Gamma-ray Burst 260306A = EP260306a

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• GCN Circular #43929
  Wenxiong Li, Runduo Liang (NAOC), Iair Arcavi, Ido Keinan (TAU), David Sand (U of Arizona) We observed the position of the EP/WXT trigger 01709258759 with a Las Cumbres 1m telescope at McDonald Observatory, Texas, 20 mins after the Einstein Probe WXT trigger. We took 2x300s exposures in the broad optical w band. We find an uncataloged source at RA=145.9490, Dec=15.6937 within the EP/WXT error circle and measure the following preliminary photometry calibrated to the r band: MJD 61105.08 Mag 21.6
• GCN Circular #43930
  G.L. Huang, Z.X. Li (IHEP), H. Zhou (PMO, CAS) and 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 EP260306a. The transient triggered EP-WXT (ID: 01709258759) at 2026-03-06T01:29:20 (UTC).
  The WXT position of the source is R.A. = 145.955 deg, DEC = 15.685 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. = 145.9478 deg, DEC = 15.6963 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).
  The location of the reported optical counterpart (GCN 43929) is consistent with the location of the uncatalogued X-ray source detected by FXT.
  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 #43931
  Noémie Globus (UNAM), Benjamin Schneider (LAM), Rosa L. Becerra (UNAM), Alan M. Watson (UNAM), Camila Angulo (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Asuka Kuwata (UNAM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM) and Antonio de Ugarte Postigo (LAM) report:
  We imaged the field of EP260306a (Huang et al., GCN Circ. 43930) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-03-06 02:41 to 03:34 UTC (from 1.20 to 2.10 hours after the trigger) and obtained 23, 12 and 39 minutes of simultaneous exposure in the r, i, and z filters.
  The data were reduced, coadded, calibrated, and analysed with the COLIBRÍ ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
  We detect the optical counterpart reported by Li et al. (GCN Circ. 43929) at preliminary magnitudes of:
  r = 23.27 +/- 0.25,
  z = 21.58 +/- 0.12.
  Comparing our observations with those reported by Li et al. (GCN Circ. 43929), the transient has faded, and we estimate a temporal decay index alpha ≈ 1.
  A preliminary fit to the SED suggests a high-z event, with a tentative photometric redshift between z=4 and z=5, but cannot rule out an intrinsically redder source at a lower redshift. We encourage spectroscopy.
  We notice that at the position of the optical counterpart, there is a faint galaxy with g = 24.91 +/- 0.28, r = 23.90 +/- 0.17, i = 23.54 +/- 0.24, z = 23.30 +/- 0.26, and photo-z = 0.72 +/- 0.27 in the Legacy Survey catalog (Dey et al., 2019).
  Furthermore, at a distance of 17 arcsec from the source, there is a nearby (photo-z = 0.06 +/- 0.01) bright galaxy with g = 18.04 +/- 0.01, r = 17.34 +/- 0.01, i = 17.02 +/- 0.01, and z = 16.76 +/- 0.01, with a chance coincidence probability Pcc~2%. Both of these galaxies may be unrelated if the event is located at high redshift.
  Additional observations in g, r, i, z bands and analysis are still ongoing.
  We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.
  COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.
• GCN Circular #43933
  Y. N. Ma, H. L. Li, Z. H. Yao, Y. L. Qiu, C. Wu, L. P. Xin, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. R. Xu, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team.
  SVOM/VT performed a Target of Opportunity observation of EP260306a detected by EP/WXT (Huang et al., GCN 43930). SVOM/VT began observing the field at 2026-03-06T03:45:23 UTC, 2.268 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
  With X-band data available, the optical counterpart (Li et al., GCN 43929; Globus et al., GCN 43931) was detected in both VT_B and VT_R bands. The AB magnitudes are:

  mid time (h) | exposure time (s) | band | mag (AB) 
  -------------|-------------------|------|--------------
      2.608    |      48*50        | VT_B |  23.1+-0.2
      2.642    |      53*50        | VT_R |  21.7+-0.1 
  

  Our photometry was 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 #43934
  A. J. Levan (Radboud), P. G. Jonker (Radboud), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), F. E. Bauer (UTa), J. Chácon (PUC), L. Cotter (UCD), R. A. J. Eyles-Ferris (Leicester), A. Martin-Carrillo (UCD), J. Quirola-Vasquez (Radboud), report on behalf of a larger collaboration:
  We observed the optical counterpart (Li et al., GCN 43929; Globus et al., GCN 43931; Ma et al., GCN 43933) of the fast X-ray transient EP260306a (Huang et al., GCN 43930) using the Gemini-North telescope equipped with the GMOS-N spectrograph.
  Four exposures of 600 s each were obtained, using grating R400, a slit width of 1 arcsec, and covering the wavelength range 5200-9900 Å. The observation mid time was 2026 March 6.489 UT (10.2 hr after the EP/WXT trigger).
  Continuum is significantly detected over the red part of the spectrum, down to ~7020 Å, where a spectral break is apparent. This is due to a combination of the H I damping wing and the onset of the Lyman forest at z ~ 4.77. A number of absorption features, including fine-structure lines, are clearly visible, which we interpret as Si II, Si II*, C II, Si IV, C IV, Fe II, Al II, all at a common redshift z = 4.773, establishing it as the redshift of EP260306a.
  We note that, given their colors and g-band detection, the two nearby objects noted by Globus et al. (GCN 43931) are both in the foreground of the transient.
  We acknowledge the excellent support from the Gemini North staff in the execution of this challenging observation.
• GCN Circular #43937
  G.L. Huang, Z.X. Li (IHEP), H. Zhou (PMO, CAS) and Z.-X. Ling (NAO, CAS) on behalf of the Einstein Probe (EP) team:
  The fast X-ray transient EP260306a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Huang et al., GCN 43930). The refined analysis of the WXT data shows that the event started at T0=2026-03-06T 01:28:20(UTC), and lasted for approximately 60 seconds, after which the WXT light curve was interrupted due to the autonomous follow-up observation. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density (NH) of 3.24×10^20 cm^-2, a photon index of 4.01(-2.04/+3.04) and an additional absorber (z=0) of NH=6.32(-4.37/+6.21)×10^21cm^-2 to represent the comprehensive absorption of the host galaxy (z=4.773, Levan et al., GCN 43934) and the foreground galaxy (z=0.72, Globus et al., GCN 43931). The derived average unabsorbed 0.5-4 keV flux is 3.98×(-2.97/+69.04)×10^(-9) erg/s/cm^2.
  The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2026-03-06T01:34:41(UTC, T0+261s). The exposure time of this observation is 4010s. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 145.9497, DEC = 15.6942 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the WXT position and position of the optical counterpart (Li et al, GCN 43929; Globus et al., GCN 43931; Ma et al., GCN 43933; Levan et al., GCN 43934). The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 3.24×10^20 cm^-2, a photon index of 2.36(-0.38/+0.41) and an additional absorber of NH=1.53(-1.11/+1.19)×10^21 cm^-2. The derived average unabsorbed 0.5-10 keV flux is 1.37(-0.16/+0.20)×10^(-12) erg/s/cm^2. All uncertainties are reported at the 90% confidence interval.
  Further FXT follow-up observations have been arranged.
  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 #43959
  A. Volnova (IKI), E. Klunko (ISTP), N. Pankov (IKI), A. Pozanenko (IKI) report on behalf of IKI-GRB-FuN:
  We observed the field of the fast X-ray transient EP260306a (Huang et al., GCNs 43930, 43937) using the AZT-33IK 1.5m telescope of the Mondy observatory on March 7, 8, and 9 taking several 120-seconds frames in the R band. In the stacked frame of the three epochs we do not detect the optical counterpart reported previously (Li et al., GCN 43929; Globus et al., GCN 43931; Ma et al., GCN 43933; Levan et al., GCN 43934). Preliminary photometry of the galaxy and observational details are the following:

  Date        UT start  t-T0      Exp.    Filter Obj.   Err.  UL      Site/Telescope
                       (mid,days) (n*s)                      (3sigma)
  2026-03-07  15:44:24  1.61672   33*120  R      n/d    n/d   22.8    Mondy/AZT-33IK
  2026-03-08  14:30:42  2.57040   40*120  R      n/d    n/d   22.1    Mondy/AZT-33IK
  2026-03-09  16:24:33  3.64947   40*120  R      n/d    n/d   23.2    Mondy/AZT-33IK
  

  The photometry is based on nearby USNO-B1.0 stars (R2-magnitudes) and is not corrected for Galactic extinction.