EP250428b
GCN Circular 40277
Subject
EP250428b: Einstein Probe detection of a fast X-ray transient
Date
2025-04-29T17:30:36Z (a month ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
M. J. Liu, Q. Y. Wu, T. Zhao, Y. J. Song (NAO, CAS), H. Z. Wu (HUST), C. C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:
We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250428b (GCN Notice ID 01799135422). The WXT position of the source is R.A. = 220.479 deg, DEC = 2.09 deg (J2000) with an uncertainty of 2.7 arcmin in radius (90% C.L. statistical and systematic). A preliminary analysis of the EP-WXT data shows that the transient began at around 2025-04-28T06:05:31(UTC) and lasted for a few hundreds of seconds, with a peak flux of around 3 x 10^-10 erg/cm^2/s. The averaged WXT spectrum can be fitted by an absorbed power law model with a Galactic hydrogen column density of 3.77 x 10^20 cm^-2 and a photon index of 1.9 (+0.2, -0.2). The averaged unabsorbed 0.5-4 keV flux is 3.3 (+2.2, -1.3) x 10^-11 erg/cm^2/s. The uncertainties are at the 90% confidence level for the above parameters.
A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed from 2025-04-29T13:41:27 (UTC). Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 220.4586 deg, DEC = 2.1002 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.
The contact TA of EP250428b is Q. Y. Wu. Please contact him via email qywu@bao.ac.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 40278
Subject
EP250428b: Global MASTER-Net observations report
Date
2025-04-29T18:09:39Z (a month 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-Kislovodsk robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the EP250428b ( EP Team et al., GCN 40277) errorbox 285 sec after notice time and 1 days 41608 sec after trigger time at 2025-04-29 17:38:59 UT, with upper limit up to 17.6 mag. Observations started at twilight. The observations began at zenith distance = 64 deg. The sun altitude is -15.2 deg.
The galactic latitude b = 53 deg., longitude l = 355 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2856874
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
128038 | 2025-04-29 17:38:59 | MASTER-Kislovodsk | (14h 39m 56.30s , +02d 18m 36.2s) | C | 60 | 17.1 |
128993 | 2025-04-29 17:54:54 | MASTER-Kislovodsk | (14h 39m 58.93s , +02d 19m 39.5s) | C | 60 | 17.5 |
129172 | 2025-04-29 17:57:52 | MASTER-Kislovodsk | (14h 39m 52.83s , +02d 19m 59.5s) | C | 60 | 17.6 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 40281
Subject
EP250428b: LAST optical upper limit
Date
2025-04-29T21:33:20Z (a month ago)
From
Ruslan Konno at Weizmann Institute of Science <ruslankonno@gmail.com>
Via
Web form
R. Konno (WIS), S. Garrappa (WIS), E. A. Zimmerman (WIS), A. Horowicz (WIS), E. O. Ofek (WIS), S. Ben-Ami (WIS), D. Polishook (WIS), O. Yaron (WIS), S. Fainer (WIS), A. Krassilchtchikov (WIS), Y. M. Shani (WIS), E. Segre (WIS), A. Gal-Yam (WIS), S. Spitzer (WIS), and K. Rybicki (WIS) on behalf of the LAST Collaboration
We report observations of the X-ray transient EP250428b (Liu et al., GCN 40277) with the Large Array Survey Telescope (LAST, Ofek et al. 2023; Ben-Ami et al. 2023). We observe the field of EP250428b using four telescopes, each with a FoV of 7.4 deg^2 and no filter (clear - similar to the GAIA Bp band).
We began observations at 2025-04-29 18:25:34.194 UTC (T-T0 = 36.3h). We coadd a total of 160x20s exposure images and perform image subtraction using a reference image of the field. We do not detect any new optical source up to a limiting magnitude of 21.66 (AB) within the reported error regions of FXT and WXT.
LAST is a survey telescope array of the Weizmann Astrophysical Observatory (https://www.weizmann.ac.il/wao/).
GCN Circular 40282
Subject
EP250428b: SAO RAS optical upper limit
Date
2025-04-29T23:39:13Z (a month ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. S. Moskvitin, O. I. Spiridonova (SAO RAS)
report on behalf of the GRB follow-up team.
We observed the field of the X-ray transient EP250428b (Liu et al.,
GCN 40277) with the SAO RAS 1-m telescope Zeiss-1000 on April 29,
20:47:38--21:28:24 UT (t_mid - T0 = 1.6267 days).
We obtained 7 x 300 sec. images in Rc band.
Within the FXT error circle we detect several sources which are presented
in the PanSTARRS images:
R.A. Decl. R_mag +/- err
#1 14:41:50.4 +02:06:18.0 20.20 +/- 0.10
#2 14:41:50.7 +02:06:13.4 22.86 +/- 0.21
#3 14:41:49.8 +02:05:55.9 22.57 +/- 0.17
Within the FXT error circle we did not detect any new sources
down to the limiting magnitude of R_lim = 23.2 (calibrated against R2
magnitudes of nearby UNSO-B1 stars and not corrected for the MW
extinction).
GCN Circular 40283
Subject
EP250428b: SVOM/VT upper limit
Date
2025-04-30T01:39:45Z (a month ago)
From
Liping Xin at NAOC, SVOM <xlp@nao.cas.cn>
Via
Web form
L. P. Xin, Y. L. Qiu, H. L. Li, C. Wu, Z. H. Yao(NAOC), M. J. Liu, Q. Y. Wu, T. Zhao, Y. J. Song (NAO, CAS), H. Z. Wu (HUST), C. C. Jin (NAO, CAS), X. H. Han,Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC) report on behalf of the SVOM team and the Einstein Probe (EP) team:
SVOM/VT conducted ToO follow-up observations of the EP250428b(Liu et al., GCN 40277). The observation started on 2025-04-29T17:26:22 UT, 2025 Apr 29, in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channel simultaneously.
With the effective exposure time of 2500 seconds, no any new sources were detected in EP/FXT error box (Liu et al., GCN 40277) in stacked images with the 3 sigma upper limit of VT_B=23.2 mag and VT_R=23.0 mag, compared to Legacy survey, at the mid time of 1.487 days after the transient time.
This result is consistent with the reports (Konno et al., GCN 40281, Moskvitin et al., GCN 40282).
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 40284
Subject
EP250428b: GROWTH-India telescope optical upper limit
Date
2025-04-30T07:54:42Z (a month ago)
From
anirudhsalgundi@gmail.com
Via
Web form
A. Salgundi (IITB), V. Swain (IITB), D. Eappachen (IIA), A.P. Saikia (IITB), V. Bhalerao (IITB), G.C. Anupama (IIA), S. Barway (IIA) and K. Angail (IAO) report on behalf of the GIT team:
We observed the field of the EP250428b (Liu et al., GCN #40277) with the 0.7m GROWTH-India Telescope (GIT). Observation started at 2025-04-29T18:21:59 UT, about 1.51 days after the trigger. We obtained single image of 360s in r' filter.
The following upper limit was obtained:
|MJD (mid)| Filter| Exposure (s)| Limiting Magnitude (AB)|
| ----------------- | ----------- |------- | ------------------ |
60794.7652662038| r'| 360| 20.5
The magnitude is calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.
The upper limit is consistent with MASTER (Lipunov et al., GCN #40278), LAST (Konno et al., GCN #40281), SAO RAS (Moskvitin et., GCN #40282), SVOM/VT (Xin et al., GCN #40283).
The GROWTH India Telescope (GIT, Kumar et al. 2022) is a 70-cm telescope with a 0.7-degree field of view, set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge funding by the IITB alumni batch of 1994, which partially supports the operations of the telescope. Telescope technical details are available at https://sites.google.com/view/growthindia/.
GCN Circular 40285
Subject
EP250428b: refined analysis of the EP-FXT follow-up observation
Date
2025-04-30T09:16:55Z (a month ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Q. Y. Wu, M. J. Liu, T. Zhao, Y. J. Song (NAO, CAS), H. Z. Wu (HUST), C. C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:
After the EP-WXT detection of the fast X-ray transient EP250428b (Liu et al., GCN 40277), a follow-up observation was performed by EP-FXT at 2025-04-29T14:04:00 (UTC), about 32 hours after the detection, with an exposure time of 3.0 ks. On-ground analysis of the EP-FXT data identified an uncatalogued X-ray source within the EP-WXT error circle, at the coordinates (J2000): R.A., Dec. = 220.4575, 2.0989 deg, with an uncertainty of 10 arcsec (radius, 90% confidence level, including both statistical and systematic errors). The unabsorbed 0.5-10 keV flux is estimated to be 1.2 (+0.4, -0.4) x 10^-13 erg/cm^2/s, derived using an absorbed power law model with the hydrogen column density fixed to the Galactic value of 3.77 x 10^20 cm^-2 and the photon index fixed at 2.0, due to the limited photon statistics. All uncertainties quoted above are at the 90% confidence level.
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 40290
Subject
EP250428b: FTW optical and NIR observations
Date
2025-04-30T17:02:17Z (a month ago)
From
Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>
Via
Web form
Malte Busmann (LMU), Brendan O’Connor (Carnegie Mellon U.), Daniel Gruen (LMU), and Antonella Palmese (Carnegie Mellon U.) report:
We observed the localization of EP250428b (Liu et al., GCN 40277) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i and J band simultaneously for 10 x 180 s starting at 2025-04-30T01:13:27 UT (1.86 days after the trigger). We performed difference imaging in the r and i band with templates from the DESI Legacy Surveys and do not detect any new sources in the improved EP/FXT localization (Wu et al., GCN 40285). The 3 sigma upper limits are
r > 23.9 mag
i > 23.5 mag.
These upper limits are consistent with previous reports (Lipunov et al., GCN 40278; Konno et al., GCN 40281; Moskvitin et al., GCN 40282; Xin et al., GCN 40283; Salgundi et al., GCN 40284).
The magnitudes are calibrated against the PS1 catalog and not corrected for Galactic extinction.
We thank Christoph Ries from the Wendelstein Observatory staff for obtaining these observations.
GCN Circular 40309
Subject
EP250428b: Pan-STARRS follow-up, upper limit on the optical counterpart, and possible host galaxy.
Date
2025-05-01T20:14:32Z (a month ago)
From
Stephen Smartt at University of Oxford <s.smartt@qub.ac.uk>
Via
Web form
J. H. Gillanders (Oxford), M. Huber, K. C. Chambers (IfA, Univ. Hawaii), S. J. Smartt, K. W. Smith, S. Srivastav, F. Stoppa (Oxford), M. Nicholl, D. Young, M. Fulton (QUB), T.-W. Chen (NCU, Taiwan) A. S. B. Schultz, T. de Boer, J. Fairlamb, G. Paek, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii).
We observed the optical counterpart of EP250428b (Liu et al., GCN 40277), using the Pan-STARRS telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560) on MJD 60795.48 (2025-04-30 11:31 UTC), 2.2 days after the EP-WXT detection (Liu et al., GCN 40277). The Pan-STARRS system consists of two 1.8m telescope units located at the summit of Haleakala on the Hawaiian island of Maui, employing an SDSS-like filter system denoted as grizy, and a broad w-filter, which is a composite of the gri-filters. We used PS1 with 6x150s exposures in both r and i. The PS reference frames were subtracted from the stacked 900s images, and no source was detected to r > 22.6 and i > 22.5.
There is a red source in the Legacy Surveys (Dey et al. 2019) DR10 at i=22.07 (r-i=0.76) morphologically classified as a galaxy (source type E), at 0.4" separation from the nominal centre of the EP250428b error circle (220.4575, 2.0989) from Wu et al. GCN 40285. The source is also visible in the Pan-STARRS 3Pi survey data and our night stack i-band. The only other source visible within 10 arcsec radius is a fainter object at 8.8 arcsec offset i=23.9 in the Lgeacy Survey. It is classified with a PSF-like morphology, although the significance of the detection would preclude a definitive stellar classification. We suggest the brigher, i=22.07, source may be the host galaxy of EP250428b.
There is also a spatially coincident source in the unWISE catalogue (at 220.4576984 +2.0988141) in the 3.4mu band at 128 +/- 7 nMgy (Vega nanomaggies), or AB mag = 19.9 +/- 0.1. The source is visible as excess flux in the legacysurvey.org browser. This could suggest a high redshift host galaxy, but we caution that there is no recorded flux in the 4.6mu band in the unWISE catalogue
The non-detection is consistent with the deeper limits reported in Busman et al. (GCN 40290) and Xin et al. GCN 40283) and also other GCN optical searches (Lipunov et al., GCN 40278; Konno et al., GCN 40281; Moskvitin et al., GCN 40282; Salgundi et al., GCN 40284).