EP250225a
GCN Circular 39475
Subject
EP250225a: Einstein Probe detected of a fast X-ray transient
Date
2025-02-26T04:23:32Z (3 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
S.Q. Jiang (NAO, CAS), B.-T. Wang (YNAO, CAS), H.Z. Wu (HUST), T. Zhao, Y.J. Song, Y. Liu (NAO, CAS) report on behalf of the Einstein Probe team:
We report on the detection of a fast X-ray transient EP250225a at 2025-02-25T12:13:50 (UTC) by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission.
The position of the source is R.A. = 141.272 deg, DEC = -4.932 deg (J2000) with an uncertainty of 2.8 arcmin (radius, 90% C.L. statistical and systematic). The transient event lasted for about 600 seconds.
The average 0.5-4 keV spectrum can be fitted with an absorbed powerlaw model, with nH fixed at the Galactic value of 3.0 x 10^20 cm^-2 and a photon index of 2.5 (-0.9/+1.1). The unabsorbed average 0.5-4 keV flux is estimated to be 4.3 (-1.3/+2.1) x 10^-11 erg/s/cm2. The uncertainties are at the 90% confidence level for the above parameters.
Further FXT observations are scheduled.
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 39477
Subject
EP250225a: GSP optical upper limit
Date
2025-02-26T05:25:33Z (3 months ago)
From
Wenxiong Li <liwenxiong1992@gmail.com>
Via
Web form
W. X. Li, S. J. Xue (NAOC), M. Andrews, J. Farah, D. A. Howell, M. Newsome, E. Padilla Gonzalez, C. McCully, and G. Terreran (Las Cumbres Observatory), on behalf of a larger collaboration, report:
Following the detection of the fast X-ray transient EP250225a by the Einstein Probe (Jiang et al., GCN 39475) with the Sinistro instrument mounted on the 1-m telescope of the LCO, we initiated observations of the fast X-ray transient location starting on 2025 Feb 26th at 04:50:50 UT (~15 hours after the EP/WXT trigger) in the r band. These observations were conducted using the 1-meter telescope at the Las Cumbres Observatory node located at the Cerro Tololo Inter-American Observatory in Chile.
No new optical source was detected in the co-added images within the EP/WXT error box down to ~21.5 mag.
These observations were taken as part of the Global Supernova Project.
GCN Circular 39478
Subject
EP250225a: Global MASTER-Net observations report
Date
2025-02-26T05:31:23Z (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 EP250225a ( EP Team et al., GCN 39475) errorbox 364 sec after notice time and 58846 sec after trigger time at 2025-02-26 04:34:36 UT, with upper limit up to 20.5 mag. The observations began at zenith distance = 30 deg. The sun altitude is -50.1 deg.
The galactic latitude b = 31 deg., longitude l = 238 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2792193
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
58876 | 2025-02-26 04:34:36 | MASTER-OAFA | (09h 24m 15.36s , -04d 59m 40.0s) | C | 60 | 20.5 |
60307 | 2025-02-26 04:58:27 | MASTER-OAFA | (09h 24m 17.17s , -05d 00m 37.2s) | C | 60 | 20.4 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 39512
Subject
EP250225a: TRT optical upper limits
Date
2025-02-27T09:15:26Z (3 months ago)
From
Zipei Zhu at NAOC <zpzhu@nao.cas.cn>
Via
Web form
Z.P. Zhu, J. An, X. Liu, S.Q. Jiang, S.Y. Fu (NAOC), S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), Z. Fan, W.X. Li, N.C. Sun, Y.N. Wang, D. Xu (NAOC) report on behalf of a large collaboration:
We observed the field of EP250225a (Jiang et al., GCN 39475) using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Cerro Tololo Inter-American Observatory, Chile. Observations started at 02:45:51 UTC on 2025-02-26, 6 x 300 s and 5 x 300s frames were obtained in Sloan-r and Sloan-z band, respectively.
No new optical source is detected in the stacked images within or beside the ~ 2.8 arc min EP/WXT error circle (Jiang et al., GCN 39475), consistent with the non-detection (Li et al., GCN 39477; Lipunov et al., GCN 39478), down to a 5-sigma limiting magnitude of r ~ 21.8 and z ~ 19.5, calibrated with Pan-STARRS sources in the field and not corrected for Galactic extinction.
GCN Circular 39516
Subject
EP250225a: NOT optical candidate counterpart
Date
2025-02-27T12:49:18Z (3 months ago)
Edited On
2025-02-27T18:00:51Z (3 months ago)
From
Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>
Via
Web form
Daniele B. Malesani (DAWN/NBI and Radboud Univ.), Andrew J. Levan (Radboud Univ. and DAWN/NBI), Antonio Martin-Carrillo (UCD), Franz E. Bauer (PUC), Peter G. Jonker (Radboud Univ.), Maria E. Ravasio (Radboud Univ.), M. Alejandra Diaz Teodori (NOT and Turku Univ.), Roar H. Rasmussen (NOT and Aarhus Univ.), Saskia Schlagenhauf (QUB), report on behalf of a larger collaboration:
We observed the field of the fast X-ray transient EP250225a (Jiang et al., GCN 39475) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were carried out in the r and z filters, with mean times 2025 Feb 26.98 and 27.00 UT (1.47 and 1.49 days after the trigger, respectively).
Template subtraction of the NOT images using the Legacy Survey as reference yields a potential faint candidate at the edge of the EP/WXT error circle, located at coordinates:
RA(J2000) = 09:25:04.07
Dec(J2000) = -04:53:40.2
This position is consistent with an object in the Legacy Survey (with magnitudes g = 23.22, r = 22.48, i = 21.53, z = 21.57). The object looks point-like both in our data (PSF 0.75" in the z band) and in the Legacy Survey. Photometry confirms a brightening of 0.50 +- 0.05 and 0.33 +- 0.10 mag in the r and z bands, respectively, when compared to the archival fluxes in the Legacy Survey.
The point-like nature of the source suggests either a transient on top of a compact host galaxy or stellar variability. Further observations are planned (and encouraged) to establish the nature of this object and its association with EP250225a.
GCN Circular 39529
Subject
EP250225a: EP-FXT follow-up observation
Date
2025-02-28T14:11:40Z (3 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
S. Q. Jiang, T. Zhao (NAO, CAS), B.-T. Wang (YNAO, CAS), H. Z. Wu (HUST), Y. J. Song, Y. Liu (NAO, CAS) report on behalf of the Einstein Probe team:
We performed one follow-up observation of the X-ray transient EP250225a (Jiang et al., GCN 39475) with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission. The observation started at 2025-02-26 11:53:56 (UTC), about 23.67 hours after the WXT detection, with an exposure of ~3 ks. No source was detected within the WXT error circle. The 0.5-10 keV upper limit at the NOT candidate position (Malesani et al., GCN 39516) is about 7e-14 erg/s/cm^2 (90% C. L.).
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 39573
Subject
EP250225a: the candidate optical counterpart is an AGN
Date
2025-03-03T15:25:46Z (3 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 Univ.), D. Mata-Sanchez (IAC and ULL), A. Martin-Carrillo (UCD), A. J. Levan (Radboud Univ.), P. G. Jonker (Radboud Univ.), report on behalf of a larger collaboration:
We observed again the variable object (Malesani et al., GCN 39516) located inside the X-ray localization of the fast X-ray transient EP250225a (Jiang et al., GCN 39475; Jiang et al., GCN 39529) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. The observation mid time was 2025 Feb 27.93 UT (2.42 days after the trigger) and a total of 5x300 s was secured in the r band.
The object is detected with a flux consistent with our previous observation (Malesani et al., GCN 39516) to within ~0.1 mag, and still brighter than the archival value from the Legacy Survey. Image subtraction confirms no significant variability between the two NOT epochs.
A spectrum of this source was secured with the Gran Telescopio Canarias (GTC). Observations started on 2025 Feb 28.98 UT (3.47 days after the trigger), and consisted of a single 1800 s exposure with grism R1000R, yielding usable data in the wavelength range 5000-9000 AA.
Detection of two broad features (FWHM ~ 3500 km s^-1), identified as Hbeta and Hgamma, as well as of narrow [O III] 5008, allows us to measure a redshift z = 0.587. The large velocities and the point-like PSF indicate that this is an active galactic nucleus.
Given the slow variability and the AGN nature of this source, its relationship to EP250225b is unclear, and the two may well be unrelated.
We acknowledge significant support from the observing staff at the NOT, in particular, Alejandra Diaz Teodori (NOT and Turku Univ.), Roar Rasmussen (NOT and Aarhus Univ.), and Saskia Schlagenhauf (QUB), and at the GTC, in particular Antonio Cabrera-Lavers (GTC).
GCN Circular 40159
Subject
EP250225a: SVOM/VT optical brightening
Date
2025-04-17T07:50:33Z (a month ago)
Edited On
2025-04-17T13:23:21Z (a month ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
X. H. Han(NAOC), L. Zhang(IHEP), X. L. Chen(YNU), Y. D. Hu(GXU), L. P. Xin, Y. L. Qiu, H. L. Li, C. Wu, Z. H. Yao, Y. Xu, P. P. Zhang, W. J. Xie, J. Wang, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC) report on behalf of the SVOM mission team.
SVOM/VT conducted ToO follow-up observations of the fast X-ray transient EP250225a (Jiang et al., GCN 39475). The observation started on 2025 Feb 26 09:37:03 UT in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channel simultaneously.
The candidate (Malesani et al., GCN 39516; Malesani et al., GCN 39573) was clearly detected in stacked images of both channels.
The brightness in AB magnitude was estimated to be:
Mid time (hour) | Band | Exposure Time (second) | Magnitude | Magnitude error
21.80 | VT_B | 3000 | 22.31 | 0.13
21.80 | VT_R | 3000 | 21.50 | 0.10
The source was brightening with about 0.4 mag within 3.3 hours in both channels during our observations. This result is consistent with the report of the brightening compared with the archival brightness in the Legacy survey (Malesani et al., GCN 39516).
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.