Gamma-ray Burst 250304A = EP 250304a

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• GCN Circular #39580
  W. Chen, W. X. Wang (NAO, CAS), Y. J. Zhang (THU), C. Y. Dai (NJU), Y.Liu (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 EP250304a. The transient triggered EP-WXT (ID: 01709132237 ) at 2025-03-04 01:32:30 (UTC). The WXT position of the source is R.A. = 208.394 deg, DEC = -42.818 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. = 208.3965 deg, DEC = -42.8067 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 #39581
  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 EP250304a ( EP Team et al., GCN 39580) errorbox 148 sec after notice time and 4018 sec after trigger time at 2025-03-04 02:39:28 UT, with upper limit up to 19.4 mag. The observations began at zenith distance = 59 deg. The sun altitude is -41.7 deg.
  The galactic latitude b = 18 deg., longitude l = 315 deg.
  Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2799155
  We obtain a following upper limits.

  Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
  _________|_____________________|_____________________|____________________________________|_____|_______|_______|________
  
      4049 | 2025-03-04 02:39:28 |         MASTER-OAFA | (13h 52m 43.16s , -42d 51m 39.4s) |   C |    60 | 19.4 |        
  

  Filter C is a clear (unfiltred) band.
  The observation and reduction will continue. The message may be cited.
• GCN Circular #39583
  X. Liu, J. An, N.C. Sun (NAOC), S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), S.Q. Jiang, S.Y. Fu, Z.P. Zhu, D. Xu, Z. Fan, W.X. Li, Y.N. Wang (NAOC) report on behalf of a large collaboration:
  We observed the field of EP250304a (Chen et al, GCN 39580), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Cerro Tololo Inter-American Observatory, Chile. Observations started at 02:18:49 UTC on 2025-03-04, i.e., ~1.12 hr after the EP/WXT trigger. A series of B-/V-/R-/I- band frames were obtained.
  An uncatalogued and varying optical source is detected within the EP/FXT error circle (Chen et al, GCN 39580) at coordinates

  R.A. (J2000) = 13:53:34.68
  Dec. (J2000) = -42:48:16.71
  

  with an uncertainty of ~ 1 arcsec.
  Preliminary photometry shows that the source has B = 20.5 +/- 0.1 mag at 1.39 hr post-trigger, and R = 20.8 +/- 0.1 mag at 2.4 hr post-trigger, calibrated with SkyMapper DR2 catalog converted using Lupton (2005) equations and not corrected for Galactic extinction.
  We thus think this source is the optical counterpart of EP250304a.
• GCN Circular #39584
  K.L. Page, P.A. Evans (U.Leicester) and J. DeLaunay (PSU) report on behalf of the Swift XRT Team:
  On 2025 March 04 at 02:59 UT, Swift started observing EP250304a, 5.2 ks after the Einstein Probe trigger (GCN Circ. 39580). A fading X-ray source was identified, at a position of RA, Dec = 208.39408, -42.8046, which is equivalent to

  RA (J2000): 13h 53m 34.58s
  Dec (J2000): -42d 48′ 16.7″
  

  with an uncertainty of 4.1 arcsec (radius, 90% confidence). This is consistent with the FXT position (GCN Circ. 39580), as well as the TRT optical counterpart given in GCN Circ. 39583.
  The mean observed X-ray flux in the first snapshot was (3.5 +/- 0.4) x 10^-12 erg cm^-2 s^-1 (0.3 - 10 keV). By the time of the second snapshot, at 10.3 ks after the trigger, the source was no longer detected, having faded to a 3-sigma upper limit of 2.1 x 10^-12 erg cm^-2 s^-1.
• GCN Circular #39585
  A. Saccardi (CEA/Irfu), Z. P. Zhu (NAOC), B. Schneider (LAM), D. Xu (NAOC), L. Izzo (INAF/OAC and DARK/NBI), A. J. Levan (Radboud and Warwick), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), N. R. Tanvir (U. Leicester), S. D. Vergani (CNRS - Paris Observatory/LUX) report on behalf of the Stargate collaboration:
  We observed the optical counterpart (Liu et al., GCN 39583; see also Page et al., GCN 39584) of the fast X-ray transient EP250304a (Chen et al., GCN 39580) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph.
  In a 30-s r-band image taken with the acquisition camera (mid-time 2025 Mar 4.302 UT, or 5.7 hr after trigger), we detect the optical counterpart with a magnitude r = 20.97 +/- 0.03 (AB), calibrated against nearby stars from the SkyMapper catalog (Wolf et al. 2018, doi:10.4225/41/593620ad5b574).
  Our spectra cover the wavelength range 3000-21000 AA, and consist of 4 exposures of 600 s each. The observation mid time was 2025 Mar 4.3248 UT (6.254 hr after the GRB).
  In a preliminary reduction of the spectra, we detect continuum over the entire covered wavelength range. We detect emission lines in the VIS arm, which we identify as Halpha and [O III] 5007 AA at the common redshift of z = 0.200. We also identify the Mg II doublet (2796,2804) in absorption at a consistent redshift.
  The spectrum continues to rise towards the blue end down to the cut-off of the UVB arm (~3100 AA). This spectral shape is qualitatively similar to what was seen in EP250108a / AT2025kg (e.g., Zhu et al., GCN 38908). We encourage further follow-up of this potentially interesting target.
  We acknowledge expert support from the ESO staff in Paranal, in particular Diego Parraguez.
• GCN Circular #39587
  S. P. R. Shilling (Lancaster U.) reports on behalf of the Swift UVOT team:
  Swift/UVOT observed the field of EP250304a (Chen et al., GCN 39580) for a total of 2 ks in the U-band starting at 02:48:13 UT, 1.25 hours after the detection by Einstein Probe WXT.
  An optical source consistent with the EP-FXT position (Chen et al., GCN 39580) is detected and appears to be rising in brightness. The preliminary detection magnitudes reported below are calculated using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373). The magnitude at the start of the observation is 19.29 +/- 0.09 (1.25 hours post trigger) and rises to 18.91 +/- 0.15 (~2.75 hours post trigger).
  We note that an X-ray source at this position was independently observed by the Swift/XRT (Page et al., GCN 39584). We also note that, in addition to the Swift/UVOT detection, an optical counterpart at this position has been detected by the TRT (Liu et al., GCN 39583) but has not been detected by MASTER (Lipunov et al., GCN 39581). Finally, that the source has a spectroscopic redshift, observed with the VLT/X-shooter, of z=0.2 (Saccardi et al., GCN 39585).
• GCN Circular #39589
  R. Brivio, M. Ferro (INAF-OAB), D. B. Malesani (DAWN/NBI and Radboud), P. D’Avanzo, S. Covino, D. Fugazza (INAF-OAB) report on behalf of the REM team:
  We observed the field of EP250304a (Chen et al., GCN 39580) with the REM 60 cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the J, H, and K bands, started on 2025 March 04 at 01:46:18 UT (i.e. 0.23 hr after the burst), and lasted for about 2 hours.
  From preliminary analysis, we do not find any NIR counterpart at the position of the reported optical afterglow (Liu et al., GCN 39583; Saccardi et al., GCN 39585) down to the following 3sigma limit:
  J > 17.5 (Vega; calibrated against the 2MASS catalogue),
  at a mid-time of 1.46 hours after the trigger.
• GCN Circular #39591
  Y.J. Zhang (THU), C. Y. Dai (NJU), W. Chen, W. X. Wang (NAO, CAS), Y. Liu (NAO, CAS) report on behalf of the Einstein Probe team:
  The X-ray transient EP250304a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Chen et al., GCN 39580), and followed up by several telescopes (Xu et al, GCN 39583, Page et al., GCN 39584, Saccardi et al., GCN 39585, Shilling et al., 39587), with an optical counterpart detected at a redshift of 0.200 (Saccardi et al., GCN 39585). Refined analysis of the WXT data shows that the event started at T0=2025-03-04T01:29:49 (UTC) and lasted for about 1200 s. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 5 x 10^20 cm^-2 and a photon index of 2.2 (-/+0.1). The derived average unabsorbed 0.5-4 keV flux is 5.3(-/+0.4) x 10^(-10) erg/s/cm^2.
  The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 200 seconds after T0. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 208.3953, DEC = -42.8050 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent positionally with the WXT transient. The FXT observation suffered from significant pile up at the begining of the observation. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 5 x 10^20 cm^-2 and a photon index of 2.11 (-/+0.05). The derived average unabsorbed 0.5-10 keV flux is 1.75 (-/+0.06) x 10^(-10) erg/s/cm^2 during the time interval from 500 to 1200 seconds after the start of the observation. Further FXT observation started about 4000 seconds after the trigger and showed an average unabsorbed 0.5-10 keV flux of 6.5 (-2.3/+3.5) x 10^(-12) erg/s/cm^2 with an exposure time of about 300 seconds. 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 #39594
  Ankur Ghosh, Soebur Razzaque (CAPP, University of Johannesburg), Alexander Moskvitin, Yulia Sotnikova (SAO RAS), Naveen Dukiya (ARIES), Rahul Gupta (NASA GSFC) on behalf of a larger collaboration.
  We observed the field of the EP 250304A triggered by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Chen et al., GCN 39580) in r filter of the 1-meter Sinistro telescope at the Las Cumbres Observatory Global Telescope (LCOGT) node located at South African Astronomical Observatory (SAAO), South Africa. The 1-m Sinistro telescope is equipped with a 4K x 4K CCD (FOV: 26 x 26 arcmin, scale: 0.39 arcsec/pixel). Observations began on March 04, 2025, starting 19.80 hours after the GRB trigger.
  We clearly detect the optical transient (OT) reported by GCNs (Xu et al, GCN 39583, Page et al., GCN 39584, Saccardi et al., GCN 39585, Shilling et al., 39587) in our r band image.

  |Date|		|UTstart|	|t-T0 (hours)|	|Exp (sec)|	|Filter|	|Magnitude| 
  --------------------------------------------------------------------------------------------------      
  2025-03-04	21:20:52.80	19.80		1 x 1000 	r		r = 20.00 +/- 0.08
  

  The field was calibrated against nearby APASS stars, with magnitudes converted using Lupton (2005) equations, and has not been corrected for Galactic extinction.
• GCN Circular #39600
  M. E. Ravasio (Radboud Univ.), E. Burns (LSU), and P.G. Jonker (Radboud Univ.) report on behalf of the Fermi-GBM Team:
  At the starting time T0=2025-03-04T01:29:49 UTC of the EP-WXT event EP250304A (Zhang et al., GCN 39591), Fermi was in South Atlantic Anomaly (SAA). Fermi-GBM had exited SAA approximately 50 seconds after T0. There was no Fermi-GBM onboard trigger around EP-WXT time.
  The GBM targeted search [1], the most sensitive, coherent search for GRB-like signals, was run in the time interval [T0-50;T0+500] s, seeking signals between 64 ms and 32.768 s in duration. No signal consistent with the EP transient both temporally and spatially is identified, as confirmed by visual inspection of the data.
  Assuming a “soft” spectral template (Band function with Epeak = 70 keV, alpha = -1.9, beta = -3.7), and a duration of 8.192 s, we derive a sky-averaged flux upper limit of 3.9e-08 erg/cm2/s in the energy band 10-1000 keV.
  [1] Goldstein et al. 2019 arXiv:1903.12597
• GCN Circular #39615
  F. Carotenuto (INAF/OAR), J. Bright (University of Oxford), P. G. Jonker (Radboud University) report on behalf of a larger collaboration:
  We observed the Fast X-ray Transient EP250304a (Chen et al., GCN 39580) with the MeerKAT radio telescope at 3.0 GHz for a total of 1 hour starting on March 7th 2024 at 04:36 UTC. J1939-6342 and PKS 1320-446 were used as flux and complex gain calibrators, respectively. Using the South African Radio Astronomy Observatory Science Data Pipeline image, we find an unresolved source at the position of the optical counterpart of the FXT (Liu et al., GCN 39583, Saccardi et al., GCN 39585) with a flux density of ~100 uJy/beam. The rms noise in the field is 6.5 uJy/beam. Further MeerKAT observations are planned.
  We thank the staff at the South African Radio Astronomy Observatory for the rapid scheduling of these observations.
• GCN Circular #39669
  R. Margutti (UC Berkeley), S. Pedrami (UC Berkeley), R. Chornock (UC Berkeley), Y. Yao (UC Berkeley), T. Laskar (U of Utah) report:
  We observed EP250304a (Chen et al., GCN 39580) with the Chandra X-ray Observatory starting on 2025-03-10, 03:13:38 UT (i.e., T0+6.2d, observer frame; 20ks exposure) under a DDT proposal (PI Margutti). An X-ray source is detected with high confidence (>8 sigma Gaussian equivalent) at a location consistent with the EP transient (Zhang et al., GCN 39591). The spectrum is well fitted by a simple power law with photon index Gamma = 2.1 +/- 0.5, similar to previous Swift-XRT observations of this source (Page et al., GCN 39584). For a Gamma=2 index, the inferred 0.3-10 keV flux is ~1.6e-14 erg/s/cm2. These Chandra observations imply that the source has entered a phase of shallower X-ray flux decay compared with the earlier Swift-XRT observations, in close similarity with low-luminosity GRBs. In the soft X-rays, the evolution of EP250304a closely resembles that of the prototypical event GRB060218 (e.g., Campana et al., 2006).
  We thank the entire CXO team for scheduling these very fast observations, which enabled unique constraints on the later time evolution of the EP source.
• GCN Circular #39701
  R. C. Chen (NJU), L. Lan (NAOC), C. W. Wang, W. J. Tan (IHEP), L. P. Xin, Y. L. Qiu, H. L. Li, Z. H. Yao, Y. Xu, P. P. Zhang, W. J. Xie, J. Wang, X. H. Han, Y. J. Xiao, H. B. Cai, J. Y. Wei (NAOC), J. T. Palmerio (CEA/Irfu) report on behalf of the SVOM mission team.
  SVOM/VT conducted ToO follow-up observations of the X-ray transient EP250304a (Chen et al., GCN 39580) at the location of Swift-XRT (Page et al., GCN 39584) and Swift/UVOT (Shilling et al., GCN 39587) in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channel simultaneously.
  The optical counterpart of EP250304a (Trigger ID: 01709132237, Chen et al., GCN 39580; Liu et al., GCN 39583; Saccardi et al., GCN 39585; Shilling et al., GCN 39587) was clearly detected in VT_R and VT_B images.
  The brightness in AB magnitude was estimated to be:

  Mid time UTC | Band | Magnitude | Magnitude error
  2025-03-08T14:31:28.502 | VT_B | 21.59 | 0.07
  2025-03-08T14:31:28.502 | VT_R | 21.60 | 0.10
  2025-03-05T10:27:01.518 | VT_B | 21.35 | 0.06
  2025-03-05T10:27:01.518 | VT_R | 21.59 | 0.10
  

  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 #39709
  J. H. Gillanders (Oxford), M. Huber, K. C. Chambers (IfA, Univ. Hawaii), S. J. Smartt, K. W. Smith, S. Srivastav (Oxford), M. Nicholl, D. Young, M. Fulton (QUB), T.-W. Chen (NCU, Taiwan) A. S. B. Schultz, T. de Boer, J. Fairlamb, G. Paek, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii).
  We observed the field of EP250304a (Chen et al., GCN 39580), using the Pan-STARRS telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560) on MJD 60745.55 (2025-03-11 13:12 UTC), approximately ~7.5 days after the EP-WXT detection (Chen et al., GCN 39580). The Pan-STARRS system consists of two 1.8m telescope units located at the summit of Haleakala on the Hawaiian island of Maui, employing an SDSS-like filter system denoted as grizy, and a broad w-filter, which is a composite of the gri-filters.
  Our observation consisted of 2x300s exposures in both the r and i filters. The images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target stacked images in the i-band only (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4). At this declination (Dec = -42), we possess a proprietary i-band reference stack with a total exposure time of 1170s. We also possess an r-band reference stack, but this has just 90s total exposure time.
  From these difference images, we measure an i-band AB magnitude, i~21.4. Previously reported r/R-band magnitudes by Liu et al., (GCN 39583; R = 20.8 +/- 0.1 at 2.4 hours post-burst) and Saccardi et al., (GCN 39585; r = 20.97 +/- 0.03 at 5.7 hours post-burst) indicate that the optical transient has not faded substantially in the ~week since explosion. A faint source is visible in the r-band images, but with only a 90s reference stack we cannot determine if this corresponds to host or transient flux.
• GCN Circular #39851
  L. Izzo (INAF-OACN and DARK/NBI), A. Martin-Carrillo (UCD), D. B. Malesani (Radboud and DAWN/NBI), A. J. Levan (Radboud and Warwick), P. G.Jonker (Radboud Univ.), L. Cotter (UCD), J. van Dalen (Radboud Univ.), G. Corcoran (UCD), K. Wiersema (Herts), F. E. Bauer (PUC), report on behalf of a larger collaboration:
  We observed the optical counterpart (Liu et al., GCN 39583; Page et al., GCN 39584; Saccardi et al., GCN 39585; Shilling et al., GCN 39587; Gupta et al., GCN 39593; Ghosh et al., GCN 39594) of the fast X-ray transient EP250304a (Chen et al., GCN 39580) using the ESO VLT UT4 (Yepun) equipped with the MUSE spectrograph on 2025-03-22 at 02:45:04 UT (about 18.05 days after trigger, 15 days in the rest frame). A series of 4x470 s exposures were taken for this observation.
  From a preliminary reduction, the continuum is well detected over the wavelength range of 5000 to 9000 AA. Broad features, typical of broad-lined supernovae expanding at relativistic speeds, can be clearly identified, with SNID finding a good match with SN1998bw at 14 days after the explosion.
  Further observations are planned.
  We acknowledge the excellent support from the ESO staff in Paranal, in particular Thallis Pessi and Francisco Caceres.
• GCN Circular #40143
  Authors: Yuhan Yao, AJ Nayana, Eli Wiston, Raffaella Margutti, Ryan Chornock, Wenbin Lu (UC Berkeley), Elizabeth Mahony (CSIRO S&A, Marsfield), Tanmoy Laskar (U of Utah)
  We observed the Fast X-ray Transient (FXT) EP250304a discovered by Einstein Probe (Chen et al. GCN 39580; Zhang et al. GCN 39591) at the optical counterpart position (Liu et al., GCN 39583) with the Australia Telescope Compact Array at 5.5 and 9 GHz in continuum mode on 2025 March 21st 13:01-17:48 UT (i.e. 17.6 days after the EP trigger) under NAPA program C3623 (PI: Yao). At the redshift of z=0.2 (Saccardi et al., GCN 39585), EP250304a has been associated with a type Ic broad line (Ic-BL) supernova (Izzo et al. GCN 39851).
  At the optical transient position, we did not detect a radio source at 5.5 GHz. The rms noise in the final map is 15 microJy/beam. The 3-sigma luminosity limit at z=0.2 is Lnu < 5.2 x 10^{28} erg s^{-1} Hz^{-1}.
  Compared with known Ic-BL SNe with low-luminosity GRB counterparts, the radio upper limit of EP250304a is fainter than that of GRB171205a/SN2017iuk at a similar phase (Leung et al. 2021, MNRAS, 503, 1847), but consistent with those of GRB980425/SN1998bw (Kulkarni et al. 1998, Nature, 395, 663), GRB060218/SN2006aj (Soderberg et al. 2006, Nature, 442, 1014), and GRB100316D/SN2010bh (Margutti et al. 2013, ApJ, 778, 18).