EP250227a
GCN Circular 39532
Subject
EP250227a: Einstein Probe detected of a fast X-ray transient
Date
2025-02-28T15:52:07Z (3 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
W.F. Wen (SZTU), J. H. Wu (GZHU), H. Z. Wu (HUST), B. -T. Wang (YNAO, CAS), Z. X. Ling(NAO, CAS) report on behalf of the Einstein Probe team:
We report on the detection of a fast X-ray transient EP250227a (trigger ID: 11916647689) at 2025-02-27 04:39:40 (UTC) by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The position of the source is R.A. = 204.133 deg, DEC = -13.820 deg (J2000) with an uncertainty of 2.7 arcmin (radius, 90% C.L. statistical and systematic). The transient event lasted for about 40 seconds.
The 0.5-4.0 keV spectrum can be fitted with an absorbed powerlaw model, with nH fixed at 1.87 x 10^20 cm^-2 and a photon index of -0.07(+2.18/-1.22). The unabsorbed 0.5-4.0 keV flux is 2.1 (+2.6/-1.2) x 10^(-9) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
We performed a follow-up target of opportunity observation with EP-FXT. The observation began at 2025-02-27 15:19:07 (UTC) with an exposure time of 3020 seconds, about 10.7 hours after the detection by EP-WXT. Within the WXT error cirlce, an uncatalogued X-ray source was detected at R.A. = 204.1444 deg, DEC = -13.8216 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The X-ray spectrum can be fitted with an absorbed powerlaw with the nH fixed at the Galactic value and a photon index of 1.2 (-0.8, +0.8). The derived unabsorbed 0.5-10 keV flux is 2.4 (-1.4, +5.2) x 10^-13 erg/s/cm2. 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 39533
Subject
EP250227a: NOT optical observations
Date
2025-02-28T16:36:03Z (3 months ago)
Edited On
2025-02-28T16:51:21Z (3 months ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
D. B. Malesani (DAWN/NBI and Radboud Univ.), A. Martin-Carrillo (UCD), A. J. Levan (Radboud Univ and Warwick Univ.), F. E. Bauer (PUC), P. G. Jonker (Radboud Univ), M. Alejandra Diaz Teodori (NOT and Turku Univ.), Roar H. Rasmussen (NOT and Aarhus Univ.), Saskia Schlagenhauf (QUB), D. Xu (NAOC), report for a larger collaboration:
We obtained observations of the field of the fast X-ray transient EP250227a (Wen et al. GCN 39532) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations started on 2025 Feb 28.126 UT (22.37 hr after the EP trigger) and were carried out in the r and z filters.
No new source is visible within the EP/FXT X-ray error circle (Wen et al. GCN 39532), down to a 3-sigma limiting magnitude r > 23.7 AB, calibrated against nearby Pan-STARRS objects. At the edge of the error circle, we note the presence of a source, visible also in the Legacy Survey and whose brightness is consistent with the archival value. While this is a potential host galaxy for EP250227a, its chance association probability is not negligible.
We also performed image subtraction on our stacked r-band image using the Legacy Survey as template. A single variable source is identified at coordinates (inconsistent with those of the EP/FXT error circle):
R.A.(J2000) = 13:36:29.37
Dec.(J2000) = -13:49:00.16
This source is located about 1” to the NE of the nucleus of a galaxy visible in the Legacy Survey with magnitude r = 18.8. We measure for the variable source r = 22.5 +/- 0.1 AB.
The nature of this source and its association with EP250227a are unclear at the moment. Further observations are planned and deep r-band photometry of the field is encouraged.
GCN Circular 39534
Subject
EP250227a: Optical upper limits with Kinder observations
Date
2025-02-28T16:56:52Z (3 months ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
Web form
A. Aryan (NCU), A. K. H. Kong (NTHU), T.-W. Chen, H.-Y. Hsiao, C.-S. Lin (all NCU), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), Y. J. Yang, A. Sankar. K, Y.-C. Pan, C.-C. Ngeow, W.-J. Hou, M.-H. Lee, H.-C. Lin, C.-H. Lai, J.-K. Guo (all NCU), S. Yang, L. L. Fan, 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 EP250227a (Wen et al., GCN 39532) using the 40cm SLT telescope at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al. 2024, arXiv:2406.09270). The first SLT epoch of observations in the i band started at 15:53 UT on the 27th of February 2025 (MJD = 60733.662), ~11.22 hrs after the EP trigger, while the first SLT epoch of observations in the r band started at 16:28 UT on the 27th of February 2025 (MJD = 60733.686), ~11.79 hrs after the EP 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 did not detect any uncataloged optical counterpart candidate within the EP-WXT localization error circle of radius 2.7 arcminutes.
We further employed the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform PSF photometry on our stacked frames. The details of the observations and measured three sigma upper limits (in the AB system) were as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
SLT | i | 60733.662 | 11.22 | 300 * 6 | >21.1 | 1".47 | 1.80
SLT | r | 60733.686 | 11.79 | 300 * 12 | >21.5 | 1".46 | 1.49
The non-detection of any prominent optical counterpart is consistent with Malesani et al., (GCN 39533).
The presented upper limits were calibrated using the field stars from the Pan-STARRS1 catalog and were not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_i = 0.12 mag and A_r = 0.16 mag, respectively, in the direction of the transient (Schlafly & Finkbeiner 2011).
GCN Circular 39536
Subject
EP250227a: TRT optical upper limit
Date
2025-02-28T17:28:36Z (3 months ago)
From
sqjiang at NAOC <sqjiang@bao.ac.cn>
Via
Web form
S.Q. Jiang (NAOC), S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), J. An, X. Liu, S.Y. Fu, Z.P. Zhu, D. Xu, Z. Fan, W.X. Li, Y.N. Wang, N.C. Sun (NAOC) report on behalf of a large collaboration:
We observed the field of EP250227a detected by EP (Wen et al., GCN 39532), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Cerro Tololo Inter-American Observatory, Chile. Observations started at 04:10:11.093 UT on 2025-02-28, i.e., 0.98 days after the EP trigger, and a series of 300 s frames were obtained in the R band.
No uncatalogued optical source is detected in the stacked R-band image within the EP/FXT error circle (Wen et al., GCN 39532), down to the 5-sigma limiting magnitude of R ~ 22.1, calibrated with LS-DR10 stars and not corrected for Galactic extinction, being consistent with the non-detections by NOT (Malesani et al., GCN 39533) and SLT (Aryan et al., GCN 39534).
GCN Circular 39538
Subject
EP250227a: Global MASTER-Net observations report
Date
2025-02-28T17:54:50Z (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 EP250227a ( EP Team et al., GCN 39532) errorbox 5473 sec after notice time and 1 days 45921 sec after trigger time at 2025-02-28 17:25:01 UT, with upper limit up to 17.1 mag. The observations began at zenith distance = 74 deg. The sun altitude is -46.8 deg.
The galactic latitude b = 47 deg., longitude l = 320 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2795063
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
132351 | 2025-02-28 17:25:01 | MASTER-Tunka | (13h 36m 31.38s , -13d 41m 08.1s) | C | 60 | 17.1 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 40132
Subject
EP250227a: Further NOT optical observations
Date
2025-04-10T09:17:11Z (2 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), P. G. Jonker (Radboud), A. Martin-Carrillo (UCD), A. J. Levan (Radboud and Warwick), M. A. Diaz Teodori (NOT and Turku Univ.), report on behalf of a larger collaboration:
We obtained ten 300-s observations in the r band of the field of the fast X-ray transient EP250227a (Wen et al., GCN 39532) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC imager. The mid time was 2025 Apr 5.09 UT (36.9 days after the EP trigger).
In Malesani et al. (GCN 39533), based on observations taken 22.4 hr after the EP trigger, we noted a possible optical transient (not consistent with the EP/FXT localisation) within the EP/WXT error circle, revealed by image subtraction using the Legacy survey as template. The new observation was obtained with the same instrument, filter, and pixel scale as our early one, in order to provide a better reference. We then carried out again image subtraction using both ISIS (Alard 2000, A&AS, 144, 363) and PyZOGY (Zackay et al. 2016, doi:10.3847/0004-637X/830/1/27).
With the new template, no credible residual is detected at the position of the early optical candidate (Malesani et al., GCN 39533), which we thus consider to be spurious. We set an upper limit r > 23.8 AB to any transient within the EP/WXT error circle, at a time of 22.4 hr after the trigger.
In our April 5 observation, one faint object is detected inside the EP/FXT error circle (at coordinates RA = 13:36:34.67, Dec = -13:49:23.18, magnitude r = 24.52 +- 0.23 AB). Its probability of a chance association with the X-ray localization is large. As a result, no connection to EP250227a can be established.