GRB 260618A — All Circulars

C. Malacaria (INAF-OAR) and R. Sonawane (IISER, TVM) report 
on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 06:45:34.28 UT on 18 June 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 260618A (trigger 803457939/260618282).
Final Real-time localization was reported in GCN 44973.
A possible afterglow is  detected by EP (Yang et al., GCN 44978, 
Jiang et al., GCN 44991).

The GBM light curve consists of a variable emission episode with a duration (T90)
of about 140 s (50-300 keV). The time-averaged spectrum
from T0-17 to T0+150 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.4 +/- 0.1 and the cutoff energy, 
parameterized as Epeak, is 43 +/- 2 keV.

The event fluence (10-1000 keV) in this time interval is
(5.9 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+25 s in the 10-1000 keV band is 2.7 +/- 0.3 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html

For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"

L.B. He, S.Q. Jiang, J. An, X. Liu, Z.P. Zhu, D. Xu (NAOC), S.Y. Fu, A.D. Zhu, L. Lei (HUST), J.Z. Liu (XAO) report:

We observed the field of EP260618a detected by EP/WXT (Yang et al., GCN 44978; Jiang et al., GCN 44991), possibly associated with GRB 260618A (Fermi GBM Team, GCN 44973) using the JinShan 100C telescope located at Altay, Xinjiang, China.

The previously reported optical counterpart (Becerra et al., GCN 44988; Malesani et al., GCN 44990; Amram et al., GCN 44994) is detected in our stacked images with a brightness of r ~ 22.4 mag at a median time of 1.429 days since the EP trigger, i.e. 1.504 days since the Fermi GBM trigger, calibrated with Pan-STARRS DR2 stars in the field and not corrected for Galactic extinction.

L.B. He, S.Q. Jiang, J. An, X. Liu, Z.P. Zhu, D. Xu (NAOC), S.Y. Fu, A.D. Zhu, L. Lei (HUST), J.Z. Liu (XAO) report:

We observed the field of EP260618a detected by EP/WXT (Yang et al., GCN 44978; Jiang et al., GCN 44991), possibly associated with GRB 260618A (Fermi GBM Team, GCN 44973) using the JinShan 100C telescope located at Altay, Xinjiang, China.

The previously reported optical counterpart (Becerra et al., GCN 44988; Malesani et al., GCN 44990; Amram et al., GCN 44994) is detected in our stacked images with a brightness of r ~ 22.4 mag at a median time of 1.429 days since the EP trigger, i.e. 1.504 days since the Fermi GBM trigger, calibrated with Pan-STARRS DR2 stars in the field and not corrected for Galactic extinction.

S. Q. Jiang (NAO, CAS), J. Yang (ZZU), H. Sun (NAO, CAS) on behalf of the Einstein Probe (EP) team:

EP-FXT performed a follow-up observation of the X-ray transient EP260618a (Yang et al., GCN 44978), which is possibly associated with GRB 260618A (Fermi GBM team, GCN 44973). The observation started at 2026-06-18T18:09:47 (UTC), about 9.58 hours after the initial WXT detection, with an exposure time of 3 ks. An uncatalogued source was detected within the WXT error circle at R.A. = 276.6799, DEC = 23.8439 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT spectrum can be fitted with an absorbed powerlaw with a hydrogen column density fixed at the Galactic value of 1.32 x 10^21 cm^-2 and a photon index of 2.09 +/- 0.13. The derived unabsorbed 0.5-10 keV flux is 1.28 (+0.13, -0.10) x 10^-12 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

The position of the uncatalogued X-ray source is consistent with the optical counterpart (Becerra et al., GCN 44988; Malesani et al., GCN 44990). Further EP-FXT follow-up observations will be arranged.

Launched on 2024 January 9, Einstein Probe is a space-based X-ray observatory designed to monitor the dynamic X-ray sky and perform rapid follow-up observations of newly discovered transients (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

D. B. Malesani (DAWN/NBI and Radboud), J. A. Quirola-Vasquez (Radboud), J. Sanchez-Sierras (Radboud), G. Corcoran (UCD), A. J. Levan (Radboud and Warwick), A. Martin-Carrillo (UCD), P. G. Jonker (Radboud), J. Chacon (PUC), report on behalf of a larger collaboration:

We observed the field of the fast X-ray transient EP260618a (Yang  et al., GCN 44978), possibly the afterglow of GRB 260618A (Fermi GBM team, GCN 44973), using an LCO 1m telescope located at the Teide Observatory (Spain) equipped with the SINISTRO instrument. We obtained 6x300 s exposures in the SDSS-r filter starting on 2026-06-18 at 21:56:12 UT (13.56 hr after the trigger).

The object reported by Becerra et al. (GCN 44988) is detected in the stack of our images, with a magnitude of r = 21.93 +/- 0.16 (AB), calibrated against the Pan-STARRS catalogue and not corrected for Galactic extinction.

We note that our magnitude indicates little or no fading compared to the COLIBRÍ observation, which was carried out about 6.5 hr later (Becerra et al., GCN 44988). Further follow-up is encouraged.

Rosa L. Becerra (UNAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), 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), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), and Alan M. Watson (UNAM) report:

We imaged the field of the EP260618a (Yang et al., GCN Circ. 44978), possibly associated with GRB 260618A (Fermi GBM team GCN Circ. 44973), using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-06-19 03:44 to 04:55 UTC (from 19.15 to 20.34 hours after the trigger) and obtained 55 minutes of simultaneous exposure in the r and z filters.

The data were reduced and coadded with the ASU COLIBRÍ 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, which is particularly significant in this direction (A_r = 1.1 mag; Schlafly & Finkbeiner 2011) due to the proximity of the field to the Galactic plane.

We detect an uncatalogued source, absent in Pan-STARRS template images and and located within the WXT error region (Yang et al., GCN Circ. 44978) at: 

RA(J2000) = 18:26:43.13 = 276.6797 degrees,

Dec(J2000) = +23:50:37.8 = 23.8438 degrees,

with an uncertainty of 0.5 arcsec.

The preliminary magnitudes derived for this source are:

r = 21.90 +/- 0.04,

z = 21.49 +/- 0.08.

We therefore suggest that this source is the optical counterpart of EP260618a.

Further observations and analysis are ongoing.

We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.

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.

J. Yang (ZZU), S. Q. Jiang (NAO, CAS), H. Sun (NAO, CAS) on behalf of the Einstein Probe (EP) team:

We report on the detection of an X-ray transient, designated EP260618a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission through the ground data analysis. The transient was detected in one WXT observation starting at 2026-06-18T08:34:46 (UTC). No significant variability was detected during the 2.7 ks exposure. The position of the transient is R.A. = 276.676 deg, Dec. = 23.849 deg (J2000), with an uncertainty radius of 2.5 arcmin (90% confidence level, including both statistical and systematic uncertainties). There is no historical X-ray source in the WXT error circle.

The average 0.5-4 keV spectrum can be fitted by an absorbed power law with a fixed Galactic hydrogen column density of 1.32^21 cm^-2 and a photon index of 1.4 (-0.4/+0.5). The derived average unabsorbed 0.5-4 keV flux is 6.9 (-1.7/+2.1) x 10^(-11) erg cm^-2 s^-1.

The position of EP260618a falls within the error circle of GRB 260618A reported by Fermi/GBM (GCN 44973), suggesting that EP260618a may be the X-ray afterglow of GRB 260618A. Follow-up observation with the Follow-up X-ray Telescope (FXT) onboard EP has been scheduled.

Launched on 2024 January 9, Einstein Probe is a space-based X-ray observatory designed to monitor the dynamic X-ray sky and perform rapid follow-up observations of newly discovered transients (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

The Fermi GBM team reports the detection of a likely LONG GRB

At 06:45:34 UT on 18 Jun 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260618A (trigger 803457939.281242 / 260618282).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 281.5, Dec = 21.1 (J2000 degrees, equivalent to J2000 18h 45m, 21d 06'), with a statistical uncertainty of 6.3 degrees.

The angle from the Fermi LAT boresight is 72.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260618282/quicklook/glg_skymap_all_bn260618282.png

The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260618282/quicklook/glg_healpix_all_bn260618282.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260618282/quicklook/glg_lc_medres34_bn260618282.gif