EP250421a
GCN Circular 40220
Subject
EP250421a: Nickel optical upper limit
Date
2025-04-23T23:13:36Z (10 months ago)
From
Weikang Zheng at UC Berkeley <weikang@berkeley.edu>
Via
email
Katherine Mora, Eyouel Abate, Ansel Parke, WeiKang Zheng
and Alex Filippenko (UC Berkeley) report on behalf of the
KAIT GRB team:
We observed the field of EP250421a (Zhao et al., GCN 40198)
with the 1-m Nickel telescope located at Lick observatory,
California. Observations were performed in the R band with
600s x 6 exposures. In our coadd image, we do not detect the
optical afterglow (Lee et al., GCN. 40192; Fu et al., GCN
40193; Liu et al., GCN 40197), with an upper limit of R >
20.5 (Vega) at a mid-time of 14.9 hours after the trigger.
GCN Circular 40218
Subject
EP250421a: Upper limits from Fermi-GBM Observations
Date
2025-04-23T14:26:25Z (10 months ago)
From
mariaedvige.ravasio@ru.nl
Via
Web form
M. E. Ravasio (Radboud Univ.), E. Burns (LSU), C. Malacaria (INAF-OAR) and P.G. Jonker (Radboud Univ.) report on behalf of the Fermi-GBM Team:
Fermi-GBM had full spatial coverage of the transient EP250421a detected by EP-WXT (Zhao et al., GCN 40198). There was no Fermi-GBM onboard trigger around the EP starting time T0=2025-04-21T16:16:28 UTC.
The GBM targeted search [1], the most sensitive, coherent search for GRB-like signals, was run in the time interval [T0-50;T0+500] s, seeking signals between 64 ms and 32.768 s in duration. A candidate was found at T0+380 s, but its location is not consistent with the EP transient. No signal consistent with the EP transient, both temporally and spatially, is identified, as confirmed also by visual inspection of the data.
Assuming a “soft” spectral template (Band function with Epeak = 70 keV, alpha = -1.9, beta = -3.7), and a duration of 8.192 s, we derive a flux upper limit of 3.7e-08 erg/cm2/s in the energy band 10-1000 keV.
[1] Goldstein et al. 2019 arXiv:1903.12597
GCN Circular 40207
Subject
EP250421a: NOT optical observations
Date
2025-04-22T13:46:09Z (10 months ago)
From
Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>
Via
Web form
D. B. Malesani (DAWN/NBI and Radboud), J. Sanchez-Sierras (Radboud), A. Martin-Carrillo (UCD), J. Quirola-Vásquez (Radboud), P. G. Jonker (Radboud), A. J. Levan (Radboud and Warwick), D. Xu (NAOC), Z. P. Zhu (NAOC), H. Dawson (IAP Potsdam), report on behalf of a larger collaboration:
We observed the field of the fast X-ray transient EP250421a (Zhao et al., GCN 40198) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were carried out under poor seeing (1.8") in the r and z filters (3x300 s, each) starting on 2025-04-21 at 22:04:12 UT (5.79 hr after the trigger).
At the location of the optical counterpart reported by Lee et al. (GCN 40192), Fu et al. (GCN 40193), and Liu et al. (GCN 40197), also consistent with the EP/FXT and Swift/XRT positions (Zhao et al., GCN 40198; Salvaggio et al., GCN 40204), we only marginally detect a source with r = 23.8 +/- 0.3 (AB), calibrated against nearby Pan-STARRS objects.
GCN Circular 40205
Subject
EP250421a: REM optical upper limit
Date
2025-04-22T09:22:46Z (10 months ago)
From
Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>
Via
Web form
R. Brivio, M. Ferro, D. Fugazza, P. D'Avanzo, S. Covino (INAF-OAB) report on behalf of the REM team:
We observed the field of EP250421a (Zhao et al., GCN 40198) with the REM 60 cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the g, r, i, and z bands, started on 2025 April 21 at 23:08:27 UT (i.e. 6.9 hr after the burst), and lasted for about 1 hour.
From preliminary inspection, we do not detect any counterpart at the position of the optical afterglow (Lee et al., GCN. 40192; Fu et al., GCN 40193; Liu et al., GCN 40197) down to the following 3sigma limit:
r > 20.6 (AB; calibrated against the Pan-STARRS catalogue),
at a mid-time of 7.4 hours after the trigger.
GCN Circular 40204
Subject
EP250421a: Swift/XRT counterpart detection
Date
2025-04-22T09:04:31Z (10 months ago)
From
Chiara Salvaggio at INAF OABrera <chiara.salvaggio@inaf.it>
Via
email
C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB), R. Brivio (INAF-OAB), M.
Ferro (INAF-OAB), K. Page (U. Leicester), D. Malesani (DAWN/NBI and
Radboud) and P.A. Evans (U. Leicester) report on behalf the Swift-XRT team:
Swift/XRT has performed follow-up observations of EP-WXT detected transient
EP250421a (trigger 01709135179, Zhao et al. GCN 40198).
We searched for X-ray sources in 1.8 ks of Photon Counting (PC) mode data,
from T0+5.8 ks to T0+11.7 ks after the EP-WXT trigger.
We found an uncatalogued X-ray source within the estimated 3-sigma EP-WXT
error region. Using 1772 s of XRT PC mode data and 2 UVOT images, we find
an enhanced XRT position: RA, Dec = 172.420,-24.67568 which is equivalent
to:
RA (J2000): 11h 29m 40.80s
Dec (J2000): -24° 40' 32.4"
with an uncertainty of 2.7 arcsec (radius, 90% confidence).
This position is 35 arcsec from the EP-WXT position, and consistent with
the X-ray counterpart found in EP/FXT data (Zhao et al., GCN 40198) and
with the optical counterpart reported by Lee et al. (GCN 40192), Fu et al.
(GCN 40193), Liu et al. (GCN 40197).
The source has a mean count rate of 0.272 ct/s and shows signs of fading at
2.8 sigma significance.
The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/EP/EP_FIELD00035/
This circular is an official product of the Swift-XRT team.
GCN Circular 40200
Subject
EP250421a: Global MASTER-Net observations report
Date
2025-04-22T05:36:49Z (10 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 EP250421a ( EP Team et al., GCN 40198) errorbox 2313 sec after notice time and 46767 sec after trigger time at 2025-04-22 05:15:55 UT, with upper limit up to 20.0 mag. The observations began at zenith distance = 43 deg. The sun altitude is -68.7 deg.
The galactic latitude b = 35 deg., longitude l = 281 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2849886
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
46798 | 2025-04-22 05:15:55 | MASTER-OAFA | (11h 29m 43.36s , -24d 18m 41.7s) | C | 60 | 20.0 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 40198
Subject
EP250421a: Einstein Probe detection of an X-ray transient
Date
2025-04-22T04:36:14Z (10 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
G. Y. Zhao (SYSU), C. Zhou (HUST), R.-Z. Li (YNAO, CAS), W. Chen, Z. X. Ling (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 EP250421a. The transient triggered EP-WXT (ID: 01709135179) at 2025-04-21T16:16:28 (UTC) and WXT detected a rise of about 70 s. The WXT position of the source is R.A. = 172.4130 deg, DEC = -24.6830 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 about 245 s after the EP/WXT trigger. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 172.4201 deg, DEC = -24.6758 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic).
The WXT spectrum can be fitted by an absorbed power law model with a photon index of 2.91(+2.76, -1.72) and an absorbing column density of 4.1e21 cm^-2. The unabsorbed 0.5-4 keV flux is about 1.3 (+14.7,-0.81) e-9 erg/cm^2/s.
The FXT spectrum can be fitted by an absorbed power law model with a photon index of 2.11 (+0.15, -0.14) and an absorbing column density of 8.3e20 cm^-2. The unabsorbed 0.5-10 keV flux is about 1.39 (+0.07, -1.39) e-11 erg/cm^2/s.
The contact TA of EP250421a is Chang Zhou. Please contact her via email d202180110@hust.edu.cn if needed.
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 40197
Subject
EP250421a: Mephisto optical counterpart confirmation
Date
2025-04-22T04:28:53Z (10 months ago)
From
liuxk@ynu.edu.cn
Via
Web form
Xiangkun Liu, Xinlei Chen, Yi Zhang, Runnan Jiang, Brajesh Kumar, Guowang Du, Yuan Fang, Jinghua Zhang, Xingzhu Zou, Yu Pan, Yuanpei Yang, Xiaowei Liu (all SWIFAR, YNU) report on behalf of the Mephisto Team:
The field of EP250421a (EP-WXT trigger ID: 01709135179) was observed with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. Simultaneous uvgriz band photometric observations were conducted starting from 16:26:55 2025-04-21 UT (~5 mins after the trigger) and 7 frames with 45s exposure time for ugi bands and 6 frames with 45s exposure times for vrz bands were taken, followed by another 2 frames with 300s exposure time for all ugi/vrz bands. We detected an optical transient candidate (Lee et al., GCN 40192; Fu et al., GCN 40193) in the stacked riz band images in both sets of observations but no detection in uvg bands. In our priliminary photometry there is clear indication of the decay of the source as listed below.
Start_Time(UT) | Band | Exp(s) | Mag/LimMag (AB)
---------------------------|--------|---------|------------
2025-04-21T16:26:55 | u | 45*7 | >21.15
2025-04-21T16:38:05 | v | 45*6 | >21.39
2025-04-21T16:26:55 | g | 45*7 | >22.02
2025-04-21T16:38:05 | r | 45*6 | 20.47 +/- 0.10
2025-04-21T16:26:55 | i | 45*7 | 19.07 +/- 0.07
2025-04-21T16:38:05 | z | 45*6 | 18.62 +/- 0.13
Start_Time(UT) | Band | Exp(s) | Mag/LimMag (AB)
---------------------------|--------|---------|------------
2025-04-21T17:12:15 | u | 300*2 | >21.41
2025-04-21T16:59:53 | v | 300*2 | >21.64
2025-04-21T17:12:15 | g | 300*2 | >22.27
2025-04-21T16:59:53 | r | 300*2 | 21.21 +/- 0.16
2025-04-21T17:12:15 | i | 300*2 | 20.28 +/- 0.12
2025-04-21T16:59:53 | z | 300*2 | 19.15 +/- 0.14
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Mephisto (Multi-channel Photometric Survey Telescope) is a 1.6-m wide-field multi-channel telescope, the first of its type in the world, capable of imaging the same field of view in three optical bands simultaneously. It provides real-time, high-quality colors of stellar objects. The on-site telescope assemblage and commissioning were carried out in September 2022. The first light in all three channels was achieved on 2023 December 21.
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GCN Circular 40193
Subject
EP250421a: JinShan likely optical counterpart
Date
2025-04-21T18:54:51Z (10 months ago)
From
Dong Xu at NAOC/CAS <dxu@nao.cas.cn>
Via
Web form
S.Y. Fu (HUST), S.Q. Jiang, J. An, Z.P. Zhu, X. Liu, D. Xu (NAOC), J.Z. Liu (XAO) report on behalf of a large collaboration:
We observed the field of EP250421a (ID: 01709135179) detected by EP/WXT, using the 100cm-C telescope (100C) of the JinShan project, located at Altay, Xinjiang, China. Observations started at 16:25:07.32 UT on 2025-04-21, i.e., 8.64 min after the EP/WXT trigger, and a series of frames were obtained in the Sloan r-bands.
An uncatalogued optical source is detected within the EP/WXT error circle at coordinates
R.A.(J2000) = 11:29:40.80
Dec.(J2000) = -24:40:31.84
with an uncertainty of ~0.6 arcsec. The source has r ~ 20.6 mag, calibrated with nearby PanSTARRS stars and not corrected for Galactic extinction. We think the source is likely the optical counterpart of EP250421a.
We acknowledge the excellent support from T.Q. Chen and Z.K. Feng for enabling these observations.