EP250228a
GCN Circular 39525
Subject
EP250228a: Einstein Probe detection of an X-ray transient
Date
2025-02-28T07:09:00Z (4 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
H.Y. Liu (NAO, CAS), J.Q. Peng (IHEP, CAS), W. F. Wen(SZTU), J. H. Wu (GZHU), C.C. Jin (NAO, CAS) report on behalf of the Einstein Probe team:
We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250228a. The transient triggered EP-WXT (ID: 01709132122) at 2025-02-28T03:59:09 (UTC). The WXT position of the source is R.A. = 103.607 deg, DEC = -41.581 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).
A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error cirlce, an uncatalogued X-ray source was detected at R.A. = 103.6021 deg, DEC = -41.5715 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.
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 39526
Subject
EP250228a: Global MASTER-Net observations report
Date
2025-02-28T08:18:36Z (4 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 EP250228a ( EP Team et al., GCN 39525) errorbox 229 sec after notice time and 12029 sec after trigger time at 2025-02-28 07:19:38 UT, with upper limit up to 19.2 mag. The observations began at zenith distance = 74 deg. The sun altitude is -36.6 deg.
The galactic latitude b = -17 deg., longitude l = 252 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2794721
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
12059 | 2025-02-28 07:19:38 | MASTER-OAFA | (06h 55m 12.36s , -41d 21m 41.5s) | C | 60 | 19.2 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 39527
Subject
EP250228a: Swift/XRT localisation
Date
2025-02-28T10:52:50Z (4 months ago)
From
K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
email
K.L. Page, P.A. Evans (U.Leicester) and J. DeLaunay (PSU) report on
behalf of the Swift XRT Team:
On 2025 February 28, at 05:53 UT, Swift obtained an Target of Opportunity
observation of EP250228a (GCN Circ. 39525), collecting ~340 s of data
around 1.9 hr after the Einstein Probe trigger. The XRT data show an
uncatalogued X-ray source, consistent with the FXT source reported by the
EP team, at a position of RA, Dec = 103.60631, -41.57160, which is
equivalent to
RA (J2000): 06h 54m 25.51s
Dec (J2000): -41d 34′ 17.8″
with an uncertainty of 4.4 arcsec (radius, 90% confidence). The flux of
this source was (5.4 +/- 0.9) x 10^-12 erg cm^-2 s^-1 (0.3 - 10 keV).
GCN Circular 39531
Subject
EP250228a: updates on the EP observations
Date
2025-02-28T15:49:46Z (4 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
H.Y. Liu (NAO, CAS), J.Q. Peng (IHEP, CAS), W. F. Wen(SZTU), J. H. Wu (GZHU), C.C. Jin (NAO, CAS) report on behalf of the Einstein Probe team:
The X-ray transient EP250228a (GCN 39525) triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at 2025-02-28T03:59:09 (UTC) and an autonomous observation was performed by the Follow-up X-ray Telescope (FXT) around 45 s later. The on-ground analysis of the FXT data shows that an uncatalogued source was detected at R.A. = 103.6059, DEC = -41.5710 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with that reported by the Swift XRT Team (GCN 39527). Given that the localisation of this source is well aligned with the position of Gaia DR3 5563280076339409920, EP250228a is likely a stellar flare associated with this star. The FXT spectrum can be well fitted by a 4T apec model with kT= 6.6(+3.4/-1.3), 1.4(+0.69/-0.35), 0.63(+0.16/-0.23), and 0.17(+0.04/-0.06) keV, respectively, and the derived flux is around 1.87 (+0.09/-0.09) x 10^-11 erg/s/cm^2 in 0.5-10 keV. 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 onboard X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).
GCN Circular 39543
Subject
EP250228a: Optical observations with Kinder and Swift/UVOT
Date
2025-03-01T13:45:29Z (4 months ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
Web form
A. Aryan, Y. J. Yang (both 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), 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 EP250228a (Liu et al., GCN 39525; Liu et al., GCN39531) 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 10:54 UT on the 28th of February 2025 (MJD = 60734.454), ~6.91 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. In the stacked frame, we did not detect any uncataloged optical counterpart candidate within the EP-FXT localization error circle of radius 20 arcseconds.
Moreover, we employed the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction utilizing the "sfft" (Hu, 2022, ApJ, 936, 157) and "hotpants" (Becker A., 2015, ascl.soft. ascl:1504.004) algorithms using templates from SkyMapper DR4 (Onken et., 2024, PASA, 41, e061). We found no evidence of any prominent candidate optical counterpart in the difference images as well.
We further employed AutoPhOT to perform the PSF photometry. The details of the observations and measured 3-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 | 60734.454 | 6.91 | 300 * 12 | >20.2 | 1".20 | 2.44
SLT | r | 60734.503 | 8.08 | 300 * 24 | >19.9 | 1".43 | 2.53
The X-ray transient was also detected in Swift-XRT (Page et al., GCN39527). The Swift-XRT position coincided with a star (Gaia DR3 5563280076339409920; as updated by Liu et al., GCN39531), but we probably missed the flaring stage as we did not see any credible excess flux in the difference image. For this star, we measured an r-band magnitude of 15.33 +/- 0.03 mag and an i-band magnitude of 13.65 +/- 0.03 mag in our stacked frames. Our measured magnitudes for this source align perfectly with the archival values in the SkyMapper catalog. In addition, a preliminary U-band magnitude of 17.61 +/- 0.05 (stat) +/- 0.02 (sys) was measured from the quick-look Swift-UVOT observation.
The presented measurements were calibrated using the field stars from the SkyMapper catalog and were not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_i = 0.20 mag, and A_r = 0.27 mag in the direction of the transient (Schlafly & Finkbeiner 2011).