2026-03-25 15:12:02 UT, i.e., 4.11 days post-burst, and a series of J-band frames were obtained.

The optical counterpart (Lee et al., GCN 44070; Tanvir et al., GCN 44082; Moran et al., GCN 44083; Ahumada et al., GCN 44084; Liu et al., GCN 44087; de Ugarte Postigo et al., GCN 44091; Xu et al., GCN 44092; Pankov et al. GCN 44103; Corcoran et al., GCN 44105; Rastinejad et al., GCN 44107; Gao et al., GCN 44110) was detected in our stacked J-band frame with magnitude J ~ 17.2 (Vega) at a mid-time 99.3 hrs post-burst. The magnitude was calibrated with the nearby 2MASS catalog and without the Galactic extinction correction.

We acknowledge the excellent support from T.Q. Chen and J.F. Zhang for enabling these observations.

Lin. B. He, X. Liu (NAOC), S.Y. Fu, A.D. Zhu, L. Lei, H.Z. Wu, W.H. Lei, Y.C. Zou (HUST), J. An, S.Q. Jiang, Z.P. Zhu, D. Xu (NAOC), J.Z. Liu (XAO), Zhong-Nan Dong, Bin Ma (Sun Yat-sen University)

We observed the field of EP260321a (Huang et. al, GCN 44068) using the JinShan 100C telescope, equipped with the INS Mars640 SWIR Camera, located at Altay, Xinjiang, China. Observation started at 2026-03-25 15:12:02 UT, i.e., 4.11 days post-burst, and a series of J-band frames were obtained.

The optical counterpart (Lee et al., GCN 44070; Tanvir et al., GCN 44082; Moran et al., GCN 44083; Ahumada et al., GCN 44084; Liu et al., GCN 44087; de Ugarte Postigo et al., GCN 44091; Xu et al., GCN 44092; Pankov et al. GCN 44103; Corcoran et al., GCN 44105; Rastinejad et al., GCN 44107; Gao et al., GCN 44110) was detected in our stacked J-band frame with magnitude J ~ 17.2 (Vega) at a mid-time 99.3 hrs post-burst. The magnitude was calibrated with the nearby 2MASS catalog and without the Galactic extinction correction.

We acknowledge the excellent support from T.Q. Chen and J.F. Zhang for enabling these observations.

The IceCube Collaboration (http://icecube.wisc.edu/) reports:

IceCube has performed a search for track-like muon neutrino events arriving from the direction of the X-ray transient EP260321a which was first detected by Einstein Probe (https://gcn.nasa.gov/circulars/events/ep260321a#gcn-circular-44068) to investigate possible neutrino emission associated with the shock breakout scenario proposed for this event. The position of the updated localization (https://gcn.nasa.gov/circulars/events/ep260321a#gcn-circular-44075) was searched over a time window covering the time range between [-3 hours, +1 hours] from Einstein Probe detection (2026-03-21 09:23:07 UTC to 2026-03-21 13:23:07 UTC), during which time IceCube was recording good quality data. The chosen time window is motivated by the shock breakout scenario, to account for possible high-energy neutrino emission produced during the shock propagation within the stellar envelope and in the immediate aftermath of the breakout.
In this case, we report a p-value of 1.0, consistent with background expectation. We accordingly derive a time-integrated muon-neutrino flux upper limit for this source of E^2 dN/dE = 2.8 x 10^-2 GeV cm^-2 at 90% CL, under the assumption of an E^-2 power law. 90% of events IceCube would detect from a source at this declination with an E^-2 spectrum have energies in the approximate energy range between 1 TeV and 6 PeV.

The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc@icecube.wisc.edu.

[1] IceCube Collaboration, R. Abbasi  et al., ApJ 910 4 (2021)

Rui-Chen Gao, Chun Chen, Duo-Le Cao, Zhong-Nan Dong, Wei-Sen Huang, Jin-Ji Li, Jia-Qi Lin, Pu Lin, Yun Shi, Hao-Nan Yang, Yan Yu, P H Thomas Tam, Rong-Feng Shen, Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team:

We observed the field of the fast X-ray transient EP260321a(Huang et. al, GCN 44068), using the Sun Yat-sen University (SYSU) 80 cm infrared telescope. Our observations were carried out on 2026 Mar 24 18:57 UT, 78.4 hours after the trigger, using stacked images in the J band.

We clearly detect the source at the position of the optical counterpart (Lee et al., GCN 44070; Tanvir et al., GCN 44082; Moran et al., GCN 44083, Ahumada et al., GCN 44084, Liu et al., GCN 44087, de Ugarte Postigo et al., GCN 44091, Xu et al., GCN 44092, Pankov et al. GCN 44103, Corcoran et al., GCN 44105, Rastinejad et al., GCN 44107), and we measure J = 17.489 +/- 0.232 Vega mag. The photometry was calibrated against nearby stars from the 2MASS catalog and is reported in the Vega system, without correction for Galactic extinction.

Further observations are ongoing.

The SYSU 80 cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.

GCN Circular 44108 is a duplicate submission of GCN Circular 44107.

GCN Circular 44108 is a duplicate submission of GCN Circular 44107.

Jillian Rastinejad, Gokul Srinivasaragavan (UMD), and Tomas Ahumada (NOIRLab) report on behalf of a larger collaboration:

Following the detection of the putative optical counterpart (Lee et al., GCN 44070; Tanvir et al., GCN 44082; Moran et al., GCN 44083, Ahumada et al., GCN 44084, Liu et al., GCN 44087, de Ugarte Postigo et al., GCN 44091, Pankov et al. GCN 44103) of EP 260321a (Huang et al., GCN 44068), we obtained optical spectroscopy using the Gemini Multi-Object Spectrograph (GMOS) mounted on Gemini-South, under Program GS-2026A-Q-119. The mean time of the observation was 23:50 UT on 2026 Mar 23 (3.5 days post-burst). The total exposure time was 4x600 s using the B480 grating, and covered the wavelength range 3500-7500 AA.

We clearly detect emission lines due to the underlying galaxy, which are well-matched to z = 0.0343 (Tanvir et al., GCN 44082). We also observe broad absorption features across a rest-frame wavelength range of 4200-5000 AA (assuming z=0.0343), consistent with the broad absorption lines observed in Type Ic-BL supernovae. Our observations confirm the presence of a supernova noted by Xu et al., GCN 44092 and Corcoran et al., GCN 44105.

We thank the Gemini staff for the rapid scheduling and execution of these observations.

G. Corcoran (UCD), A. Martin-Carrillo (UCD), L. Izzo (INAF/OACN), A. de Ugarte Postigo (LAM), D. B. Malesani (DAWN/NBI and Radboud), M. De Pasquale (U. of Messina), N. R. Tanvir (U. Leicester), R. A. J. Eyles-Ferris (U. Leicester), A.J. Levan (Radboud) report on behalf of the Stargate collaboration:

We observed the optical counterpart (Tanvir et al., GCN 44082; see also Lee et al., GCN 44070; Aryan et al., GCN 44081; Ahumada et al., GCN 44084; Liu et al., GCN 44087, Sankar et al., GCN 44089, de Ugarte Postigo et al., GCN 44091, Xu et al., GCN 44092, Pankov et al., GCN 44103), of EP260321a (Huang et al., GCNs 44068, 44075) with the FORS2 spectrograph on the ESO VLT UT1 (Antu) beginning on 2026-03-24 at 00:52 UT (2.515 days post-trigger). Our observations consisted of 3x300 s exposures in both the 300V and 300I grisms, spanning overall the wavelength range 3300-11000 AA.

From the 30 s R-band acquisition image we measure a magnitude of r = 18.31 +/- 0.09 mag (AB) calibrated using nearby stars from the Pan-STARRS catalogue, not corrected for Galactic extinction and not corrected by the host galaxy contribution at that position.

The acquired spectrum shows a good match to the spectrum of GRB-SN 2006aj (at 5 days pre-peak) using SNID SAGE (Stoppa in prep., https://github.com/FiorenSt/SNID-SAGE). Based on this similarity, we identify the supernova associated with EP260321a as a broad-lined type-Ic SN similar to other SNe accompanying GRBs and FXTs. This object is now named SN 2026gzf.

A figure comparing our spectrum (blue line) to SN 2006aj (red line) is available at the following link: https://shorturl.at/DpROp.

Further analyses and observations are ongoing.

We acknowledge the excellent expert support from the observing staff in Paranal, in particular Linda Schmidtobreick, Alonso Luna Ruiz Fernandez, Lorena Faundez, and Marcela Espinoza.

N. Pankov (IKI), A. Pozanenko (IKI), E. Shekotikhin (IKI), E. Klunko (ISTP), A. Volnova (IKI) report on behalf of IKI-GRB-Fun collaboration:

We observed the localization of the EP260321A (Huang et. al, GCN 44068) with the AZT-33IK telescope of the Mondy observatory. We obtained series in the R filter on epochs 2026-03-21, 2026-03-22 and 2026-03-23. The source reported by the Kinder (Lee et al., GCN 44070) and observed till now by (Aryan et al., GCN 44081; Ma et al., GCN 44074;  Aguilar-Ruiz et al., GCN 44076; Liang et al., GCN 44079; Aryan et al., GCN 44081; Tanvir et al., GCN 44082; Moran et al., GCN 44083; Ahumada et al., GCN  44084; Liu et al., GCN 44087; Sankar et al., GCN 44089; de Ugarte Postigo et al., GCN 44091; Xu et al., GCN 44092) is clearly visible in the co-add images on all epochs. The early brightening of the source is evident after subtracting the epochs (i.e., 2026-03-23 minus 2026-03-22). 

The photometry of the host galaxy + the source and observation details are listed below:

Date       UT start  t-T0         Exp.   Filter OT+host Err.  UL(3sigma)
                     (mid, days)  (s)
2026-03-21 14:50:00  0.11785      30x120   R     18.09  0.02  22.7
2026-03-22 16:02:29  1.16819      30x120   R     17.77  0.02  22.7
2026-03-23 14:48:46  2.11561      28x120   R     17.29  0.02  22.8

The photometry was calibrated using nearby USNO-B1.0 stars (R2 mags) and has not been corrected for the Galactic extinction. The photometry of the 2026-03-23 epoch might be affected by the host very slightly, while for other epochs the photometry is significantly contaminated by the host.

D. Xu (NAOC), G. Corcoran (UCD), J. An (NAOC), L. Izzo (INAF/OACN), N. Habeeb (Leicester), N. R. Tanvir (Leicester), A. J. Levan (Radboud and Warwick), C. C. Thoene (AbAO), S. Schulze (Weizmann), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBI), A. Saccardi (CEA/Irfu), A. Martin-Carrillo (UCD), G. Pugliese (API), A. de Ugarte Postigo (LAM), A. L. Thakur (INAF-IAPS) report on behalf of the Stargate collaboration:

We observed the variable point source, AT2026gzf, (Tanvir et al., GCN 44082; see also Lee et al., GCN 44070; Aryan et al., GCN 44081; Ahumada et al., GCN 44084; Liu et al., GCN 44087) associated with X-ray transient EP260321a (Huang et al., GCN 44068) with the X-shooter spectrograph on the ESO VLT UT3 (Melipal) beginning on 2026-03-23 at 00:38 UT (36.1 hr post-trigger). Our observations consisted of 4x600 s exposures, and span the wavelength range 3000-20600 AA.

Bright continuum flux is visible in all three arms of the spectrum. In addition to the blue continuum and narrow emission lines reported for the previous MUSE observation (Tanvir et al., GCN 44082), our X-shooter spectrum shows broad features below 5000 AA, similar to those seen in the early spectra of core-collapse supernovae, and inconsistent with those seen in typical young stellar populations. However, we caution that at this early phase there are limited templates for comparison, combined with some contribution from any underlying cluster light, and a search for matches within GELATO and SNID does not, at this time, yield a firm classification. 

Furthermore, the detection of narrow absorption features due to Ca II at z = 0.0344 conclusively excludes a Milky Way origin for this transient.

We acknowledge excellent support of the ESO observing staff at Paranal, in particular Alonso Luna Ruiz Fernandez, Elyar Sedaghati, Lorena Faundez and Marcela Espinoza.

D. Xu (NAOC), G. Corcoran (UCD), J. An (NAOC), L. Izzo (INAF/OACN), N. Habeeb (Leicester), N. R. Tanvir (Leicester), A. J. Levan (Radboud and Warwick), C. C. Thoene (AbAO), S. Schulze (Weizmann), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBI), A. Saccardi (CEA/Irfu), A. Martin-Carrillo (UCD), G. Pugliese (API), A. de Ugarte Postigo (LAM), A. L. Thakur (INAF-IAPS) report on behalf of the Stargate collaboration:

We observed the variable point source, AT2026gzf, (Tanvir et al., GCN 44082; see also Lee et al., GCN 44070; Aryan et al., GCN 44081; Ahumada et al., GCN 44084; Liu et al., GCN 44087) associated with X-ray transient EP260321a (Huang et al., GCN 44068) with the X-shooter spectrograph on the ESO VLT UT3 (Melipal) beginning on 2026-03-23 at 00:38 UT (36.1 hr post-trigger). Our observations consisted of 4x600 s exposures, and span the wavelength range 3000-20600 AA.

Bright continuum flux is visible in all three arms of the spectrum. In addition to the blue continuum and narrow emission lines reported for the previous MUSE observation (Tanvir et al., GCN 44082), our X-shooter spectrum shows broad features below 5000 AA, similar to those seen in the early spectra of core-collapse supernovae, and inconsistent with those seen in typical young stellar populations. However, we caution that at this early phase there are limited templates for comparison, combined with some contribution from any underlying cluster light, and a search for matches within GELATO and SNID does not, at this time, yield a firm classification. 

Furthermore, the detection of narrow absorption features due to Ca II at z = 0.0344 conclusively excludes a Milky Way origin for this transient.

We acknowledge excellent support of the ESO observing staff at Paranal, in particular Alonso Luna Ruiz Fernandez, Elyar Sedaghati, Lorena Faundez and Marcela Espinoza.

Antonio de Ugarte Postigo (LAM), Leonardo García García (UNAM), Rosa L. Becerra (UNAM), Alan M. Watson (UNAM), Noémie Globus (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (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), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), 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 EP260321a (Huang et al., GCN Circ. 44068, 44075) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope, following our previous observation (Aguilar-Ruiz et al. GCN Circ. 44076). We observed in two epochs between 2026-03-23 04:40 to 10:04 UTC (between 40.17 and 45.57 hours after the trigger) and obtained  imaging in the g, r, i, z and y bands.

The data were reduced and coadded with the ASU COLIBRÍ 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.

From our first observation, at a mid epoch T+16.33 h after the EP detection, to the second epoch of last night, at a mid epoch of T+43.61 h, the transient reported by Lee et al. (GCN Circ. 44070), has increased in brightness in r-band by 1.3+/-0.1 mag consistently with the reported evolution of the source (Lee et al. GCN Circ. 44070, Aryan et al. GCN Circ. 44081, Tanvir et al. GCN Circ. 44082, Moran et al. GCN Circ. 44083, Ahumada et al. GCN Circ. 44084, Liu et al. GCN Circ. 44087 and Sankar et al. GCN Circ. 44089). Our latest photometric measurement is r = 18.67 +/- 0.02 mag but we caution that the value is likely affected by the contribution of the underlying galaxy.

Further observations are planned.

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.

A. Sankar.K, A. Aryan, T.-W. Chen, W.-J. Hou, C.-S. Lin (all NCU), S. Yang (HNAS), A. K. H. Kong (NTHU), S. J. Smartt, J. Gillanders (both Oxford), M. Nicholl (QUB), T. Moriya (NAOJ), M. Saillenfest (LTE, Paris Observatory), Y.-N. Lee (NTNU), M.-H. Lee, A. Dutta, Y.-H. Lee, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), Y. J. Yang (NYUAD), Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz  M. Huber, K. Chambers (all IfA, Hawaii) report:

We obtained further optical observations of the field of the fast X-ray transient EP260321a (Huang et al., GCN 44068, 44075) using the 1m LOT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86; Aryan et al. 2025, ApJS, 281, 20).

The new LOT observations started at UTC 12:42 on March 23rd 2026 (MJD 61122.5295), corresponding to 48.20 hr after the EP-WXT trigger. At the refined EP/FXT position, coincident with the blue variable source first noted by Lee et al. (GCN 44070) and further discussed by Tanvir et al. (GCN 44082), Ahumada et al. (GCN 44084), and Liu et al. (GCN 44087), we detect the source in the g band. We used the AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform PSF photometry for this object:

Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude      | Seeing | Airmass
LOT       | g      | 61122.5295  | 48.20     |  300 * 1     | 18.30 +/- 0.03 | 1".02  | 1.14

The presented magnitude is calibrated using field stars from the ATLAS-RefCat2 catalog from MAST (Tonry et al. 2018, ApJ, 867, 105), is reported in the AB system, and is not corrected for the expected Galactic foreground extinction of A_g = 0.08 mag in the direction of the transient (Schlafly & Finkbeiner 2011).

Compared with our previous LOT measurement of g = 18.77 +/- 0.04 mag at 23.99 hr post-trigger (Aryan et al., GCN 44081), the source has brightened by approximately 0.47 mag. This is also consistent with, and extends, the re-brightening trend reported by Liu et al. (GCN 44087), indicating that the source is still rising.

Our time resolved monitoring of this source during the six hours after the FXT detection with the LOT (Lee et al., GCN 44070, Aryan et al., GCN 44081) indicates a sharp decline of 0.3 mag within the first 2 hours followed by a continued, monotonic rise, now at g = 18.3 at t-t0 = 48.2 hr. With the spectroscopic observation from Tanvir et al. (GCN 44082) indicating that the source may be associated the z = 0.0343, we propose the LOT optical lightcurve could be explained by the  shock cooling and a rise from a supernovae associated with the blue point like source visible in the archive images, also proposed by Liu et al. (GCN 44087). At M_g ~ -17.7, this is now at supernova-like luminosities, which would be consistent with Huang et al. (GCN 44075) interpretaiton of the EP260321a as shock breakout signature. 

The luminosity of the blue progenitor source would be very large for a single star, lying between M_r = -15 to -16. Elevated, high luminosity has been seen in LBV type precursors (e.g. SN1961V, Kochanek et al. 2011, ApJ 737, 76; and other LBV precursors, Smith et al. 2010, AJ 139, 1451), but this case would be extreme.  We note that long term Pan-STARRS monitoring shows this source to be variable over 7 years, with flux variations in the w-band difference imaging around 21 < w < 22. Further spectroscopic observations are required to determine if the transient shows high expansion velocities and lines that definitively place it in the z=0.0343 host. 

X. Liu, D. Xu (NAOC), S. Tinyanont, R. Anutarawiramkul, P. Butpan, K. Noysena (NARIT), J. An, Z.P. Zhu, N.C. Sun, W.X. Li, Z. Fan, S.Q. Jiang, L.B. He (NAOC), S.Y. Fu (HUST), J.Z. Liu (XAO) report on behalf of a large group:

Upon the alert of EP260321a detected by Einstein Probe (Huang et al., GCN 44068), we observed the field of EP260321a using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at New South Wales, Australia (SBO), and then using the 100B telescope of the JinShan project, located at Altay, Xinjiang, China.

The TRT observation started at 12:52:13 UT on 2026-03-21, i.e., 21.9 mins after the EP-WXT trigger. No uncatalogued optical transient is detected within the EP-FXT error cicle (Huang et al., GCN 44068) down to R ~ 20.5 mag.

We noticed the brightening of the catalogued blue point-like source (R.A. 09:59:42.880, Dec. 00:25:06.29, J2000), if compared with PanSTARRS and Legacy Survey, in the outskirt of the galaxy, SDSS J095942.88+002506.2, with a redshift z = 0.0343 in the DESI Legacy Survey DR1 spectroscopic catalogue, mentioned by Kinder (Lee et al., GCN 44070) and then spectroscopically confirmed at VLT/MUSE (Tanvir et al., GCN 44082). The point source has R ~ 19.0 (Vega) at a median time of 27.6 mins post-trigger, calibrated with PanSTARRS in which the source has r = 19.85 +/- 0.03 mag (AB).

The JinShan 100B telescope observed the field in g- and r- bands in the nights of 2026-03-21 and 2026-03-22.

In the first night, the blue source decayed a bit to g ~ 18.9 (AB and hereafter) at 2.3 hrs post-trigger and r ~ 19.3 mag at 2.8 hrs post-trigger. And the g-band brightening, if subtracted using the PanSTARS templates, is more prominent than in the r-band, suggesting the transient itself is also quite blue. Together with the rather deep ZTF pre-EP upper limit (Ahumada et al., GCN 44084), the TRT observation, and the Kinder first observation (Lee et al., GCN 44070), the transient shows a fast rising around the EP trigger time and then starts to decay around the first Kinder observation.

The decaying behavior is consistent with the cooling of Shock Break-Out (SBO) of a SN-like event (Huang et al., GCN 44075) and a thermal-like spectrum at VLT/MUSE (Tanvir et al., GCN 44082).

In the second night, the blue source re-brightened to g ~ 18.5 at 28.5 hrs post-trigger and r ~ 18.8 mag at 29.0 hrs post-trigger, which are also brighter than the second Kinder observation that is a bit earlier than the second JinShan (Aryan et al. GCN 44081).

This indicates that a SN-like main component is emerging and will likely continue to brighten to a peak in the coming days.

T. Ahumada (NOIRLab), X. J. Hall (CMU), D. A. Perley (LJMU) on behalf of  the Zwicky Transient Facility 

We report the Zwicky Transient Facility detection of ZTF26aaonmha (AT 2026gzf), the candidate optical counterpart to EP260321a (Huang et al., GCNs 44068, 44075) announced by Lee et al (GCN 44070, see also Aryan et al., GCN 44081, Tanvir et al. GCN 44082). 

The ZTF detection was observed at 2026-03-22 07:35:25 (~19 hr after the EP-trigger) in the ZTF-g band at a magnitude of 19.7 +/- 0.2 mag. The region was also observed in ZTF-r band at 2026-03-22 07:12:13, but resulted in a non-detection limit of 19.6 mag.

The source has no previous activity in the ZTF survey, with the most recent upper limit of g = 20.2 mag on 2026-03-21 08:06:02 (4.5 hr before the X-ray detection). The ZTF non-detection prior to the EP detection, together with the post-EP ZTF trigger, strongly supports the optical association.

We additionally note that the source is detected in the Rubin alert stream, although we caution that the alerts show a dipole structure and could be spurious detections. The variability in the Rubin data fluctuates in around r = 23.8 mag, with the most prominent pre-EP detection at r = 21.1 mag, ~25 days before the X-ray flare.

The combined Rubin and ZTF lightcurve can be found at https://babamul.caltech.edu/objects/LSST/314003014107006318

We are pursuing and encouraging follow-up observations to classify the source and validate the X-ray and optical association. 
 
Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan; Operations are conducted by Caltech's Optical Observatory (COO) and Caltech/IPAC. 

S. Moran, A. Kumar, G. Ramsay, D. O'Neill, K. Ulaczyk, S. Moran, B. P. Gompertz, R. Starling, M. Wortley, M. Kennedy, K. Ackley, M. Dyer, J. Lyman, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, B. Godson, T. Killestein, M. Pursiainen, on behalf of GOTO collaboration:

We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) that serendipitously covered the field of EP/WXT detected EP260321a (WXT01709259023; Huang et al., GCNs 44068, 44075). Observations covering the localisation area were taken at 2026-03-22 11:25:46 UT (+22.92h post trigger) utilising GOTO-South. Each observation consisted of 4x45s exposures in the GOTO L-band (400-700 nm).

We detect the previously reported variable source (Lee et al. GCN 44070, Tanvir et al. GCN 44082) with L = 19.51 ± 0.1 AB mag.

GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester, the University of Birmingham and the Instituto de Astrofisica de Canarias (IAC).

N. R. Tanvir (Leicester), L. Izzo (INAF/OACN), A. J. Levan (Radboud and Warwick), D. Xu (NAOC), G. Corcoran (UCD), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), G. Pugliese (API), A. de Ugarte Postigo (LAM), K. Wiersema (Hertfordshire), report on behalf of the Stargate collaboration:

We observed the field of EP260321a (Huang et al., GCNs 44068, 44075) with the ESO VLT UT1 (Antu) and UT4 (Yepun), obtaining R-band imaging with the FORS2 instrument (mid-time 00:44 UT on 2026-02-22; 12.23 hr after the EP trigger), and integral field spectroscopy with the MUSE instrument on UT4 (mid-time 05:29 UT on 2026-02-22; 16.98 hr after the EP trigger).

Within the refined EP/FXT error circle (Huang et al., GCN 44075), the blue, historically variable source coincident with the outskirts of a galaxy at z = 0.0343, noted by Lee et al. (GCN 44070), is prominent. We measure a magnitude of R = 19.5, which is within the range of archival values, but is significantly brighter than reported by Lee et al. (GCN 44070). The source itself is consistent with being marginally resolved in 0.7" seeing imaging, though this may be due to contamination from the underlying galaxy. 

Our spectroscopy shows the variable source (covering the wavelength range 4750 - 9300 Å) to be characterised by a blue power-law continuum, with very strong narrow emission lines at z = 0.0343 (e.g., [O III] 5008 EW ~ 500 Å). No absorption features are detected at either z = 0 or z = 0.0343. While the emission lines are visible across the entire galaxy, they are strongly spatially concentrated at the location of the blue variable object, with fluxes approximately twenty times brighter than at the galaxy nucleus. This makes it unlikely for the source to be a random foreground superposition, suggesting that it lies itself within the galaxy at z = 0.0343.

At this redshift, our measured flux corresponds to an absolute magnitude of approximately M_R = -16.8, and the past variability (Lee et al., GCN 44070) appears to be consistent with previous outbursts to a comparable flux level. It is possible that past variability was due to LBV-like outbursts from a star embedded within a dense, luminous cluster (as evidenced by the strong emission line component). The short-term variability following the outburst (Lee et al., GCN 44070; Aryan et al., GCN 44081), coupled with the location within the revised FXT error circle, suggest that this source is the optical counterpart of EP260321a. It is unclear if the current outburst is related to a further LBV flare or the terminal collapse of the star, as may be suggested by the possible shock breakout properties of the X-ray outburst (Huang et al., GCN 44075). Under this interpretation, this would make EP260321a the first example of an X-ray transient progenitor detected prior to explosion.

We encourage further monitoring of this object at all wavelengths in order to clarify its enigmatic nature.

We acknowledge excellent support of the ESO observing staff at Paranal, in particular Abel de Burgos Sierra, Alonso Luna Ruiz Fernandez, Elyar Sedaghati, Enrico Congiu, Israel Blanchard, and Rodrigo Palominos. We also thank the visitor observers on UT4, Silvia Piranomonte and Eleonora Parlanti, for generously allowing the execution of our observation.

A. Aryan, M.-H. Lee, T.-W. Chen, W.-J. Hou (all NCU), S. Yang (HNAS), A. K. H. Kong (NTHU), T. Moriya (NAOJ), S. J. Smartt, J. Gillanders (both Oxford), Y. J. Yang (NYUAD), A. Dutta, Y.-H. Lee, A. Sankar.K, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, C.-S. Lin H.-C. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), Z. N. Wang, D. C. Qiang, L. L. Fan (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 EP260321a (Huang et al., GCN 44068) using the 1m LOT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen et al., 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first LOT epoch of observations started at 11:34 UTC on the 22nd of March 2026 (MJD 61121.482), 23.07 hr after the EP-WXT trigger. 

We used Kinder pipeline (Yang et al. A&A 646, A22) to perform template subtraction with the DESI Legacy Survey (Dey et al. 2019, AJ 157, 168) DR10 image. In the difference image, we still detected this stellar source SDSS J095942.88+002506.2 (Lee et al., GCN 44070), but no other new transient has emerged.

Moreover, we used AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform PSF photometry for this variable star (without template subtraction). 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/avg.) Airmass
LOT       | r      | 61121.482   | 23.07     | 300  * 6     | 19.18 +/- 0.04 | 0".96  | 1.27
LOT       | g      | 61121.520   | 23.99     | 300  * 2     | 18.77 +/- 0.04 | 1".11  | 1.17
Our 3-sigma upper limit for the observed field is r = 23.4 mag.

EP260321a was identified as a possible supernova shock-breakout candidate in the EP refined analysis (Huang et al., GCN 44075). However, no optical counterpart has been reported to date (Lee et al., GCN 44070; Ma et al., GCN 44074; Aguilar-Ruiz et al., GCN 44076; Liang et al., GCN 44079). For comparison, in SN 2008D the optical counterpart was detected only ~1.4 hr after the initial X-ray detection (Soderberg et al. 2008). Given the similar X-ray duration to SN 2008D and the continued absence of any optical counterpart, we suspect that the EP260321a X-ray emission more likely originates from the variable star itself. If it is a Galactic X-ray binary, the spectrum suggests a supersoft (WD) accretor, but the X-ray flux is weak; a strong local stellar flare is also plausible.

The presented magnitudes are calibrated using the field stars from the ATLAS-RefCat2 catalog from MAST (Tonry J. L. et al. 2018, ApJ, 867, 105) and are not corrected for the expected Galactic foreground extinctions of A_r = 0.06 mag and A_g = 0.08 mag, in the direction of the transient (Schlafly & Finkbeiner 2011). The methodology, details on the Lulin observatory telescopes, and a compilation of our optical follow-up campaign for FXTs discovered within the first year of operation of the Einstein-Probe mission can be found in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.

Runduo Liang, Wenxiong Li (NAOC), Iair Arcavi (TAU), Ido Keinan (TAU), David Sand (U of Arizona)

We observed the position of EP260321a (GCN 44068, GCN 44075) with a Las Cumbres 1m telescope at Cerro Tololo Inter-American Observatory, Chile, about 11.25 hours after the Einstein Probe WXT trigger. We took 2x300s exposures in the broad optical r-band. We didn’t detect any uncataloged source within the EP-FXT refined position down to a 3-sigma limiting r-band mag of ~20.8. 

The presented magnitudes are calibrated with Pan-STARRS sources in the field and not corrected for Galactic extinction. Additional follow-up is encouraged.

Edilberto Aguilar-Ruiz (UNAM), Antonio de Ugarte Postigo (LAM), Rosa L. Becerra (UNAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), 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), and Benjamin Schneider (LAM) report:

We imaged the field of the Einstein Probe X-ray transient EP260321a (Huang et al., GCN Circ. 44068) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-03-22 03:48 to 05:50 UTC (from 15.3 to 17.4 hours after the trigger) and obtained 24, 65, and 89 minutes of exposure, respectively, in the g, r, and z filters.

The data were reduced and coadded with the COLIBRÍ ASU 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 Legacy Survey DR10 (Dey et al. 2019) using STDWeb, we do not detect any new source at the FXT refined position (Huang et al., GCN Circ. 44075) down to the following 5-sigma limit:

g > 23.8
r > 23.8
z > 22.8

Our upper limit in r is consistent with the ones reported by Lipunov et al. (GCN Circ. 44069) and Lee et al. (GCN Circ. 44070). We do see additional flux from the superposed star, also in agreement with Lee et al.

In order to test the shock breakout scenario proposed by Huang et al. (GCN Circ. 44075), we compared our results against the light curve of the AT2016gkg (Bersten et al. 2018), shifted at the redshift of the galaxy 2dFGRS TGN352Z077 at z=0.034 (DESI Legacy Survey, Dey et al. 2019). At the time of our observations, we expect an AB magnitude about 19.2 at 570 nm, which lies between our g and r filters. Our upper limits are about 4.6 magnitudes fainter than this, and so we are unable to confirm the presence of a shock breakout.

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.

Q. J. Huang (PMO, CAS), Z.-C. Zou (NJU), D. Y. Li, X. Mao, H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team:

The fast X-ray transient EP260321a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Huang et al., GCN 44068). The transient triggered WXT at T0=2026-03-21T12:23:07 (UTC). The WXT observation lasted for approximately 432 seconds, and was interrupted due to the autonomous follow-up observation. The WXT 0.5-4 keV spectrum was soft, and can be fitted with an absorbed black body with the absorption fixed at the Galactic value of 2.7 × 10^(20) cm^-2 and a temperature of 164 (-29/+40) eV. The derived average unabsorbed 0.5-4 keV flux is 8.0 (-1.9/+2.2) × 10^(-11) erg/s/cm^2.

The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2026-03-21T12:35:02 (UTC, T0+12min). The exposure time of this observation is 4266s. On-ground analysis of the FXT data found an uncatalogued fading source within the WXT error circle at R.A., Dec. = 149.9287, 0.4177 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). A significant decay was detected in the FXT light curve during the initial phase of FXT observation. The average 0.5-10 keV spectrum can be fitted with an absorbed black body with Galactic absorption and a temperature of 121 (-3/+3) eV. The derived average unabsorbed 0.5-10 keV flux is 9.8 (-0.3/+0.3) × 10^(-12) erg/s/cm^2. All quoted errors are at the 90% confidence level. We note that within the FXT error circle, there is a galaxy 2dFGRS TGN352Z077, at a redshift of 0.0345. If the transient is associated with this galaxy, the corresponding luminosity of the WXT detection would be approximately 2.2 × 10^(44) erg/s. Given its very soft spectrum, rapid decay and the corresponding luminosity at the assumed redshift, we tend to consider EP260321a as possibly a supernova shock breakout candidate.

No optical counterpart of EP260321a has been detected so far (Lipunov et al., GCN 44069; Lee et al., GCN 44070; Ma et al., GCN 44074) , and further mutil-band follow-up observations are encouraged to explore the nature of EP260321a.

A follow-up observation with the EP-FXT was planned, and 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).

Y. N. Ma, H. L. Li, Z. H. Yao, Y. L. Qiu, C. Wu, L. P. Xin, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. R. Xu, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team. 

SVOM/VT performed a Target of Opportunity observation of EP260321a detected by EP/WXT (Huang et al., GCN 44068). SVOM/VT began observing the field at 2026-03-21T13:10:39 UTC, 0.673 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. 

With X-band data available, no uncataloged source is found within the error box of FXT (Huang et al., GCN 44068) compared with DESI Legacy Survey at the mid time of 1.92 hours after trigger, VT_R > 22.8 mag with exposure time of 53*50 sec, VT_B > 23.0 mag with exposure time of 54*50 sec.
              
The stellar source and the galaxy reported by Kinder (Lee et al., GCN 44070) are detected in our observations, with no significant variability.

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.

M.-H. Lee, A. Aryan, T.-W. Chen, W.-J. Hou (all NCU), S. Yang (HNAS), S. J. Smartt, J. Gillanders (both Oxford), A. K. H. Kong (NTHU), Y. J. Yang (NYUAD), Y.-H. Lee, A. Sankar.K, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, C.-S. Lin H.-C. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), Z. N. Wang, D. C. Qiang, L. L. Fan (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 EP260321a (Huang et al., GCN 44068 using the 1m LOT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first LOT epoch of observations started at 13:38 UTC on the 21st of March 2026 (MJD 61120.568), 1.13 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. We utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction with the DESI Legacy Survey (Dey et al. 2019, AJ 157, 168) DR10 image using the 'SFFT' (Hu et al. 2022, ApJ, 936, 157) algorithm. In the difference image, we detected an obvious residual at RA, DEC = 09:59:42.82, 00:25:05.84. We noticed that this position coincides with a stellar source that sits close on top of a resolved galaxy detected in SDSS, Pan-STARRS1 and Legacy Surveys (SDSS J095942.88+002506.2). The stellar source has  varying magnitudes in different catalogs (PS1 r = 20.42 +/- 0.25, SDSS r = 21.07 +/- 0.05, DESI r = 19.07). This is likely an unrelated variable star. 
 
Moreover, we used AutoPhOT to perform PSF photometry at the residual's position. 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      | 61120.568   | 1.13      | 300  * 6     | 20.97 +/- 0.04     | 1".02       | 1.09

Our 3-sigma upper limit for the observed field is r = 23.1 mag using the SDSS catalogue as the reference.

The galaxy SDSS J095942.88+002506.2 is at z = 0.0343, as measured in 2dF and in the DESI Legacy Survey DR1 spectroscopic catalogue (TARGETID: 39627799320858502) lies within the EP FXT localisation region. If EP260321 is associated with this galaxy, then our detection limits imply there is no new source brighter than an absolute magnitude of M ~ -12.6 mag at t-t0 = 1.13 hours after the FXT trigger. A supernova or kilonova would still be rapidly rising in this phase; further observations are encouraged. 

The presented magnitude is calibrated using the field stars from the ATLAS-RefCat2 catalog from MAST (Tonry J. L. et al. 2018, ApJ, 867, 105) and is not corrected for the expected Galactic foreground extinction of A_r = 0.06 mag in the direction of the transient (Schlafly & Finkbeiner 2011). The methodology, details on the Lulin observatory telescopes, and a compilation of our optical follow-up campaign for FXTs discovered within the first year of operation of the Einstein-Probe mission can be found in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.

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-Tunka robotic telescope  [1]  located in Russia (Applied Physics Institute, Irkutsk State University) was pointed to the EP260321a ( EP Team et al., GCN 44068) errorbox  194 sec after notice time and 2105 sec after trigger time at 2026-03-21 13:05:23 UT, with upper limit up to  19.1 mag. Observations started at twilight.  The observations began at zenith distance = 56 deg. The sun  altitude  is -16.9 deg. 

The galactic latitude b = 41 deg., longitude l = 239 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=3200651

We obtain a following upper limits.  

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

    2135 | 2026-03-21 13:05:23 |        MASTER-Tunka | (09h 57m 47.48s , +00d 56m 51.7s) |   C |    60 | 18.8 |        
    2135 | 2026-03-21 13:05:23 |        MASTER-Tunka | (09h 57m 34.12s , +01d 01m 05.4s) |   C |    60 | 19.0 |        
    2208 | 2026-03-21 13:06:36 |        MASTER-Tunka | (09h 57m 50.16s , +00d 58m 39.4s) |   C |    60 | 18.8 |        
    2208 | 2026-03-21 13:06:36 |        MASTER-Tunka | (09h 57m 36.78s , +01d 02m 53.3s) |   C |    60 | 19.1 |        
    2283 | 2026-03-21 13:07:50 |        MASTER-Tunka | (09h 57m 44.76s , +00d 57m 41.4s) |   C |    60 | 18.9 |        
    2283 | 2026-03-21 13:07:50 |        MASTER-Tunka | (09h 57m 31.35s , +01d 01m 55.6s) |   C |    60 | 19.1 |        
    2354 | 2026-03-21 13:09:02 |        MASTER-Tunka | (09h 57m 45.04s , +00d 58m 40.3s) |   C |    60 | 18.8 |        
    2354 | 2026-03-21 13:09:02 |        MASTER-Tunka | (09h 57m 31.61s , +01d 02m 54.6s) |   C |    60 | 19.0 |        
    2429 | 2026-03-21 13:10:17 |        MASTER-Tunka | (09h 57m 50.55s , +00d 57m 39.6s) |   C |    60 | 18.8 |        
    2429 | 2026-03-21 13:10:17 |        MASTER-Tunka | (09h 57m 37.10s , +01d 01m 53.7s) |   C |    60 | 19.0 |        
    2502 | 2026-03-21 13:11:30 |        MASTER-Tunka | (09h 57m 45.69s , +00d 56m 38.6s) |   C |    60 | 18.9 |        
    2502 | 2026-03-21 13:11:30 |        MASTER-Tunka | (09h 57m 32.22s , +01d 00m 52.6s) |   C |    60 | 19.1 |        
    2575 | 2026-03-21 13:12:42 |        MASTER-Tunka | (09h 57m 37.50s , +01d 01m 04.8s) |   C |    60 | 19.1 |        
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

Q.J. Huang (PMO, CAS), Z. C. Zou (NJU), X. Mao, D. Y. Li, 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 EP260321a. The transient triggered EP-WXT (ID: 01709259023) at 2026-03-21T12:30:18 (UTC). The WXT position of the source is R.A. = 149.894 deg, DEC = 0.397 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. = 149.9260 deg, DEC = 0.4181 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).