EP260321a
GCN Circular 44412
Subject
EP260321a/SN 2026gzf: spectroscopy at Mondy observatory
Date
2026-04-27T15:01:46Z (6 months ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin (SAO RAS), I. Zaznobin (IKI), M. Eselevich (ISTP),
A. Volnova (IKI), N. Pankov (IKI), A. Pozanenko (IKI),
report on behalf of IKI-GRB-FuN:
We observed the optical counterpart of the fast X-ray transient
EP260321a (Huang et al., GCN 44068; GCN 44075) identified early
as SN 2026gzf (Lee et al., GCN 44070; Ma et al., GCN 44074;
Aguilar-Ruiz et al., GCN 44076; 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;
Pankov et al., GCN 44103; Corcoran et al., GCN 44105;
Rastinejad et al., GCN 44107; Gao et al., GCN 44110; Zhang et al.,
GCN 44171; Li et al., GCN 44284; Gupta et al., GCN 44348;
also detected as NIR source: He et al., GCN 44117; Volnova et al.,
GCN 44295) with AZT-33IK telescope (Mondy) equipped with the ADAM
spectrograph (Afanasiev et al., Astrophysical Bulletin, 2015,
Vol. 71, Issue 4, p. 479; Burenin et al., Astronomy Letters, 2016,
Vol. 42, Issue 5, p. 295).
We obtained 4 x 900 sec spectra with VPHG600G grism
(range 3600--7250AA, dispersion 3.6 A/pix) on 2026-04-13,
15:02:08--16:02:47 UT(t_mid - T0 = 23.1265 days).
In the spectra we clearly detected host galaxy emission lines
of Balmer series, [O III] 4959A, 5007A, 4363A, [O II] 3727A,
[S II] 6717A and 6731A, He I 5876A at the redshift of z = 0.0343
which is consistent with other reported measurements (Tanvir et al.,
GCN 44082; Xu et al., GCN 44092; Corcoran et al., GCN 44105;
Rastinejad et al., GCN 44107).
We also detected broad lines of supernova SN 2026gzf which we
interpreted as Fe II and Si II with the velocity of about 10000 km/s.
Comparison with the SNID (Blondin & Tonry, 2007, ApJ, 666, 1024)
showed best fit with SN 1997ef, SN 2006aj, SN 2002ap and SN 1998bw
supernovae.
GCN Circular 44403
Subject
EP260321a: Upper limits from ATCA radio observations
Date
2026-04-23T17:14:05Z (6 months ago)
From
Nayana A J at University of California, Berkeley <nayan89deva@gmail.com>
Via
Web form
Nayana A.J., Eli Wiston, Raffaella Margutti, Itai Sfaradi, Ryan Chornock (UC Berkeley) report:
We observed the field of EP260321a (Huang et al., GCN 44068) with the Australia Telescope Compact Array (ATCA) using the BIGCAT system. The observations were conducted on 2026 March 30.37 UT in the 15 mm band, utilizing a total instantaneous bandwidth of approximately 8 GHz, divided into four 2 GHz-wide sub-bands, each imaged independently. The resulting images have central observing frequencies of 17, 19, 21, and 23 GHz. In our preliminary analysis, we do not detect radio emission at the position of the optical counterpart (Lee et al., GCN 44070; Tanvir et al., GCN 44082). The corresponding 3-sigma flux density upper limits are 0.21, 0.15, 0.075, and 0.06 mJy at 17, 19, 21, and 23 GHz, respectively.
We thank the ATCA operations staff for their prompt scheduling of these observations. The Australia Telescope Compact Array is part of the Australia Telescope National Facility, funded by the Australian Government for operation as a National Facility managed by CSIRO.
GCN Circular 44357
Subject
EP260321a: VLA radio follow-up
Date
2026-04-20T21:56:06Z (6 months ago)
From
todwyer1@jh.edu
Via
Web form
Tanner O'Dwyer (JHU), Alessandra Corsi (JHU), Gokul Srinivasaragavan (UMD), S. Bradley Cenko (NASA GSFC), Shreya Anand (Stanford.U), Anna Ho (Cornell), Jesper Sollerman (U.Stockholm), Bei Zhou (FemiLab & U.Chicago), Marc Kamionkowski (JHU), Jillian Rastinejad (UMD)
Following GCN 44239 we re-observed the field of EP260321a (Huang et al., GCN 44068) with the Karl G. Jansky Very Large Array (VLA) in its A-configuration. Our observations were conducted at about 22 days post trigger at a nominal central frequency of 6 GHz. We report no significant variation of the radio source at a position compatible with that of SN 2026gzf (Corcoran et al., GCN 44105, Rastinejad et al., GCN 44107).
GCN Circular 44348
Subject
EP260321A/AT2026gzf: Optical observation from 1.3m DFOT
Date
2026-04-19T09:06:27Z (6 months ago)
From
ANSHIKA GUPTA at ARIES <anshika05180@gmail.com>
Via
Web form
Anshika Gupta, Pankaj Pawar, Divyanshu Janghel, Dhruv Jain, Debalina Kar, and Kuntal Misra (ARIES) report:
We observed the field of EP260321A/AT2026gzf detected by EP-WXT (Huang, GCN 44068) with the 1.3m Devasthal Fast Optical Telescope (DFOT), located at the Devasthal Observatory of the Aryabhatta Research Institute of Observational Sciences (ARIES), India. Observations began on 2026-04-16 at 14:22:58 UT, i.e., ~26.07 days after the EP-WXT trigger. We have taken multiple frames with an exposure time of 300s in the R filter. We stacked the images after the alignment. We detect an optical
afterglow in our stacked image. We obtain the following preliminary magnitude in the stacked image:
Date Start_UT T_start-T0 (days) Filter Exp time (s) Magnitude
=================================================================
2026-04-16 14:22:58 ~26.07 R 300s*4 17.14 +- 0.01
The optical detection of the burst is consistent with Lipunov et al., GCN 44069; Lee et al., GCN 44070; 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; Pankov et al. GCN 44103; Corcoran et al., GCN 44105; Rastinejad et al., GCN 44107; Gao et al., GCN 44110; He et al., GCN 44117; Zhang et al., GCN 44171; Li et al., GCN 44284; Volnova et al., GCN 44295.
The magnitude is not corrected for the Galactic extinction in the direction of the burst. Photometric calibration is performed using the standard stars from the USNO-B1.0 catalog.
GCN Circular 44295
Subject
EP260321a: CMO SAI J,K observations
Date
2026-04-14T11:27:29Z (6 months ago)
From
Alina Volnova at IKI RAS <alinusss@gmail.com>
Via
Web form
A.A. Volnova (IKI), A.M. Tatarnikov (SAI MSU), S.G. Zheltoukhov (SAI MSU), A.S. Pozanenko (IKI), N.S. Pankov (IKI) report on behalf of IKI-GRB-FuN:
We observed the field of the fast X-Ray transient EP260321a (Huang et al., GCNs 44068, 44075) at z = 0.0343 (Tanvir et al., GCN 44082) with SAI-2.5 (2.5m) telescope of the Caucasian Mountain Observatory of Sternberg Astronomical Institute (CMO SAI), equipped with the IR camera ASTRONIRCAM. The observations in the J and K filters were carried out on 2026-04-02, and in J filter on 2026-04-07. The Supernova reported previously (Lee et al., GCN 44070; 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; Pankov et al. GCN 44103; Corcoran et al., GCN 44105; Rastinejad et al., GCN 44107; Gao et al., GCN 44110; He et al., GCN 44117; Zhang et al., GCN 44171; Li et al., GCN 44284) is clearly detected in stacked frames.
Preliminary photometry and observational details are the following:
Date UT_start t-T0 Exp. Filter Obj. Err. UL Site/Telescope
(mid,days) (n*s) (3sigma)
2026-04-02 21:11:47 12.36569 6*96 J 16.76 0.04 20.5 CMO SAI/SAI-2.5
2026-04-02 20:58:27 12.35698 20*29 K 16.50 0.06 19.1 CMO SAI/SAI-2.5
2026-04-07 17:53:10 17.23089 11*96 J 16.45 0.04 21.1 CMO SAI/SAI-2.5
The photometry is based on several nearby stars from the 2MASS-PSC and is not corrected for the Galactic extinction.
The supernova continues to rise in the J-band and is most likely near the maximum of the SN light curve.
GCN Circular 44284
Subject
EP260321a: SVOM/VT optical observations and color changes around peak
Date
2026-04-13T10:24:49Z (6 months ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, Y. N. Ma, C. Wu, L. P. Xin, Y. L. Qiu, 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, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.
SVOM/VT performed several ToO observations of EP260321a triggered by Einstein Probe (Huang et al., GCN 44068) in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously from 2026-03-21 (Ma et al., GCN 44074), 2026-03-23, 2026-03-26, 2026-03-28, 2026-04-01, 2026-04-04, 2026-04-07, 2026-04-10 and 2026-04-12.
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; Sankar et al. GCN 44089; 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; He et al., GCN 44117; Zhang et al., GCN 44171) showed a rise to a peak after trigger in both channels, followed by a decline. The peak in VT_B was at approximately 10.7 days post trigger.
The preliminary measurements are in the AB magnitude and are not corrected for Galactic extinction:
Mid time | Band | Exposure Time | Brightness
1.906 hr VT_B 53*50 sec 19.70+/-0.05 mag
1.906 hr VT_R 53*50 sec 19.98+/-0.06 mag
19.75 d VT_B 17*50 sec 17.48+/-0.03 mag
19.75 d VT_R 17*50 sec 17.21+/-0.02 mag
One should be cautious that the brightness mentioned above may be contaminated by the host.
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 44250
Subject
EP260321a: Chandra X-ray Non-detection
Date
2026-04-09T22:11:06Z (7 months ago)
From
Brendan O'Connor at Carnegie Mellon University <boconno2@andrew.cmu.edu>
Via
Web form
Brendan O'Connor (CMU) and Eleonora Troja (U Rome) on behalf of a larger collaboration:
We observed the position of EP260321a (Huang et al., GCN 44068) with the Chandra X-ray Observatory (CXO) under GO program 27400339 (PI: O'Connor). Observations were carried out with ACIS-S starting on 2026-04-05 at 22:00 UT, corresponding to ~15.4 d after the EP trigger, for a total of 19.81 ks. We do not detect any coincident X-ray source at this time. With the exception of GW170817, this observation is capable of excluding all known GRB X-ray afterglows from the Swift era, including GRB 060218 which is also located at ~150 Mpc.
Further observations are planned.
We thank the staff of the Chandra X-ray Observatory, including Pat Slane, Dan Schwartz, Jack Steiner, and Vinay Kashyap, for rapidly scheduling these observations.
GCN Circular 44239
Subject
EP260321a: VLA radio detection
Date
2026-04-09T01:06:49Z (7 months ago)
From
todwyer1@jh.edu
Via
Web form
Tanner O'Dwyer (JHU), Alessandra Corsi (JHU), Gokul Srinivasaragavan (UMD), S. Bradley Cenko (NASA GSFC), Shreya Anand (Stanford.U), Anna Ho (Cornell), Jesper Sollerman (U.Stockholm), Bei Zhou (FemiLab & U.Chicago), Marc Kamionkowski (JHU), Jillian Rastinejad (UMD)
We observed the field of EP260321a (Huang et al., GCN 44068) with the Karl G. Jansky Very Large Array (VLA) in its A-configuration. Our observations were conducted at about 13.6 days post trigger at a nominal central frequency of 6 GHz. We report the detection of a radio source with a S/N of about 4 at a position compatible with that of SN 2026gzf (Corcoran et al., GCN 44105, Rastinejad et al., GCN 44107). Our image RMS is about 5 uJy. Further observations are planned to also evaluate potential contribution of the host galaxy to the observed emission.
GCN Circular 44229
Subject
EP260321a: Upper limits from VLA radio observations
Date
2026-04-07T14:50:36Z (7 months ago)
From
James Leung at U. Toronto / HUJI <jamesk.leung@utoronto.ca>
Via
Web form
J. K. Leung (U. Toronto/HUJI), L. Izzo (INAF-OACN), F. de Colle (UNAM), and M. R. Drout (U. Toronto) report on behalf of a larger collaboration:
We report radio observations of EP260321a (Huang et al., GCN 44068) conducted on the Karl G. Jansky Very Large Array (VLA). The observations were centred at the position 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, Gao et al., GCN 44110, He et al., GCN 44117, Zhang et al., GCN44171), identified now as the broad-lined type-Ic supernova SN 2026gzf (Corcoran et al., GCN 44105, Rastinejad et al., GCN 44107).
The radio observations were taken from UT05:12-07:42 on 2026-03-27 (~5.7 days post-trigger) at the central observing frequencies of 6, 10, 15 and 22 GHz. In our preliminary analysis, we detect no radio source at the position of the putative optical counterpart to EP260321a, giving a 3-sigma non-detection limit of <0.03 mJy/beam at each observing frequency band.
We thank the VLA and NRAO staff for quickly scheduling and executing these observations. Further observations are planned.
The National Radio Astronomy Observatory and Green Bank Observatory are facilities of the U.S. National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
GCN Circular 44227
Subject
EP260321a: uGMRT observations
Date
2026-04-07T11:11:53Z (7 months ago)
From
Aishwarya L Thakur at INAF-IAPS, Rome <aishth@outlook.com>
Via
Web form
G. Gianfagna (INAF-IAPS), A. Balasubramanian (IIA),
G. Bruni (INAF-IAPS), D. Eappachen (IIA),
L. Piro, A. L. Thakur (INAF-IAPS), G.C. Anupama, D.K. Sahu, Aru Beri (IIA), Varun Bhalerao, Aditya Pawan Saikia (IITB) report:
We observed the Fast X-ray Transient EP260321a (Huang et al, GCN 44068, GCN 44075) with the upgraded Giant Metrewave Radio Telescope (uGMRT). Observations were performed on March 26 2026 (at a mean epoch of ~5 days after the EP trigger) for a total of 2 hours in Band 5 under a joint allocation for programs 49_076 (PI Eappachen) and 49_114 (PI Gianfagna).
Data were reduced and imaged with the SPAM pipeline. The final image rms in Band 5 is ~90 uJy/beam.
We do not detect any radio emission at the position of the optical counterpart (Lipunov et al. GCN 44069, Lee et al. GCN 44070, 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, Pankov et al. GCN 44103, Corcoran et al. GCN 44105, Rastinejad et al. GCN 44107, Gao et al. GCN 44110, He et al. GCN 44117, Zhang et al. GCN 44171).
The three-sigma upper limit on the radio counterpart is thus 270 uJy/beam at 1.1 GHz.
Additional uGMRT observations are planned.
We thank the staff of the GMRT who made these observations possible. GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research.
GCN Circular 44171
Subject
EP260321a :Xinglong optical observations
Date
2026-04-01T05:05:57Z (7 months ago)
From
Xinglong Observatory at National Astronomical Observatories (NAOC), Chinese Academy of Sciences (CAS) <xinglong@nao.cas.cn>
Via
Web form
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)
GCN Circular 44117
Subject
EP260321a: JinShan near-infrared observations
Date
2026-03-26T07:29:39Z (7 months ago)
Edited On
2026-03-26T16:50:52Z (7 months ago)
From
liuxing@nao.cas.cn
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of liuxing@nao.cas.cn
Via
Web form
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.
GCN Circular 44114
Subject
EP260321a: Upper limits from a neutrino search with IceCube
Date
2026-03-25T21:30:19Z (7 months ago)
From
Sam Hori at IceCube/U Wisc-Madison <sahori@wisc.edu>
Via
email
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)
GCN Circular 44110
Subject
EP260321a: J-band observations by the SYSU 80 cm infrared telescope
Date
2026-03-25T09:38:42Z (7 months ago)
From
Rui-Chen Gao <ayxzxn0100@gmail.com>
Via
Web form
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
Subject
EP260321a: Gemini-South confirmation of a supernova (duplicate submission)
Date
2026-03-24T18:32:10Z (7 months ago)
Edited On
2026-03-25T18:17:44Z (7 months ago)
From
Jillian Rastinejad at University of Maryland <jillianrastinejad2024@u.northwestern.edu>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov>
Via
email
GCN Circular 44107
Subject
EP260321a: Gemini-South confirmation of a supernova
Date
2026-03-24T18:32:10Z (7 months ago)
From
Jillian Rastinejad at University of Maryland <jillianrastinejad2024@u.northwestern.edu>
Via
email
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.
GCN Circular 44105
Subject
EP260321a: VLT/FORS2 spectroscopy confirmation of an associated type Ic-BL supernova SN 2026gzf
Date
2026-03-24T15:23:31Z (7 months ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
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.
GCN Circular 44103
Subject
EP260321A: Mondy optical observations
Date
2026-03-24T12:34:33Z (7 months ago)
From
Nicolai Pankov at IKI <colinsergesen@gmail.com>
Via
Web form
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.
GCN Circular 44092
Subject
EP260321a / AT2026gzf: VLT/X-shooter detection of supernova-like spectral features at z = 0.0344
Date
2026-03-23T17:12:25Z (7 months ago)
Edited On
2026-03-24T14:23:15Z (7 months ago)
From
nrt3@le.ac.uk
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
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.
GCN Circular 44091
Subject
EP260321a: COLIBRÍ further optical observations and brightening
Date
2026-03-23T16:06:10Z (7 months ago)
From
Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Via
email
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.
GCN Circular 44089
Subject
EP260321a: Continued brightening from Kinder follow-up of the emerging supernova candidate
Date
2026-03-23T15:36:09Z (7 months ago)
From
Janet Chen at National Central University <janetstars@gmail.com>
Via
Web form
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.
GCN Circular 44087
Subject
EP260321a: Early TRT rising followed by JinShan decaying and then re-rising
Date
2026-03-23T11:26:51Z (7 months ago)
From
liuxing@nao.cas.cn
Via
Web form
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.
GCN Circular 44084
Subject
EP260321a: ZTF and Rubin detections of the candidate optical counterpart to EP260321a
Date
2026-03-22T23:06:20Z (7 months ago)
From
Tomas Ahumada <tomas.ahumada@noirlab.edu>
Via
Web form
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.
GCN Circular 44083
Subject
EP260321a: GOTO detection of blue variable source
Date
2026-03-22T21:30:52Z (7 months ago)
From
Shane Moran at University of Leicester <shane.moran@leicester.ac.uk>
Via
Web form
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).
GCN Circular 44082
Subject
EP260321a: VLT imaging and spectroscopy show the variable source to be at z = 0.0343
Date
2026-03-22T19:50:53Z (7 months ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
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.
GCN Circular 44081
Subject
EP260321a: further Kinder optical observations
Date
2026-03-22T15:49:58Z (7 months ago)
From
Janet Chen at National Central University <janetstars@gmail.com>
Via
Web form
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.
GCN Circular 44079
Subject
EP260321a: Las Cumbres upper limit
Date
2026-03-22T13:12:22Z (7 months ago)
From
aujust@mail.ustc.edu.cn
Via
Web form
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.
GCN Circular 44076
Subject
EP260321a: COLIBRÍ optical upper limit on any supernova shock breakout emission
Date
2026-03-22T07:01:20Z (7 months ago)
From
Rosa L. Becerra at Instituto de Astronomía, UNAM <rbecerra@astro.unam.mx>
Via
Web form
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.
GCN Circular 44075
Subject
EP260321a: refined analysis of the EP-WXT and EP-FXT observations, implying a possible supernova shock breakout candidate
Date
2026-03-22T04:32:32Z (7 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
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).
GCN Circular 44074
Subject
EP260321a: SVOM/VT optical upper limit
Date
2026-03-22T02:34:27Z (7 months ago)
From
Yinuo Ma <mayn@bao.ac.cn>
Via
Web form
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.
GCN Circular 44070
Subject
EP260321a: Kinder observations detect a blue variable star and set limits on a source from the z =0.034 galaxy within the error circle
Date
2026-03-21T17:18:54Z (7 months ago)
From
Janet Chen at National Central University <janetstars@gmail.com>
Via
Web form
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.
GCN Circular 44069
Subject
EP260321a: Global MASTER-Net observations report
Date
2026-03-21T13:54:25Z (7 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-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
GCN Circular 44068
Subject
EP260321a: Einstein Probe detection of an X-ray transient
Date
2026-03-21T13:01:43Z (7 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
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).