EP250220a
GCN Circular 39387
Subject
EP250220a: EP-WXT detection of a fast X-ray transient
Date
2025-02-20T15:47:51Z (3 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Y. L. Wang (NAOC, CAS), D. F. Hu (PMO, CAS), B. B. Zhang (BNU), P. Y. Han, C. X. Zhang (HUST), Y. Liu (NAOC, CAS) report on behalf of the Einstein Probe team:
We report on the detection of a fast X-ray transient by EP-WXT, designated EP250220a. The WXT position of the source is R.A. = 113.400 deg, DEC = 39.795 deg (J2000) with an uncertainty of 2.81 arcmin in radius (90% C.L. statistical and systematic). The flare starts from 2025-02-20T02:11:59 (UTC), lasting for about 150 seconds. The average 0.5-4 keV spectrum can be fitted with an absorbed powerlaw model, with NH fixed at the Galactic value of 7.13 x 10^20 cm^-2 and a photon index of 1.8 (-0.9/+1.0). The unabsorbed average 0.5-4 keV flux is estimated to be 5.8 (-2.2/+3.3) x 10^-11 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
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 39388
Subject
EP250220a: Global MASTER-Net observations report
Date
2025-02-20T16:18:36Z (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-Tunka robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Applied Physics Institute, Irkutsk State University) was pointed to the EP250220a ( EP Team et al., GCN 39387) errorbox 228 sec after notice time and 49332 sec after trigger time at 2025-02-20 15:54:11 UT, with upper limit up to 12.9 mag. The observations began at zenith distance = 17 deg. The sun altitude is -46.1 deg.
The galactic latitude b = 25 deg., longitude l = 179 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2784319
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
49363 | 2025-02-20 15:54:11 | MASTER-Tunka | (07h 33m 25.59s , +40d 01m 57.1s) | C | 60 | 12.9 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 39393
Subject
EP250220a: Xinglong optical upper limit
Date
2025-02-21T02:21:20Z (3 months ago)
From
Xinglong Observatory at National Astronomical Observatories (NAOC), Chinese Academy of Sciences (CAS) <xinglong@nao.cas.cn>
Via
Web form
Yu-Zhang(NAOC), Junjie-Jin(NAOC), Haiyang-Mu(NAOC), Feng-Xiao(NAOC), Qi-Zhao(NAOC), Zhou-Fan(NAOC), Hong-Wu(NAOC) report on behalf of a large collaboration:
Following the detection of EP250220a by EP-WXT (Y. L. Wang et al., GCN 39387), we observed the field of EP250220a using the 2.16-m telescope at Xinglong Observatory, NAOC. We obtained 6x600s clear-band frames with a median time of 2025-02-20T12:10:32 i.e., 10 hr after the EP trigger. No uncatalogued optical transient is detected in the stacked images within the 2.81 arcmin EP/WXT error circle (Y. L. Wang et al., GCN 39387), down to 5-sigma limiting magnitudes of clear ~ 22.6, calibrated with Pan-STARRS sources in the field. Also there is no apparent brightening for the catalogued sources within the error circle.
GCN Circular 39400
Subject
EP250220a: Optical upper limit with Kinder observations
Date
2025-02-21T08:46:24Z (3 months ago)
From
Janet Chen at National Central University <janetstars@gmail.com>
Via
Web form
L. L. Fan (HNAS), A. Aryan (NCU), A. K. H. Kong (NTHU), T.-W. Chen, W.-J. Hou (both NCU), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), Y. J. Yang, A. Sankar. K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, H.-C. Lin, C.-H. Lai, H.-Y. Hsiao, C.-S. Lin, J.-K. Guo (all NCU), S. Yang, 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 EP250220a (Wang et al., GCN 39387) using the 1m LOT 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 started at 11:23:50 UTC on the 20th of February 2025 (MJD = 60726.475), about 9.20 hr after the EP WXT 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. We do not find any evidence of a new and uncataloged source in the stacked images within the 2.81 arcminute error circle of EP-WXT localization.
Moreover, we employed the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction using the "sfft" (Hu, 2022, ApJ, 936, 157) and "hotpants" (Becker A., 2015, ascl.soft. ascl:1504.004) algorithms using a template from Pan-STARRS1 (Chambers et al. 2016, arXiv:1612.05560). We found no evidence of any prominent candidate optical counterpart in the difference image as well.
We further employed AutoPhOT to perform the PSF photometry. The details of the observations and measured 3-sigma upper limit (in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
LOT | r | 60726.475 | 9.20 | 300 * 6 | > 22.65| 1".12 | 1.13
The non-detection of any optical counterpart in our observations is consistent with Xinglong observations, as reported by Zhang et al. (GCN 39393).
The presented magnitudes are calibrated using the field stars from the Pan-STARRS1 catalog and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E(B-V) = 0.12 mag in the direction of the transient (Schlafly & Finkbeiner 2011).
GCN Circular 39401
Subject
EP250220a: Liverpool Telescope optical upper limits
Date
2025-02-21T09:31:44Z (3 months ago)
From
Rob Eyles-Ferris at U of Leicester <raje1@leicester.ac.uk>
Via
Web form
R. A. J. Eyles-Ferris, P. T. O’Brien and R. L. C. Starling (U of Leicester) report:
We observed the field of the X-ray transient EP250220a (Wang et al., GCN 39387) with the 2m Liverpool Telescope using the IO:O instrument. We obtained 4x100s exposures in the SDSS g’ filter starting at 2025-02-20 22:14:10 UT and 6x100s exposures in the SDSS r’ filter starting at 2025-02-20 22:22:20 UT, approximately 20 hours after the X-ray detection.
We performed image subtraction on the stacked images using reference images from Pan-STARRS and, due to the crowded field and therefore extensive subtraction artefacts, compared the stacked and reference images manually. Consistent with Zhang et al. (GCN 39393) and Fan et al. (GCN 39400), we identify no new sources within the error region of the EP/FXT candidate. We derive 3-sigma upper limits of g’ > 21.5 and r’ > 21.8 with photometry calibrated to Pan-STARRS and not corrected for Galactic extinction.
GCN Circular 39402
Subject
EP250220a: 1.6m Mephisto observations
Date
2025-02-21T09:36:39Z (3 months ago)
From
Brajesh Kumar at SWIFAR, YNU <brajesh@ynu.edu.cn>
Via
Web form
Helong Guo, Guowang Du, Xinlei Chen, Xiangkun Liu, Xiaotong Chen, Yiheng Xie, Brajesh Kumar, Yuan Fang, Edoardo P. Lagioia, Yu Pan, Xingzhu Zou, Xiaowei Liu (all SWIFAR, YNU) report on behalf of the Mephisto Team:
The field of EP250220a detected by EP Team (Wang et al., GCN 39387) was observed with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. Simultaneous uvgr band photometric observations were conducted starting from 13:08:36 2025-02-20 UT (~10.93 hr after the trigger) and 3 frames with 300s exposure times were taken. No reliable uncatalogued source was detected in our stacked images, consistent with Zhang et al. (GCN 39393) and Fan et al. (GCN 39400). The 3 sigma upper limits are listed below.
Start_Time(UT) | Band | Exp(s)| LimMag (AB)
--------------------|------|-------|------------
2025-02-20T13:33:57 | u | 300*3 | >22.85
2025-02-20T13:08:36 | v | 300*3 | >22.85
2025-02-20T13:33:57 | g | 300*3 | >23.10
2025-02-20T13:08:36 | r | 300*3 | >23.04
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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 39408
Subject
EP250220a: Liverpool Telescope upper limits
Date
2025-02-21T14:53:53Z (3 months ago)
From
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 EP250220a (Wang et al., GCN 39387) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 6x90s exposures in the SDSS r’ and i’ filters starting at 2025-02-20 21:40:49.049 UT, approximately 19.5 hours after the trigger.
The images were stacked and the background was subtracted relative to Pan-STARRS-1, as well as Legacy Sky Survey reference imaging using PSF matching with the help of PSFEx. We do not detect any new object at the EP/WXT error region of the transient: the 3-sigma limiting magnitudes on the stacked images are r > 21.7 mag and i > 21.8 mag, in line with Zhang et al. (GCN 39393), Fan et al. (GCN 39400), Eyles-Ferris et al. (GCN 394001) and Guo et al. (GCN 394002). However, we note that there is a bright, saturated star inside the error region, impacting our observations as well as the reference fields.
GCN Circular 39420
Subject
EP250220a: EP-FXT follow-up observations
Date
2025-02-22T08:11:55Z (3 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
D. F. Hu (PMO, CAS), Y. L. Wang (NAOC, CAS), P. Y. Han, C. X. Zhang (HUST), B. B. Zhang (BNU), Y. Liu (NAOC, CAS) report on behalf of the Einstein Probe team:
Following the detection of the fast X-ray transient EP250220a (Wang et al., GCN 39387), we performed two follow-up observations with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission.
The first FXT observation started at 2025-02-20 14:59:57 (UTC), about 13 hours after the WXT detection, with an exposure of ~ 3 ks. Within the WXT error circle, one faint X-ray source was detected by one of the FXT modules, at the position of R.A. = 113.4516 deg, DEC = 39.8098 deg (J2000), with an uncertainty of 10 arcsec in radius (90% C. L. statistical and systematic). The detection is relatively weak and the spectrum is not well fitted.
The second FXT observation started at 2025-02-21 10:12:09 (UTC), about 32 hours after the WXT detection, with an exposure of ~ 7 ks. No source was detected within the WXT error circle. Assuming a photon index of 1.1 and Galactic absorption with NH = 7.13 x 10^20 cm^-2, we derive an upper limit of 1.90e-14 erg/s/cm^2 (0.5-10 keV; 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 39421
Subject
EP250220a: FTW optical and NIR observations and upper limits
Date
2025-02-22T11:03:18Z (3 months 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 90% EP-FXT localization area of EP250220a (Wang et al., GCN 39387; Hu et al., GCN 39420) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i and J band simultaneously for 59 x 180 s starting at 2025-02-20T22:20:37 UT (0.84 days after the trigger) and 2025-02-21T20:54:36 UT (1.78 days after the trigger). We performed difference imaging with templates from the DESI Legacy Surveys in the r band and templates from PS1 in the i band as well as between our two epochs. We detect no new or evolving source in the EP-FXT localization area. The three sigma mean depths of our observations in the r band are
2025-02-20: r > 25.0 mag
2025-02-21: r > 24.4 mag.
This is consistent with the non-detections by Lipunov et al. (GCN 39338), Zhang et al. (GCN 39393), Eyles-Ferris et al. (GCN 39401), Guo et al. (GCN 39402) and Bochenek and Perley (GCN 39408).
The magnitudes are calibrated against the PS1 catalog and provided in the AB system. They are not corrected for Galactic extinction.
We thank Michael Schmidt from the staff of the Wendelstein Observatory for obtaining these observations.