EP250827b
GCN Circular 41838
Subject
EP250827b: VLA Observations
Date
2025-09-16T13:27:15Z (2 months ago)
From
Genevieve Schroeder at Cornell University <genevieveschroeder@u.northwestern.edu>
Via
Web form
G. Schroeder (Cornell), D. A. Perley (LJMU), X. J. Hall (CMU), A. Y. Q. Ho (Cornell), Gokul Srinivasaragavan (UMD) report:
We observed the location of the optical transient AT2025wkm, associated with EP250827b (Schroeder et al., GCN 41635) with the NSF’s Karl G. Jansky Very Large Array under program 25B-363 (PI Perley) on 2025 September 13 at a mid time of 13:02 UT (~17.3 days post discovery) at a mid frequency of 10 GHz (4 GHz bandwidth).
Based on preliminary analysis, we detect a ~6 sigma source with a flux density of ~28 microJy. At a redshift of z = 0.120 (Sevilla et al., GCN 41639), this corresponds to a luminosity of ~1e28 erg/s/Hz. This is a factor of ~6 less luminous than the radio emission associated with SN1998bw (Kulkarni et al. Nature, 395, 663), and a factor of ~5 more luminous than the radio emission associated with SN2006aj (Soderberg et al. 2006, Nature, 442, 1014) at a similar observing frequency and epoch. If the radio emission can be attributed solely to star formation, this would indicate a radio star formation rate of ~3 Msol/year. Additional followup is planned to determine variability of the source.
We thank the VLA staff for quickly approving and executing these observations.
GCN Circular 41705
Subject
EP250827b: Gemini Spectroscopic Confirmation of AT2025wkm as a Ic-BL Supernova
Date
2025-09-04T17:53:57Z (2 months ago)
Edited On
2025-09-05T13:21:23Z (2 months ago)
From
Brendan O'Connor at Carnegie Mellon University <boconno2@andrew.cmu.edu>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Brendan O'Connor at Carnegie Mellon University <boconno2@andrew.cmu.edu>
Via
Web form
Brendan O’Connor (CMU), Xander J. Hall (CMU), Antonella Palmese (CMU), Daniel Gruen (LMU), Gokul Srinivasaragavan (UMD), Anna Y. Q. Ho (Cornell):
We observed the optical transient AT2025wkm, associated with EP250827b (Schroeder et al., GCN 41635), using GMOS mounted on Gemini-North (PI: O’Connor). Observations started on 2025-09-02 at 13:36:55 UT (6.3 d) and 2025-09-04 at 10:58:00 UT (8.2 d). At each epoch, we obtained 2x600 s exposures using the B480 grating covering 3950-8210 AA.
The spectra show a steep blueward slope (likely due to shock cooling) and display broad features that are consistent with ejecta moving at a high velocity (see also Corcoran et al., GCN 41670 and Ho et al., GCN 41639). We identify blueshifted Fe II absorption features in both spectra, indicative of ejecta velocities ~ 40,000 km/s. The Fe II absorption features in combination with the lack of broad Hydrogen and Helium features associated with the transient itself allow us to confirm the classification of a Ic-BL supernova as initially reported by Corcoran et al. (GCN 41670). In addition, we identify numerous narrow emission lines, including the [O II] doublet, multiple [Ne III] lines, the entire Balmer series between Heta and Halpha, He I lines, [N II], [S II], and [Ar III], that identify the host galaxy as highly star forming.
Further observations are planned and encouraged.
We thank the staff of the Gemini Observatory, in particular Brian Lemaux, Eunchong Kim, Teo Mocnik, and Aleksandar Cikota for their assistance in planning and obtaining these observations.
GCN Circular 41673
Subject
EP 250827B: Detection of supernova counterpart by LCO.
Date
2025-09-03T15:34:43Z (2 months ago)
Edited On
2025-09-03T17:56:10Z (2 months ago)
From
ankur ghosh at CAPP, University of Johannesburg <ghosh.ankur1994@gmail.com>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of ankur ghosh at CAPP, University of Johannesburg <ghosh.ankur1994@gmail.com>
Via
Web form
Ankur Ghosh, Soebur Razzaque (CAPP, University of Johannesburg), Rahul Gupta (NASA GSFC), Alexander Moskvitin, Yulia Sotnikova (SAO RAS), Naveen Dukiya (ARIES)on behalf of a larger collaboration.
We observed the field of the EP250827B/AT 2025wkm (Schroeder et al., GCN 41635) in V, r filter of the 0.4-m SCICAM QHY600 at the Las Cumbres Observatory Global Telescope (LCOGT) node located at Haleakala Observatory, Hawai . The 0.4 m SCICAM QHY600 is equipped with 9576 x 6388 pixel CCD (FOV: 1.9 x 1.2 degrees, scale: 0.74 arcsec/pixel) but we only used the FOV of 30 x 30 arcmin for our observation. Observations began on September 03, 2025, starting 7.32 days after the GRB trigger. Further observations are currently ongoing.
We clearly detect the optical transient (OT) reported by GCNs (Schroder et al., GCN 41635; Hall et al., GCN 41636; Hall et al., GCN 41641; Eyles-Ferris et al., GCN 41655; Ma et al., GCN 41667; Eyles-Ferris et al., GCN 41669, He et al,. GCN 41671) in our r band image. Corcoran et al. GCN 41670. reported the detection of the rise of the associated Ic-BL supernova.
|Date| |UTstart| |t-T0 (days)| |Exp (sec)| |Filter| |Magnitude|
--------------------------------------------------------------------------------------------------
2025-09-03 13:58:49.728 7.32 3 x 900 r r = 19.81 +/- 0.05
The field was calibrated against nearby APASS stars, with magnitudes converted using Lupton (2005) equations, and has not been corrected for Galactic extinction.
GCN Circular 41671
Subject
EP250827b / AT 2025wkm: TRT optical observations
Date
2025-09-03T15:06:38Z (2 months ago)
Edited On
2025-09-03T15:35:24Z (2 months ago)
From
L. B. He at NAOC <helb@bao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of L. B. He at NAOC <helb@bao.ac.cn>
Via
Web form
L.B. He (NAOC), K. Noysena, K. Chanchaiworawit, S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), S.Y. Fu (HUST), J. An, X. Liu, Z.P. Zhu, S.Q. Jiang, Z. Fan, W.X. Li, N.C. Sun, Y.N. Wang, D. Xu (NAOC) report on behalf of a large collaboration:
We followed up the optical transient AT 2025wkm, which is potentially associated with EP250827b (Schroeder et al., GCN 41635). Our first epoch was carried out at the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Fresno, California, U.S.A (SRO), approximately 5.04 days after the EP X-ray flare. And we obtained several frames in the R band.
The counterpart (Schroder et al., GCN 41635; Hall et al., GCN 41636; Hall et al., GCN 41641; Eyles-Ferris et al., GCN 41655; Ma et al., GCN 41667; Eyles-Ferris et al., GCN 41669) was clearly detected in the stacked image, with R = 19.62 +/- 0.04 (Vega), calibrated with Pan-STARRS DR2 catalog and not corrected for Galactic extinction.
GCN Circular 41670
Subject
EP250827b: NOT spectroscopic observations and AT 2025wkm classification as a broad-lined type-Ic supernova
Date
2025-09-03T13:24:21Z (2 months ago)
Edited On
2025-09-04T12:27:26Z (2 months ago)
From
Gregory Corcoran at University College Dublin <gregory.corcoran@ucdconnect.ie>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Gregory Corcoran at University College Dublin <gregory.corcoran@ucdconnect.ie>
Via
Web form
G. Corcoran (UCD), A. J. Levan (Radboud and Warwick), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), P. G. Jonker (Radboud), R. A. J. Eyles-Ferris (Leicester), V. Vuolteenaho (NOT and Oulu Univ.), M. Pursiainen (Warwick), I. Worssam (Birmingham) report:
We carried out two spectroscopic observations of the optical transient AT 2025wkm (Schroeder et al., GCN 41635) likely associated with the Einstein Probe transient EP250827b, using the ALFOSC spectrograph mounted on the Nordic Optical Telescope (NOT). The observations started on 2025-09-02 at 00:31:54 and 2025-09-03 at 04:37:19 UT for the two epochs, respectively (5.76 and 6.93 days after the EP trigger). Both observations consisted of 4 x 900 s exposures and cover the wavelength range 3800-9500 AA.
Both spectra show features consistent with broad-lined type Ic supernovae at a comparable phase, though with an excess of flux on the blue end, providing a good match to the super-luminous SN 2011kl (associated with GRB 111209A; Greiner et al. 2015, doi:10.1038/nature14579) near peak. The brightness of the source, at an absolute magnitude of M_r = -18.91 and still rising (Eyles-Ferris & Starling, GCN 41669), is currently consistent with classification as both a normal Ic-BL SN and a super-luminous SN. Between the two epochs, we also identify no significant spectral evolution consistent with the constant colour observed by Eyles-Ferris & Starling (GCN 41669).
Further photometric and spectroscopic observations are planned and encouraged to better characterize this interesting source.
GCN Circular 41669
Subject
EP250827b: continued Liverpool Telescope observations
Date
2025-09-03T12:19:38Z (2 months ago)
From
Rob Eyles-Ferris at U of Leicester <raje1@leicester.ac.uk>
Via
Web form
R. A. J. Eyles-Ferris and R. L. C. Starling (U of Leicester) report on behalf of a larger collaboration:
We observed the position of EP250827b/AT 2025wkm (Schroeder et al., GCN 41635) with the 2m Liverpool Telescope using the IO:O instrument. We obtained a series of exposures in the SDSS u’g’r’i’z’ filters starting at 2025-09-03 01:45:07 UT, approximately 162.5 hours after the X-ray detection.
We performed image subtraction on the stacked g’r’i’z’ images using reference images from Pan-STARRS. AT 2025wkm is clearly detected in all our subtractions and appears to be currently rising in all bands. We measure r’ = 19.90 +/- 0.05, calibrated to Pan-STARRS and not corrected for Galactic extinction, which corresponds to an absolute magnitude of -18.91 adopting the redshift of 0.120 of Sevilla and Ho (GCN 41639). We note no significant change in colour (g’ - r’ ~ 0) when compared with our earlier epoch photometry (Eyles-Ferris and Starling, GCN 41655).
Further observations are planned.
GCN Circular 41667
Subject
EP250827b: SVOM/VT observation of AT 2025wkm
Date
2025-09-03T05:04:52Z (2 months ago)
From
Yinuo Ma <mayn@bao.ac.cn>
Via
Web form
Y. N. Ma, Z. H. Yao, L. P. Xin, Y. L. Qiu, C. Wu, H. L. Li, 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), J. Palmerio (CEA) report on behalf of the SVOM/VT team.
SVOM performed a Target of Opportunity observation of EP250827b/AT 2025wkm (Schroeder et al., GCN 41635). SVOM/VT began observing the field at 2025-09-02T09:14:40 UTC, 6.12 days after the X-ray detection, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
With X-band data availible, the source consistent with AT 2025wkm (Schroeder et al., GCN 41635; Hall et al., GCN 41636; Sevilla & Ho, GCN 41639; Hall et al., GCN 41641; Eyles-Ferris & Starling, GCN 41655) was clearly detected in both VT_B and VT_R. However, the source in VT_R was contaminated by the blooming of bright star, so the magnitude of VT_B is:
mid time (day) | exposure time (s) | band | mag (AB) | mag err
---------------|-------------------|------|----------|--------
6.24 | 37*70 | VT_B | 19.81 | 0.03
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 41655
Subject
EP250827b: Liverpool Telescope observations
Date
2025-09-02T08:06:57Z (2 months ago)
From
Rob Eyles-Ferris at U of Leicester <raje1@leicester.ac.uk>
Via
Web form
R. A. J. Eyles-Ferris and R. L. C. Starling (U of Leicester) report on behalf of a larger collaboration:
We observed the position of EP250827b/AT 2025wkm (Schroeder et al., GCN 41635) with the 2m Liverpool Telescope using the IO:O instrument. We obtained a series of exposures in the SDSS u’g’r’i’z’ filters starting at 2025-09-02 02:47:47 UT, approximately 139.5 hours after the X-ray detection.
We performed image subtraction on the stacked g’r’i’z’ images using reference images from Pan-STARRS. We clearly detect a source consistent with AT 2025wkm in all our subtractions and measure r’ = 19.97 +/- 0.04. Our measured magnitude compared to Schroeder et al. (GCN 41635) and Hall et al. (GCN 41636) suggests AT 2025wkm may still be rising in the r band. The colour of the source (g’ - r’ ~ 0.0) is consistent with the blue spectrum observed by Sevilla and Ho (GCN 41639).
Further observations are planned.
GCN Circular 41641
Subject
EP250827b: Swift observations of AT 2025wkm
Date
2025-09-01T20:57:31Z (2 months ago)
From
xjh@andrew.cmu.edu
Via
Web form
Xander J. Hall (Carnegie Mellon U.), Tomas Ahumada (Caltech), and Robert Stein (UMD) on behalf of the ZTF collaboration:
We requested Swift ToO observations of the X-ray flare EP250827b and optical transient AT 2025wkm reported by Schroeder et al. (GCN 41635).
The observation was taken with a midpoint time of 2025-08-31 at 21:13:29 UT, approximately 4.62 days after the flare, with a duration of 4ks.
The source was not detected with XRT with an upper limit of 2.5 * 10^-3 cts/s at 0.3 - 10 keV. Given the redshift reported by Sevilla & Ho (GCN 41639) of z = 0.1200 this gives a rough luminosity upper limit of 4.5 * 10^42 erg/s.
We clearly detect a source at the position of AT 2025wkm in UVOT observations. We use a synthetic galaxy model to produce subtracted magnitudes and correct for galactic extinction. We report in the UVW1 filter, a magnitude of m= 21.7 +/- 0.4 AB and in the UVM2 filter, a magnitude of m = 22.2 +/- 0.3.
We thank the Swift team, Observatory Duty Scientists and the PI for promptly scheduling and making these observations possible.
GCN Circular 41639
Subject
EP250827b: Gemini Spectroscopy of AT 2025wkm
Date
2025-09-01T20:27:47Z (2 months ago)
From
Anna Y Q Ho at Cornell University <ayh24@cornell.edu>
Via
Web form
Cassie Sevilla and Anna Ho (Cornell) report on behalf of the ZTF collaboration:
We acquired long-slit spectroscopy of AT 2025wkm (possibly associated with Einstein Probe X-ray flare EP250827b; Schroeder et al. GCN #41635) using GMOS-N at Gemini Observatory on 2025-08-31 12:15:31 UTC, under the target-of-opportunity program GN-2025B-Q-130 (PI Ho). The observation was carried out with a 1" slit, the R400 grating, and the GG455 filter. We took four 450s exposures, two at a central wavelength of 710 nm and two at 720 nm. The data was reduced using DRAGONS.
The spectrum shows a blue continuum with narrow emission lines of H alpha, H beta, [O III], and [S II] at a consistent redshift of z=0.120. The implied peak measured absolute magnitude of AT 2025wkm is M_g = -19.5 mag, corrected for Milky Way extinction. The rapid rise (~3d) to such a high luminosity is unusual for extragalactic transients.
We thank the Gemini Observatory staff for assistance in rapidly obtaining these observations.
GCN Circular 41636
Subject
EP250827b: FTW Observations of AT 2025wkm
Date
2025-09-01T19:30:16Z (2 months ago)
From
xjh@andrew.cmu.edu
Via
Web form
Xander J. Hall (Carnegie Mellon U.), Malte Busmann (LMU), Daniel Gruen (LMU), Brendan O’Connor (Carnegie Mellon U.), Antonella Palmese (Carnegie Mellon U.), Mansi Kasliwal (Caltech), Tomas Ahumada (Caltech), and Gokul Srinivasaragavan (UMD) report:
We observed the localization of the X-ray flare EP250827b and optical transient AT 2025wkm as reported by Schroeder et al. (GCN 41635), with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i, and J bands starting at 2025-08-31T02:01:59 UTC (3.82 days after the X-ray flare) for 24 x 180s. In the difference imaging with templates from PS1, we detect AT 2025wkm at
r = 20.14 +/- 0.02 AB mag,
The magnitudes are calibrated against the PS1 catalog and not corrected for Galactic extinction.
Furthermore, we see no other candidates in the difference imaging to a limiting depth of r = 23.15, which supports the association of AT 2025wkn and EP250827b. Further analysis is underway.
We thank Michael Schmidt from the Wendelstein Observatory staff for obtaining these observations.
GCN Circular 41635
Subject
EP250827b: Contemporaneous detection of EP X-ray flare EP250827b and ZTF transient ZTF25abmpngy/AT2025wkm
Date
2025-09-01T19:20:45Z (2 months ago)
From
Tomas Ahumada at Caltech <tfahumada94@gmail.com>
Via
Web form
G. Schroeder (Cornell), T. Ahumada (Caltech), A. Y. Q. Ho (Cornell), M. Kasliwal (Caltech), L.Yan (Caltech), G. Srinivasaragavan (UMD), X. J. Hall (CMU), M. Coughlin (UMN), D. Y. Li (NAO, CAS), J. Yang (ZZU), X. Tian (GXU), K.R., Ni, (CCNU), H. L., Peng (NJNU), H. Y, Liu, Y. J., Song, H. Sun, Y. Liu, C.C.Jin, W. Yuan (NAO, CAS) on behalf of the Zwicky Transient Facility and Einstein Probe (EP) Partnership
We report the contemporaneous detection of the X-ray flare EP250827b associated with the optical transient ZTF25abmpngy/AT 2025wkm (RA = 02:26:20.64, Dec = +37:29:59.90). This event was highlighted in the cross-matching of ZTF alerts with EP alerts, an experiment first announced in Ahumada et al (GCN 39791).
ZTF25abmpngy/AT 2025wkm was first detected on 2025-08-27T09:38:04 UT, 3.27 hrs after the X-ray flare, and with an r-band apparent magnitude of r = 20.33 +/- 0.11 mag. We note the transient sits on top of a potential host galaxy with no archival spectroscopic redshift.
EP250827b was first detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at RA.= 36.581, DEC.= 37.499, with an uncertainty of 2.4 arcmin in radius (90% C.L. statistical and systematic). The X-ray detection is 14.7 arcseconds away from the optical position. The X-ray transient started at 2025-08-27T06:22:27 (UT), and lasted for more than 1000 seconds as seen from the WXT light curve. The X-ray spectrum can be fitted by an absorbed power law model, with the absorption fixed at the Galactic value of 5.4x10^20 cm^-2 , and a photon index of 2.2 (-0.7, +0.8). The average unabsorbed 0.5-4keV flux is 2.6 (-0.9, +1.3) x 10^-11 erg/s/cm2. In the follow-up WXT coverage approximately 5 hours later at the position, the source was no longer detected, indicating a fast decay in the soft X-ray band.
We performed a follow-up target of opportunity observation with EP-FXT. The observation began at 2025-09-01T02:35:49 with an exposure time of 2995 seconds, about 5 days after the burst detected by EP-WXT. Within the WXT error circle, no X-ray source was detected at the optical position, setting a flux upper limit at 90% confidence level around 5 x 10^-14 erg/s/cm2. Based on the current temporal and spatial consistency, EP250827b is likely associated with AT2025wkn. However, due to the significant systematic uncertainty in the WXT position and the non-detection in the follow-up FXT observation, this association cannot be fully confirmed.
We are pursuing and encouraging follow-up observations to classify the source and validate the X-ray and optical association.
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).
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.