Gamma-ray Burst 250903B = EP250903a

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• GCN Circular #41674
  T. Zhao, Y. J. Song(NAO, CAS), Y. J. Zhang(THU), H. Sun(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 EP250903a, which was also followed by GOTO (O'Neill et al., GCN 41672). The transient triggered EP-WXT (ID: 01709201159) at 2025-09-03T14:03:01 (UTC). The WXT position of the source is R.A. = 339.120 deg, DEC = -49.863 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 circle, an uncatalogued X-ray source was detected at R.A. = 339.1097 deg, DEC = -49.8572 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).
  There are several stars within the FXT error circle. The stellar flare origin for this event can not be fully ruled out at this time. 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 #41675
  J. An (NAOC), S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), S.Y. Fu (HUST), L.B. He, X. Liu, Z.P. Zhu, S.Q. Jiang, 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 EP250903a (Zhao et al., GCN 41674), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at New South Wales, Australia (SBO). Observations started at 14:14:31 UTC on 2025-09-03, i.e., ~ 11.5 min after the EP/WXT trigger and a series of frames in the R band were obtained.
  No uncatalogued source is detected in our stacked image within or at the border of the EP-FXT error circle, down to a 5-sigma limiting magnitude of R > 20.1 (Vega) at ~ 1.21 hr post-trigger, calibrated with SKYMAPPER DR2 catalog and not corrected for Galactic extinction.
  The two relatively bright catalogued sources/stars within or at the border of the EP-FXT error circle do not show variability in brightness. Our result is consistent with that by GOTO (O'Neill et al., GCN 41672).
  We note that there are faint galaxies within the EP-FXT error circle from Legacy Survey, so deep follow-ups are encouraged.
• GCN Circular #41676
  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 EP250903a ( EP Team et al., GCN 41674) errorbox 4803 sec after notice time and 10720 sec after trigger time at 2025-09-03 17:01:41 UT, with upper limit up to 15.9 mag. Observations started at twilight. The observations began at zenith distance = 60 deg. The sun altitude is -8.9 deg.
  The galactic latitude b = -56 deg., longitude l = 342 deg.
  Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2983979
  We obtain a following upper limits.

  Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
  _________|_____________________|_____________________|____________________________________|_____|_______|_______|________
  
     10751 | 2025-09-03 17:01:41 |         MASTER-SAAO | (22h 31m 47.53s , -49d 57m 38.2s) |   C |    60 | 15.9 |        
  

  Filter C is a clear (unfiltred) band.
  The observation and reduction will continue. The message may be cited.
• GCN Circular #41680
  A. J. Levan (Radboud and Warwick), J. A. Quirola-Vasquez (Radboud), D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), A. Martin-Carrillo (UCD), R. A. J. Eyles-Ferris (Leicester), P. G. Jonker (Radbound), F. E. Bauer (SSI and UTA), and J. Chacón (PUC) report on behalf of a larger collaboration:
  We observed the location of EP250903a (Zhao et al. GCN 41674) with the Sinistro instrument mounted on the 1-m telescopes at the LCO Siding Springs Observatory. 3 x 300 s exposures were obtained in the SDSS r filter at a start time of 14:44:15 UT on 2025-09-03 (~41 minutes post trigger).
  Within the WXT and FXT error circles we locate a new source in our stacked r-band image with r~21 mag (AB) calibrated against nearby SkyMapper stars and not corrected by Galactic extinction, at a location of

  RA(J2000): 22:36:27.75
  DEC(J2000): -49:51:25.6
  

  The error is about 0.5” in each coordinate. We note the transient is offset ~1” from a Legacy survey object with magnitude r= 21.49 mag which has a photometric redshift of 0.24 +/- 0.11. In the legacy images the source appears to extend underneath the new source. Thus, our photometry likely includes some contribution from the host galaxy.
• GCN Circular #41683
  L. P. Xin, H. L. Li, Y. N. Ma, Y. L. Qiu, C. Wu, Z. H. Yao, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team.
  SVOM performed a Target of Opportunity observations of EP250903a detected by EP/WXT (Zhao et al., GCN 41674). The observations was started from 2025-09-03T15:01:54 UTC, 58 min after the EP trigger time, in VT_B (400-650 nm) and VT_R (650-1000nm) bands simultaneously. .
  The optical source reported (Levan et al., GCN 41680) was clear detected in VT stacked images. The brightness was VT_B=22.1+/-0.1 and VT_R=21.5+/-0.1 mag at the mid time of 1.1 hours after the trigger.
  Our photometry is not corrected for Galactic extinction.
  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.
• GCN Circular #41693
  Y. J. Zhang (THU), T. Zhao, Y. J. Song, H. Sun(NAO, CAS), on behalf of the Einstein Probe (EP) team:
  The X-ray transient EP250903a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Zhao et al., GCN 41674), with several multi-wavelength follow-up observations (O'Neill et al., GCN 41672; An et al., GCN 41675; Lipunov et al., GCN 41676; Levan et al., GCN 41680; Xin et al., GCN 41683). The refined WXT data shows that the event started at T0=2025-09-03T13:59:40 (UTC) and lasted for about 200 s before the interruption of the autonomous follow-up. WXT detected the source during its rising phase with a peak flux of 6 x 10^(-10) erg/s/cm^2. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 1.0 x 10^20 cm^-2 and a photon index of 1.6 (+/-0.6). The derived average unabsorbed 0.5-4 keV flux is around 1.0 (-0.5/+1.1) x 10^(-10) erg/s/cm^2.
  The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 428s after T0. On-ground analysis of the FXT data found an fading source at R.A. = 339.1101, DEC = -49.8541 (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 1.0 x 10^20 cm^-2 and a photon index of 2.6(+/-0.1). The derived average unabsorbed 0.5-10 keV flux is around 4.3(+/-0.3) x 10^(-12) 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 #41696
  A. J. Levan (Radboud and Warwick), J. A. Quirola-Vasquez (Radboud), D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), A. Martin-Carrillo (UCD), R. A. J. Eyles-Ferris (Leicester), P. G. Jonker (Radbound), F. E. Bauer (SSI and UTA), and J. Chacón (PUC) report on behalf of a larger collaboration:
  We observed again the field of the fast X-ray transient EP250903a (Zhao et al., GCN 41674). Our first observation was carried out with LCO/Sinistro (3x300 s exposure, mean epoch 0.68 hr after the trigger) and was reported by Levan et al. (GCN 41680). We obtained a second, deeper observation with LCO/Sinistro (9x300 s exposure, mean epoch 3.79 hr after the trigger), and a follow-up observation with Gemini/GMOS-S (4x60 s exposure, mean epoch 19.38 hr after the trigger).
  Besides, a more accurate X-ray localization of EP250903a is now available, thanks to both Swift/XRT (https://www.swift.ac.uk/EP/) and EP/FXT (Zhang et al., GCN 41693).
  The candidate afterglow proposed by Levan et al. (GCN 41680) is outside both the XRT and FXT error circles. Furthermore, image subtraction with respect to the Legacy Survey leaves no detectable residual. The magnitudes of this object reported by Xin et al. (GCN 41683) using SVOM/VT are also consistent with the Legacy Survey archival values (r = 21.49, g = 21.95). Given the low S/N of the original detection, the most likely explanation is that there was no flux excess, and we hereby retract this afterglow candidate.
  Inspection of the second (deeper) LCO epoch and Gemini images reveals however an object not visible in the Legacy survey, at a position consistent with both the Swift/XRT and EP/FXT X-ray localization, at coordinates:

  RA(J2000) = 22:36:27.02
  Dec(J2000) = -49:51:14.1
  

  The estimated error is 0.5". This object is seen to fade between the two observations, and is not detected in (deeper) images from the Legacy Survey. We conclude that this is the actual optical afterglow of EP250903a. We report for the transient the following r-band magnitudes, calibrated against the SkyMapper catalog and not corrected for Galactic extinction:

  | Time since trigger (hr) | Telescope | Magnitude (AB) |
  | ----------------------- | --------- | -------------- |
  | 0.68                    | LCO       | > 21.7         |
  | 3.79                    | LCO       | 23.2 +- 0.3    |
  | 19.38               	  | Gemini    | 24.3 +- 0.2    |
  | N/A                 	  | Legacy    | > 24.8         |
  

  We apologize for any inconvenience that our previous announcement may have caused.
  We acknowledge excellent support from the Gemini staff, in particular Roque Ruiz.
• GCN Circular #41698
  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 EP250903a, collecting 3.7 ks of Photon Counting (PC) mode data between T0+1.3 ks and T0+35 ks after the trigger. We have detected 1 source inside the EP/WXT error region. This has been automatically classified as follows:

    * 0 likely counterparts
    * 0 candidate counterparts
    * 1 uncatalogued X-ray source
    * 0 known X-ray sources
  

  Uncatalogued X-ray sources
  --------------------------
  Source 2 (SWIFT J223627.0-495113):
  ==================================

      RA (J2000.0):   339.1126  =  22 36 27.02
      Dec (J2000.0):  -49.8538  =  -49 51 13.7
      Error:	    7.1 (arcsec, radius, 90% confidence).
      Detect flag:    GOOD
      Distance:	    37 arcsec from the Einstein Probe/WXT position.
      Mean rate:	    (3.8 [+1.4, -1.2])e-3 ct s^-1
      Mean flux:	    (2.20 [+0.83, -0.67])e-13 erg cm^-2 s^-1
      Peak rate:	    (3.8 [+1.4, -1.2])e-3 ct s^-1
      Peak flux:	    (2.20 [+0.83, -0.67])e-13 erg cm^-2 s^-1
      ECF:	    5.78e-11 erg cm^-2 ct^-1
  		      assuming NH=1.17e+20 cm^-2, gamma=1.15
  		      determined from a spectral fit.
      XMM UL:	    1.9e-12 erg cm^-2 s^-1, (0.3-10 keV)
  		      so the source is not above this 3-sigma upper limit.
      There is no evidence for fading.
      There are 2 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 position of Source 2 is consistent with the optical counterpart detected by LCO (GCN 41680) and SVOM/VT (GCN 41683).
  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_FIELD00068.
  This circular is an official product of the Swift-XRT team.