EP250321a, GRB 250321D
GCN Circular 39800
Subject
EP250321a: Einstein Probe detection of an X-ray transient
Date
2025-03-21T08:04:38Z (3 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
D. F. Hu (PMO, CAS), H. Q. Cheng (NAO, CAS), Z. H. Yang, Q. C. ZHAO (IHEP, CAS), L. Chen, W. D. Zhang (NAO, CAS) report on behalf of the Einstein Probe 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 EP250321a. The transient triggered EP-WXT (ID: 01709133009) at 2025-03-21 06:09:59 (UTC). The WXT position of the source is R.A. = 179.262 deg, DEC = 17.386 deg (J2000) with an uncertainty of 3.1 arcmin in radius (90% C.L. statistical and systematic).
A follow-up observation with the Follow-up X-ray Telescope (FXT) has been scheduled. 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).
GCN Circular 39803
Subject
EP250321a: Global MASTER-Net observations report
Date
2025-03-21T08:36:30Z (3 months ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
V.Lipunov, E.Gorbovskoy, N.Tiurina, P.Balanutsa, , D.Vlasenko, I.Panchenko,
A.Kuznetsov, G.Antipov, A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, 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. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)
MASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP250321a ( EP Team et al., GCN 39800) errorbox 141 sec after notice time and 7118 sec after trigger time at 2025-03-21 08:08:37 UT, with upper limit up to 18.7 mag. The observations began at zenith distance = 70 deg. The sun altitude is -32.2 deg.
The galactic latitude b = 74 deg., longitude l = 250 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2819086
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
7149 | 2025-03-21 08:08:37 | MASTER-OAFA | (11h 56m 58.33s , +17d 45m 08.7s) | C | 60 | 18.7 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 39804
Subject
EP250321a: TRT optical counterpart detection
Date
2025-03-21T08:39:06Z (3 months ago)
From
sqjiang at NAOC <sqjiang@bao.ac.cn>
Via
Web form
S.Y. Fu (HUST), W.X. Li, J. An, S.Q. Jiang (NAOC), S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), X. Liu, Z.P. Zhu, Z. Fan, N.C. Sun, D. Xu, Y.N. Wang (NAOC) report on behalf of a large collaboration:
We observed the field of EP250321a detected by EP/WXT (Hu et al., GCN 39800) using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Sierra Remote Observatories in California. We obtained several 300 s frames in the R and I filters.
An uncatalogued and decaying optical source is detected within the error circle at coordinates
R.A. (J2000) = 11:57:03.02
Dec. (J2000) = +17:21:45.94
with an uncertainty of ~0.5 arcsec. The source has R ~ 19.7 mag at 1.24 hr and I ~ 18.8 mag at 1.72 hr after the EP trigger, calibrated with nearby Pan-STARRS DR2 stars and not corrected for Galactic extinction.
We think that the source is very likely the optical counterpart of EP250321a.
GCN Circular 39805
Subject
EP250321a: BOOTES-5/JGT optical afterglow detection
Date
2025-03-21T08:52:33Z (3 months ago)
Edited On
2025-03-21T17:43:29Z (3 months ago)
From
ipg@iaa.es
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of I. Perez-Garcia at Institute of Astrophysics of Andalusia <ipg@iaa.es>
Via
Web form
I. Perez-Garcia, E. J. Fernandez-Garcia, G. Garcia-Segura, M. D. Caballero-Garcia, S.-Y. Wu, R. Sanchez-Ramirez, S. Guziy and A. J. Castro-Tirado (IAA-CSIC, Granada), M. Gritsevich (Univ. of Helsinki), C. Perez del Pulgar, A. Castellon (Univ. de Malaga), Y.-D. Hu (INAF-OAB, Brera), S. Jeong (ADD, Daejeon) and D. Hiriart and W. H. Lee (UNAM), on behalf of a larger collaboration, report:
Following the detection of EP250321a by Einstein Probe (Hu et al. GCNC 39800), the BOOTES-5/JG robotic telescope at Observatorio Astronomico Nacional in San Pedro Martir (Mexico) automatically responded to this X-ray transient starting on 2025-03-21 07:27:36 UT (23 min after notification). Series of images in clear filter were gathered and we detect and uncatalogud source at RA, Dec = 11:57:03.0, +17:21:46.2 (179.26263, 17.36283) within the WXT region, for which we measure a magnitude of 19.6 +- 0.1, consistent with Fu et al. (GCNC 39804). Further analysis of the additional images is ongoing.
We thank the staff at Observatorio Astronomico Nacional in San Pedro Martir for their excellent support.
GCN Circular 39807
Subject
EP250321a: REM optical/NIR afterglow detection
Date
2025-03-21T09:16:29Z (3 months ago)
From
Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>
Via
Web form
R. Brivio, M. Ferro, P. D'Avanzo, S. Covino, D. Fugazza (INAF-OAB) report on behalf of the REM team:
We observed the field of EP250321a (Hu et al., GCN 39800) with the REM 60cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the g, r, i, z, J, H, K bands, starting on 2025 March 21 at 07:05:26 UT (i.e. ~0.92 hr after the EP trigger), and lasting for about 2 hours.
From preliminary photometry we detect the counterpart in the optical and NIR images at the position of the optical afterglow (Fu et al., GCN 39804; Perez-Garcia et al., GCN 39805) with the following magnitudes:
r = 19.9 +/- 0.2 (AB; calibrated against the Pan-STARRS catalogue)
at a mid-time of t - t0 = 1.78 hr after the trigger;
H = 15.3 +/- 0.2 (Vega; calibrated against the 2MASS catalogue)
at a mid-time t - t0 = 1.06 hr after the trigger.
GCN Circular 39809
Subject
EP250321a: VLT/X-shooter redshift z = 4.368
Date
2025-03-21T09:49:05Z (3 months ago)
From
Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>
Via
Web form
Z. P. Zhu (NAOC), B. Schneider (LAM), A. Saccardi (CEA/Irfu), D. Xu (NAOC), V. D’Elia (SSDC & INAF-OAR), A. J. Levan (Radboud and Warwick), D. B. Malesani (DAWN/NBI & Radboud), P. T. O’Brien (Leicester), J. T. Palmerio (CEA/Irfu), S. D. Vergani (CNRS, Obs. Paris/LUX), report on behalf of the Stargate collaboration:
We observed the optical and NIR counterpart (Fu et al., GCN 39804; Perez-Garcia et al., GCN 39805; Brivio et al., GCN 39807) of the Einstein Probe transient EP250321a (Hu et al., GCN 39800) using the ESO/VLT UT3 (Melipal) equipped with the X-shooter spectrograph.
Observations were conducted in rapid response mode (RRM) using the X-shooter spectrograph, and were requested as soon as we became aware of the optical counterpart. In a 10-s r-band exposure taken with the acquisition camera (102.8 min after the EP trigger), the counterpart is well detected with a magnitude r = 20.2 +/- 0.1 (AB, calibrated against a single Pan-STARRS object). Comparing with the report by Fu et al. (GCN 39804), our measurement indicates fading and confirms this object as the optical afterglow of EP250321a.
Our spectra, covering the wavelength range 3000-21000 AA, consist of 2 exposures of 600 s each. The observation mid time was 2025 Mar 21.336 UT (1.91 hr after the EP trigger).
In a preliminary reduction of the spectra, we clearly detect a continuum down to ~4930 AA, and a broad trough at ~6525 AA. These two features are consistent with Lyman limit and Lyman alpha, respectively. From the detection of a plethora of narrow absorption features, including S II, Si II, Si II*, O I, OI*, C II, C II*, Ni II, Si IV, C IV, Fe II, Fe II*, Al II, Al III, Mg II, Mg I, we infer a redshift of z = 4.368. The detection of fine-structure lines robustly confirms the physical association between this absorption system and the high-energy transient. We thus conclude that EP250321a was at z = 4.368.
We wish a happy March equinox to the whole GCN community. We acknowledge excellent support from the ESO observing staff in Paranal, in particular Cecilia Bustos, Enrico Congiu, Israel Blanchard, Julien Drevon, and Miguel Lopez.
GCN Circular 39811
Subject
EP250321a: Swift-XRT detection of an X-ray source
Date
2025-03-21T11:18:07Z (3 months ago)
From
K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
email
K.L. Page, P.A. Evans (U.Leicester) and M.H. Siegel (PSU) report on behalf
of the Swift-XRT Team:
On 2025 March 21 at 07:26 UT, Swift started observing EP250321a, 4.6 ks
after the Einstein Probe trigger (GCN Circ. 39800). A
previously-uncatalogued X-ray source was identified, at a position of RA,
Dec = 179.26259, +17.3628, which is equivalent to
RA (J2000): 11h 57m 03.02s
Dec (J2000): +17d 21' 46.1"
with an uncertainty of 2.6 arcsec (radius, 90% confidence). This
is consistent with the position of the optical and NIR source reported
(GCN Circs. 39804, 39805, 39807, 39809, 39810).
The first snapshot of data, spanning 4.6-6.2 ks after the trigger,
shows some X-ray flaring, with a mean X-ray flux of 3.7 (+0.4, -0.4) x
10^-11 erg cm-2 s-1 (observed, 0.3-10 keV).
GCN Circular 39812
Subject
EP250321a: LCO optical counterpart detection
Date
2025-03-21T12:58:42Z (3 months ago)
From
Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>
Via
Web form
I. Pérez-Fournon, F. Poidevin (IAC and ULL), D. Cano-Morales, I. Correa-Plasencia, A.E. Hernández-Díaz (ULL), D. Aguado, A. López-Oramas, D. Nespral (IAC and ULL), N.C. Sun (UCAS), W. Li, Y. Wang, and Z. Niu (NAOC)
We report Las Cumbres Observatory Global Telescope network (LCOGT) observations of EP250321a detected
by EP/WXT (Hu et al., GCN circ. 39800), and Swift-XRT (Page et al., GCN circ. 39811).
We observed the field of EP250321a with one of the two LCOGT 1-m telescopes, equipped with Sinistro cameras, located at the LCOGT node at McDonald Observatory (Texas). We obtained a 200-sec exposure in the SDSS i' filter starting at 2025-03-21 09:38:42 UT, about 3.48 hr after the EP trigger.
An uncatalogued source is detected at the position of the optical and near-IR counterpart reported
by Fu et al. (GCN circ. 39804), Perez-Garcia et al. (GCN circ. 39805), Brivio et al. (GCN circ. 39807), Zhu et al. (GCN circ. 39809), and Becerra et al. (GCN circ. 39810).
Zhu et al. (GCN circ. 39809) have reported a redshift of z = 4.368.
We measure the following magnitude, calibrated against Pan-STARRS DR2 stars and not corrected for Galactic extinction:
Date | UT start | mag | error | filter |
----------------------------------------------
2025-03-21 09:38:42 19.3 0.15 i'
This work makes use of observations from the Las Cumbres Observatory global telescope network (LCOGT observing programme IAC2025A-009, SGLF).
GCN Circular 39815
Subject
EP250321a: Kinder optical follow-up observations
Date
2025-03-21T14:45:24Z (3 months ago)
From
Janet Chen at National Central University <janetstars@gmail.com>
Via
Web form
Y.-H. Lee, A. Aryan, T.-W. Chen, Y. J. Yang, W.-J. Hou, H.-Y. Hsiao (all NCU), A. K. H. Kong (NTHU), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), M.-H. Lee, Y.-C. Pan, C.-C. Ngeow, A. Sankar. K, C.-H. Lai, C.-S. Lin, H.-C. Lin, J.-K. Guo (all NCU), S. Yang, L. L. Fan, Z. N. Wang, G. H. Sun (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 EP250321a (Hu et al., GCN 39800) using the 1m LOT (in g and r band) and 40cm SLT (in i and z band) at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al. 2024, arXiv:2406.09270). The first LOT epoch of observations in the r band started at 11:57 UT on the 21st of March 2025 (MJD 60755.498), ~5.78 hrs after the EP trigger, while the first SLT epoch of observations in the i band started at 12:23 UT on the 21st of March 2025 (MJD 60755.516), ~6.21 hrs after the EP 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. In the stacked images, we clearly detected the optical counterpart candidate proposed by Fu et al. (GCN 39804) and confirmed by several other observations (e.g., Perez-Garcia et al al., GCN 39805; Brivio et al., GCN 39807; Zhu et al., GCN 39809; Becerra et al., GCN 39810; Pérez-Fournon et al., GCN 39812).
We employed the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform PSF photometry on individual frames. The details of the observation and measured photometry from the first frame (in the AB system) were as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | Seeing | Airmass
LOT | r | 60755.498 | 5.78 | 300 * 6 | 21.86 +/- 0.07 | 1".1 | 1.63
SLT | i | 60755.516 | 6.21 | 300 * 12 | 20.75 +/- 0.10 | 1".4 | 1.36
LOT | g | 60755.520 | 6.31 | 300 * 6 | >22.04 | 1".5 | 1.41
The presented magnitudes were calibrated using the field stars from the Pan-STARRS1 catalog and were not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_r = 0.12 mag, A_i = 0.09 mag, and A_g = 0.17 mag, respectively, in the direction of the transient (Schlafly & Finkbeiner 2011).
GCN Circular 39817
Subject
EP250321a: SVOM/VT optical observation
Date
2025-03-21T15:58:18Z (3 months ago)
From
Xuhui Han at NAOC/SVOM <hxh@nao.cas.cn>
Via
Web form
X. H. Han, Z. H. Yao, L. P. Xin, Y. L. Qiu, H. L. Li, Y. Xu, P. P. Zhang, W. J. Xie, J. Wang, Y. J. Xiao, H. B. Cai, J. Y. Wei (NAOC) report on behalf of the SVOM mission team.
SVOM/VT conducted ToO follow-up observations of the X-ray transient EP250321a (Hu et al., GCN 39800) at the location of WXT position in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channel simultaneously.
The optical counterpart of EP250321a (Trigger ID: 01709133009, Fu et al., GCN 39804; Perez-Garcia et al., GCN 39805; Brivio et al., GCN 39807; Zhu et al., GCN 39809; Becerra et al., GCN 39810; Pérez-Fournon et al., GCN 39812; Lee et al., GCN 39815) was clearly detected in VT_R and VT_B images.
The brightness in AB magnitude was estimated to be:
Mid time UTC | Band | Exposure Time (second) | Magnitude | Magnitude error
2025-03-21T10:29:29 | VT_B | 2940 | 21.85 | 0.09
2025-03-21T10:29:29 | VT_R | 2940 | 19.39 | 0.03
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 39822
Subject
EP250321a: Xinglong 216 decaying optical observations
Date
2025-03-21T19:24:45Z (3 months ago)
Edited On
2025-03-25T16:56:28Z (3 months ago)
From
Xinglong Observatory at National Astronomical Observatories (NAOC), Chinese Academy of Sciences (CAS) <xinglong@nao.cas.cn>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Xinglong Observatory at National Astronomical Observatories (NAOC), Chinese Academy of Sciences (CAS) <xinglong@nao.cas.cn>
Via
Web form
Junjie-Jin (NAOC), Haiyang-Mu (NAOC), Jinlei-Zhang (NAOC), Pengliang-Du (NAOC), Feng-Xiao(NAOC),Zhou-Fan (NAOC), Hong-Wu (NAOC)
We performed optical observations of the field of a fast X-Ray transient EP250321a (Hu et al., GCN 39800) in the Free filter using 2.16-m telescope located at Xinglong, Hebei, China. In the band a 600 s exposures were taken , with a median observation time of 2025-03-02T20:32:28.
No uncatalogued optical transient is detected in the stacked images within the 3 arcmin EP/WXT error circle (Li et al., GCN 36405), down to 3-sigma limiting magnitudes of g ~ 20.5, calibrated with Pan-STARRS sources in the field. Also there is no apparent brightening for the catalogued sources within the error circle.
GCN Circular 39827
Subject
EP250321a: WFST optical observations
Date
2025-03-22T02:51:05Z (3 months ago)
From
Jin-Jun Geng at PMO <jjgeng@pmo.ac.cn>
Via
Web form
Jia-Zheng Zhu, Jin-Jun Geng, Ze-Lin Xu, Ning Jiang, Yan-Long Hua, Yi-Fang Liang, Ding-Fang Hu, Ji-An Jiang, Xue-Feng Wu, and Tian-Rui Sun report on behalf of the WFST team:
Following the detection of EP250321a by Einstein Probe (Hu et al., GCN 39800), we use the Wide Field Survey Telescope (WFST Collaboration; arXiv:2306.07590) at Lenghu Astronomical Observation Base (Qinghai province, China) to search and follow up its afterglow. We observed the target position with 3x120s exposure in r-band starting from 2025-03-21T14:17:14.
We clearly detect the optical transient (OT) reported by GCNs (Fu et al., GCN 39804; Perez-Garcia et al al., GCN 39805; Brivio et al., GCN 39807; Zhu et al., GCN 39809; Becerra et al., GCN 39810; Pérez-Fournon et al., GCN 39812; Lee et al., GCN 39815; Ghosh et al., GCN 39816; Han et al., GCN 39817; Jin et al., GCN 39822) in our image. Our preliminary results are as follows:
|Date| |UTstart| |t-T0 (hours)| |Exp (sec)| |Filter| |Magnitude|
----------------------------------------------------------------------------------------------
2025-03-21 14:17:14 8.12 3 x 120 r 22.57 +/- 0.12
We thank the WFST staff for supporting these observations.
GCN Circular 39832
Subject
EP250321a: KAIT optical observations
Date
2025-03-22T05:56:29Z (3 months ago)
From
Weikang Zheng at UC Berkeley <weikang@berkeley.edu>
Via
email
WeiKang Zheng (UCB), Xuhui Han (NAOC), Pinpin Zhang (NAOC) and
Alexei V. Filippenko (UCB) report on behalf of the KAIT GRB team:
The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at
Lick Observatory, automatically responded to EP250321a detected
by Einstein Probe (Hu et al., GCN 39800) starting at 07:05:35 UT,
about 0.93 hours after the trigger and lasted for ~4 hours. A set
of 60s exposure images were obtained in the clear (roughly R)
filters. We detect the optical afterglow (Fu et al., GCN 39804;
Perez-Garcia et al al., GCN 39805; Brivio et al., GCN 39807;
Zhu et al., GCN 39809; Becerra et al., GCN 39810; Pérez-Fournon
et al., GCN 39812; Lee et al., GCN 39815; Ghosh et al., GCN 39816;
Han et al., GCN 39817; Jin et al., GCN 39822; Zhu et al., GCN 39827;
Gill et al., GCN 39831) with a mag of 18.7 +/- 0.2 (Vega) at 0.93
hours, it decayed to ~20.7 +/- 0.3 mag at ~5.4ks. The OT started
getting brighter slowly and reach a peak of ~19.5 +/- 0.2 mag
around 9.5ks, then decayed after that.
GCN Circular 39833
Subject
EP250321a: refined analysis of the EP-WXT and EP-FXT observations
Date
2025-03-22T09:11:25Z (3 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
D. F. Hu (PMO, CAS), H. Q. Cheng (NAO, CAS), Z. H. Yang, Q. C. ZHAO (IHEP, CAS), L. Chen, W. D. Zhang (NAO, CAS) report on behalf of the Einstein Probe team:
The X-ray transient EP250321a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Hu et al., GCN 39800), and followed up by several telescopes (Lipunov et al., GCN 39803, Fu et al., GCN 39804, Perez-Garcia et al., GCN 39805, Brivio et al., GCN 39807, Zhu et al., GCN 39809, Becerra et al., GCN 39810, Page et al., GCN 39811, Pérez-Fournon et al., GCN 39812, Lee et al., GCN 39815, Ghosh et al., GCN 39816, Han et al., GCN 39817, Jin et al., GCN 39822, Zhu et al., GCN 39827, Gill et al., GCN 39831, Zheng et al., GCN 39832), with an optical counterpart detected at a redshift of 4.368 (Zhu et al., GCN 39809). Refined analysis of the WXT data shows that the event started at T0=2025-03-21T06:06:44.850 (UTC) and lasted for about 600s, with the tail of the fading light curve undetected due to the interruption of the observation. The peak flux approximately reached 4.2 x 10^(-9) erg/s/cm2 in the 0.5-4 keV band. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 4 x 10^20 cm^-2 and a photon index of 0.66 (-/+0.17). The derived average unabsorbed 0.5-4 keV flux is 1.73(-0.19/+0.21) x 10^(-9) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source about 16 ks after T0. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 179.2623, DEC = 17.3621 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is spatially consistent with the WXT transient. The FXT position was found to be consistent with the source detected by Swift-XRT (Page et al., GCN 39811) and optical/infrared counterparts as well. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 4 x 10^20 cm^-2 and a photon index of 1.98 (-/+0.06). The derived average unabsorbed 0.5-10 keV flux is 9.23 (-0.54/+0.58) x 10^(-12) erg/s/cm^2 with an exposure time of about 2990 seconds.
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 39836
Subject
EP250321a: Mondy AZT-33IK Optical Observations
Date
2025-03-22T17:46:10Z (3 months ago)
Edited On
2025-03-23T19:49:44Z (3 months ago)
From
Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>
Via
Web form
N. Pankov (HSE, IKI), E. Klunko (ISTP), A. Pozanenko (IKI) report on behalf of IKI-GRB-FuN:
We performed optical observations of the field of EP250321a (Hu et. al, GCN 39800; Page et. al, GCN 39811, Hu et. al, GCN 39833) at the redshift of z = 4.368 (Zhu et. al, GCN 39769) in the R filter with the 1.5-meter AZT-33IK telescope of the Sayan Solar Observatory (Mondy). The observations started on (UT) 2025-03-21 16:42:58, i.e. about 0.46 days since trigger. The optical afterglow (Fu et. al, GCN 39804; Perez-Garcia et. al, GCN 39805; Brivio et. al, GCN 39807; Zhu et. al, GCN 39809; Becerra et. al, GCN 39810; Pérez-Fournon et. al, GCN 39812; Lee et. al, GCN 39815; Han et. al, GCN 39817; Jin et. al, GCN 39822; Zhu et. al, GCN 39827; Gill et. al, GCN 39831; Zheng et. al, GCN 39832) is detected in the stacked image. The preliminary photometry is as follows:
Date UT start t-T0 Exp. Filter OT Err UL(3sigma)
(mid, days) (s)
2025-03-21 16:42:58 0.46042 30*120 R 22.41 0.28 22.8
The photometry is based on nearby stars of SDSS-DR12 and has not been corrected for the Galactic extinction.
SDSS-DR12
RA Dec R(Lupton transformations, 2005)
179.2552 +17.3825 18.123 +/- 0.049
179.2436 +17.3713 18.846 +/- 0.012
GCN Circular 39837
Subject
EP250321a: Liverpool Telescope optical follow-up
Date
2025-03-22T18:25:33Z (3 months ago)
Edited On
2025-03-23T12:39:49Z (3 months ago)
From
A. Bochenek at Liverpool John Moores University <a.m.bochenek@2023.ljmu.ac.uk>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of A. Bochenek at Liverpool John Moores University <a.m.bochenek@2023.ljmu.ac.uk>
Via
Web form
A. Bochenek and D. A. Perley (LJMU) report:
We observed the field of the fast X-ray transient EP250321a (Hu et al., GCN 39800) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 6x100s exposures in the SDSS r’ and i’ filters, starting at 2025-03-22 00:24:35 UT, approximately 18.24 hours after the trigger.
We report a detection in the stacked images in the i band at the position of the optical counterpart (Fu et al., GCN 39804) of the magnitude i = 22.08 ± 0.16. In the r-band stacked images, we report a non-detection with the 3-sigma limiting magnitude of r > 22.19 mag. Our results are in agreement with previous observations (Fu et al., GCN 39804; Perez-Garcia et al al., GCN 39805; Brivio et al., GCN 39807; Zhu et al., GCN 39809; Becerra et al., GCN 39810; Pérez-Fournon et al., GCN 39812; Lee et al., GCN 39815; Ghosh et al., GCN 39816; Han et al., GCN 39817; Jin et al., GCN 39822; Zhu et al., GCN 39827; Gill et al., GCN 39831; Zheng et al., GCN 39832; Pankov et al., GCN 39836).
MJD (mid) T_mid - T_0 Filter Mag. (AB)
60756.02120 18.34 h r > 22.19
60756.02976 18.55 h i 22.08 ± 0.16
The photometry was calibrated using nearby PanSTARRS secondary standards and was not corrected for extinction.
GCN Circular 39854
Subject
EP250321a: TNOT Observation of Optical Counterpart
Date
2025-03-23T16:31:37Z (3 months ago)
From
Xiaofeng Wang at Tsinghua University <wang_xf@mail.tsinghua.edu.cn>
Via
Web form
Haichang Zhu (THU), A. Iskandar(XAO), Xiaofeng Wang (THU), and Letian Wang (XAO) report the detection of the optical counterpart that is associated with the X-ray transient EP250321a (Hu et al., GCN 39800; Fu et al., GCN 39804; Perez-Garcia et al al., GCN 39805; Brivio et al., GCN 39807; Zhu et al., GCN 39809; Becerra et al., GCN 39810; Pérez-Fournon et al., GCN 39812; Lee et al., GCN 39815; Ghosh et al., GCN 39816; Han et al., GCN 39817; Jin et al., GCN 39822; Zhu et al., GCN 39827; Gill et al., GCN 39831).
We obtained the r-band images (~12.44 hours after the burst) with the 80~cm Tsinghua-Nanshan Optical Telescope (TNOT) located at Nanshan Station of Xinjiang Astronomy observatory, starting on 2025-03-21 (UT)18:36:10. From the stacked images with a total exposure time of 100sx15, we did not detect the optical afterglow down to a limiting magnitude of about 22.2 mag (MJD=60755.775).
The above photometric result is calibrated using the field stars from the Pan-STARRS catalog and is not corrected for the Galactic extinction.
GCN Circular 39870
Subject
EP250321a: Mondy AZT-33IK Continued Optical Observations
Date
2025-03-25T17:27:37Z (3 months ago)
From
Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>
Via
Web form
N. Pankov (HSE, IKI), E. Klunko (ISTP), A. Pozanenko (IKI) report on behalf of IKI-GRB-FuN:
We continued optical observations of the field of EP250321a (Hu et. al, GCN 39800; Page et. al, GCN 39811, Hu et. al, GCN 39833) at the redshift of z = 4.368 (Zhu et. al, GCN 39769) in the R filter with the 1.5-meter AZT-33IK telescope of the Sayan Solar Observatory (Mondy). The observations started on (UT) 2025-03-22 15:35:42, i.e. about 1.4 days since trigger. The optical afterglow (Fu et. al, GCN 39804; Perez-Garcia et. al, GCN 39805; Brivio et. al, GCN 39807; Zhu et. al, GCN 39809; Becerra et. al, GCN 39810; Pérez-Fournon et. al, GCN 39812; Lee et. al, GCN 39815; Han et. al, GCN 39817; Jin et. al, GCN 39822; Zhu et. al, GCN 39827; Gill et. al, GCN 39831; Zheng et. al, GCN 39832; Bochenek & Perley, GCN 39837; Zhu et. al, GCN 39854) is not detected in the stacked images. The preliminary photometry is as follows:
Date UT start t-T0 Exp. Filter OT Err UL(3sigma)
(mid, days) (s)
2025-03-22 15:35:42 1.40467 17*120 R n/d n/d 22.3
2025-03-23 16:02:48 2.45336 60*120 R n/d n/d 23.6
The photometry is based on nearby stars of SDSS-DR12 and has not been corrected for the Galactic extinction.
SDSS-DR12
RA Dec R(Lupton transformations, 2005)
179.2552 +17.3825 18.123 +/- 0.049
179.2436 +17.3713 18.846 +/- 0.012
GCN Circular 40129
Subject
IPN triangulation of GRB 250321D (consistent with the fast X-ray transient EP250321a)
Date
2025-04-09T14:22:43Z (3 months ago)
From
Dmitry Svinkin at Ioffe Institute <svinkin@mail.ioffe.ru>
Via
Web form
D. Svinkin, D. Frederiks, A. Ridnaia, A. Lysenko,
and T. Cline on behalf of the Konus-Wind team,
E. Burns on behalf of the IPN,
and
E. Bozzo, C. Ferrigno, and S. Mereghetti
on behalf of the INTEGRAL SPI-ACS GRB team,
report:
The long-duration GRB 250321D
was detected by Konus-Wind in the waiting mode and INTEGRAL (SPI-ACS)
at about 22405 s UT (06:13:25), ~200 s after EP triggered on
the fast X-ray transient EP250321a (Hu et al., GCNs 39800, 39833).
We have triangulated it to a Konus-SPI-ACS annulus centered at
RA(2000)=176.654 deg (11h 46m 37s) Dec(2000)=+10.352 deg (+10d 21' 08"),
whose radius is 11.845 +/- 11.845 deg (3 sigma).
This localization may be improved.
EP250321a is inside the annulus and is consistent with the Konus-Wind ecliptic latitude response,
lending support to the association of GRB 250321D and the transient.
A triangulation map is posted at
http://www.ioffe.ru/LEA/GRBs/GRB250321_T22405/IPN/
The burst time history and spectrum will be given in forthcoming
GCN circulars.