EP250901a
GCN Circular 41682
Subject
EP250901a: Mondy, Assy and SAO RAS optical limits
Date
2025-09-03T20:14:49Z (9 days ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin (SAO RAS), A. Pozanenko (IKI), E. Klunko (ISTP),
V. Kim, M. Krugov (FAI), O. Spiridonova (SAO RAS)
report on behalf of GRB follow-up team and IKI-GRB-FuN:
We observed the field of EP250901a (Song et al., GCNs 41627, 41651)
with the 1.5m AZT-33IK telescope of the Solar Sayan Observatory
(Mondy), 1.5m AZT-20 telescope of Assy Observatory and
1.0m Zeiss-1000 telescope of SAO RAS during the night of September 1.
Within the refined FXT error circle (Song et al., GCN 41651)
we didn't detect any new objects down to the following limits.
UT_start t_mid-T0, h exp, s filter up.lim telescope FWHM
13:51:06 2.250997 30 x 120 R 23.2 Mondy, AZT-33IK 2".0
16:17:59 4.837000 61 x 60 r 22.8(AB) Assy, AZT-20 2".6
16:57:33 5.231111 14 x 120 Rc 22.4 SAO, Zeiss-1000 2".1
These preliminary results are in an agreement with the limits
from MASTER (Lipunov et al., GCN 41629) and Mephisto (Liu et al.,
GCN 41631).
In the stacked R band image of AZT-33IR within FXT error circle
localization (Song et al., GCN 41651) we detected two objects,
both presented in the PanSTARRS DR1 images and PS1 catalog,
the 1st one is also visible in z-band image of the Legacy Survey DR10.
obj. R. A. Decl. (J2000) R_mag +/- err
#1 18:12:10.03 +18:42:25.0 22.5 +/- 0.2
#2 18:12:09.78 +18:42:28.3 23.2 +/- 0.3
Also in the stacked image of AZT-33IK we detected one object
near the border of XRT circle (SWIFT J181210.9+184216; Evans et al.,
GCN 41644) with the coordinates of
R. A. (J2000) = 18:12:11.02
Decl. (J2000) = +18:42:23.1
with uncertainness of +/- 0".4 in both coordinates,
and brightness of R = 22.39 +/- 0.16.
This object is presented in the PanSTARRS DR1 images and catalog.
The field was calibrated with nearby stars from the PanSTARRS DR1
catalog and not corrected for Galactic extinction.
RA DEC r err R err
273.042629860 18.705792060 18.284 0.004 18.048 0.009
273.048977320 18.703099150 17.400 0.004 17.204 0.008
273.046825830 18.696310020 19.130 0.017 18.940 0.017
GCN Circular 41651
Subject
EP250901a: refined analysis of the EP-WXT and EP-FXT observations
Date
2025-09-02T04:26:12Z (11 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Y. J. Song, T. Zhao, J. W. Hu, C. L. Guo, Z. X. Ling (NAO, CAS), on behalf of the Einstein Probe (EP) team:
The X-ray transient EP250901a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Song et al., GCN 41627), with several multi-wavelength follow-up observations (Lipunov et al., GCN 41629; Liu et al., GCN 41631; Evans et al., GCN 41644). The refined WXT data shows that the event started at T0=2025-09-01T12:02:50 (UTC) and lasted for about 150 s before the interruption of the autonomous follow-up. It has a peak flux of ~1 x 10^-9 erg/s/cm^2 in the 0.5-4 keV band. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 1.1 x 10^21 cm^-2 and a photon index of 1.02 (-0.79/+0.84). The derived average unabsorbed 0.5-4 keV flux is around 5.05 (-1.54/+2.40) x 10^(-10) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 250s after T0. On-ground analysis of the FXT data found an fading source at R.A. = 273.0416, DEC = 18.7052 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), within the WXT error circle. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 3.0(-0.06/+0.07) x 10^21 cm^-2 and a photon index of 2.01(-0.15/+0.17). The derived average unabsorbed 0.5-10 keV flux is around 6.03 (-0.27/+0.43) x 10^(-12) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. A historical ROSAT detection (1RXS J181210.7+184228) lies 10.1 arcsec away from the FXT position, with a flux of 3 x 10^(-13) erg/s/cm^2 at 0.5-4 keV by assumimg a power-law spectrum with NH=3x10^20 cm^-2 and photon index =1.7.
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 41644
Subject
EP250901a: Swift-XRT observations
Date
2025-09-01T22:18:32Z (11 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi (INAF-IASFPA) , A.P.
Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N.
Burrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-OAR), A. D'Ai (INAF-IASFPA),
P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), M. Ferro
(INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T. Sbarrato
(INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf of the
Swift-XRT team:
Swift-XRT has performed follow-up observations of the Einstein Probe/WXT-detected
source EP250901a, collecting 2.0 ks of Photon Counting (PC) mode data between T0+6.2
ks and T0+12 ks after the trigger. We have detected 1 source. This has been
automatically classified as an known X-ray source.
Known X-ray sources
-------------------
Source 1 (SWIFT J181210.9+184216):
==================================
RA (J2000.0): 273.0457 = 18 12 10.97
Dec (J2000.0): +18.7047 = +18 42 16.9
Error: 5.5 (arcsec, radius, 90% confidence).
Detect flag: GOOD
Distance: 2.1 arcmin from the Einstein Probe/WXT position.
Mean rate: 0.0570 +/- 0.0097 ct s^-1
Mean flux: (2.06 +/- 0.35)e-12 erg cm^-2 s^-1
Peak rate: 0.061 +/- 0.015 ct s^-1
Peak flux: (2.21 +/- 0.54)e-12 erg cm^-2 s^-1
ECF: 3.62e-11 erg cm^-2 ct^-1
assuming NH=4.95e+21 cm^-2, gamma=2.27
determined from a spectral fit.
This matches a catalogued X-ray source 1RXS J181210.7+184228
in the ROSAT/RASSFSC catalogue. Details:
Separation: 12.4" from the XRT source
Cat Rate: 2.0e-02 +/- 8.7e-03 ct s^-1
Cat Flux: 6.7e-13 +/- 2.8e-13 erg cm^-2 s^-1 (0.3-10 keV)
so the source is 2.5-sigma above the catalogued flux.
The source may be fading, at the 0.9-sigma level.
A SIMBAD object `1RXS J181210.7+184228' is 12.4" away.
There are 3 2MASS objects within the source's 3-sigma error radius.
All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with
catalogues and upper limits from other missions, we assumed a power-law spectrum
with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated.
The results of the XRT-team automatic analysis of the XRT observations, including a
position-specific upper limit calculator, are available at
https://www.swift.ac.uk/EP/EP_FIELD00067.
This circular is an official product of the Swift-XRT team.
GCN Circular 41631
Subject
EP250901a: Mephisto optical upper limits
Date
2025-09-01T18:00:45Z (11 days ago)
Edited On
2025-09-02T16:18:09Z (10 days ago)
From
liuxk@ynu.edu.cn
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov>
Via
Web form
Xiangkun Liu, Chenxu Liu, Guowang Du, Xufeng Zhu, Yuan Fang, Xingzhu Zou, Shuai Xie, Xiaowei Liu (all SWIFAR, YNU), and Chao Wu (NAOC) report on behalf of the Mephisto Team:
The field of EP250901a, detected by EP-WXT (ID: 00109201113, triggered at 2025-09-01T12:05:03 UTC) and subsequently followed up by EP-FXT (GCN 41627), 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 13:30:37 2025-09-01 UT (~1.5 hr after the trigger) and 6 frames with 45s exposure times were taken. No new optical counterpart was detected (consistent with the upper limits reported by Global MASTER-Net, GCN 41629) in the stacked images of uvgr bands. The preliminary photometry and 3 sigma upper limits are listed below.
Start_Time(UT) | Band | Exp(s) | LimMag (AB)
--------------------|------|--------|------------
2025-09-01T13:30:37 | u | 45*6 | >21.68
2025-09-01T13:30:37 | v | 45*6 | >21.77
2025-09-01T13:37:13 | g | 45*6 | >22.17
2025-09-01T13:37:13 | r | 45*6 | >22.31
-------------------------------------------------------------------------------------------------
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.
-------------------------------------------------------------------------------------------------
GCN Circular 41629
Subject
EP250901a: Global MASTER-Net observations report
Date
2025-09-01T17:18:37Z (11 days ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko,
G.Antipov, A.Sankovich, Yu.Tselik, 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.Sosnovskij (CrAO),
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-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) was pointed to the EP250901a ( EP Team et al., GCN 41627) errorbox 11710 sec after notice time and 17745 sec after trigger time at 2025-09-01 17:00:48 UT, with upper limit up to 17.5 mag. Observations started at twilight. The observations began at zenith distance = 53 deg. The sun altitude is -9.0 deg.
The galactic latitude b = 16 deg., longitude l = 46 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2981475
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
17775 | 2025-09-01 17:00:48 | MASTER-SAAO | (18h 08m 55.12s , +18d 41m 44.3s) | C | 60 | 16.8 |
17776 | 2025-09-01 17:00:48 | MASTER-SAAO | (18h 10m 43.80s , +18d 26m 21.7s) | C | 60 | 15.2 |
18076 | 2025-09-01 17:05:48 | MASTER-SAAO | (18h 09m 00.19s , +18d 41m 53.8s) | C | 60 | 17.1 |
18076 | 2025-09-01 17:05:48 | MASTER-SAAO | (18h 10m 48.74s , +18d 26m 31.7s) | C | 60 | 16.3 |
18369 | 2025-09-01 17:10:42 | MASTER-SAAO | (18h 10m 47.53s , +18d 26m 45.9s) | C | 60 | 16.7 |
18369 | 2025-09-01 17:10:42 | MASTER-SAAO | (18h 08m 58.87s , +18d 42m 08.5s) | C | 60 | 17.5 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 41627
Subject
EP250901a: Einstein Probe detection of an X-ray transient
Date
2025-09-01T13:43:28Z (11 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Y. J. Song, T. Zhao, J. W. Hu, C. L. Guo, Z. X. Ling (NAO, CAS), on behalf of the Einstein Probe (EP) 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 EP250901a. The transient triggered EP-WXT (ID: 00109201113) at 2025-09-01T12:05:03 (UTC). The WXT position of the source is R.A. = 273.025 deg, DEC = 18.676 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The estimated WXT flux is around 4e-10 erg/s/cm^2 in 0.5-4.0 keV.
A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an X-ray source was detected around 3 minutes later at R.A. = 273.0416 deg, DEC = 18.7062 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). The estimated FXT flux is around 2e-11 erg/s/cm^2 in 0.5-10.0 keV. We note that a historical ROSAT source (1RXS J181210.7+184228) positioned just 12 arcseconds from our FXT detection coordinates, with a flux about three orders of magnitude lower than the present WXT measurement.
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).