GRB 260806A — All Circulars

Anna Ho, Cassie Sevilla, Michael Camilo (Cornell), Daniel Perley (LJMU) report:

We observed the position of the Einstein Probe transient EP260806a (Li et al. GCN 45299, Yang et al. GCN 45390), with the Karl G. Jansky Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA) under joint VLA-ALMA program 26A-414 (PI Ho). The VLA observation began on 2026 September 5 UT (30 observer-frame days after discovery) at mean frequencies of 6, 10, 15, and 33 GHz, while the ALMA observation began on 2026 September 10 UT (35 observer-frame days after discovery) at mean frequencies of 104.5, 210, and 350 GHz. 

From a preliminary analysis, we detect a bright radio source with a 10 GHz flux density of ~5.5 mJy and a 104.5 GHz flux density of ~50 mJy. The source position at 10 GHz is:

RA(J2000) = 00:49:13.571
Dec(J2000) = +35:33:39.59

This is consistent with the location of the optical counterpart of EP260806a (He et al. GCN 45300, de Ugarte Postigo et al. GCN 45302, Eyles-Ferris GCN 45303, Ma et al. GCN 45305, Li et al. GCN 45305, de Ugarte Postigo et al. GCN 45306, Anumarlapudi et al. GCN45312, Abidkhanov et al. GCN 45354, Hua et al. GCN 45385, Maksut et al. GCN 45389, Pankov et al. GCN 45396) and confirms the mm detection reported by NOEMA (de Ugarto Postigo et al. GCN 45541). 

At the redshift of EP260806a, the VLA observation corresponds to a 10 GHz and 100 GHz luminosity of ~1E30 erg/s/Hz and ~1E31 erg/s/Hz, respectively. This luminosity is similar to the afterglows of relativistic jetted transients (e.g., gamma-ray bursts; Chandra & Frail 2012) while the positive spectral index across the radio bands at such a late time is similar to what has been observed in jetted tidal disruption events (e.g., Andreoni et al. 2022).  

Additional followup is planned, and we thank the VLA and ALMA staff for quickly approving and executing these observations.

H. Yang, S. Guillot (IRAP), A. Coleiro (APC) 

report on behalf of the SVOM team:

The SVOM/ECLAIRs telescope detected EP260806a (EP team, GCN 45299), starting at 2026-08-06T20:02:00 UTC (T0), through an offline targeted search with the event-by-event data downloaded via the X-band ground station. The earliest ECLAIRs detection, at T0, occurred almost three hours before the EP-WXT detection that began at 2026-08-06 22:57:37 UTC (EP team, GCN 45305).

The transient was detected during two time windows: first, between T0 and 2026-08-06T20:25:20 UTC, and later, between 2026-08-06T21:36:00 UTC and 2026-08-06T22:04:40 UTC. These intervals were interrupted by periods during which the transient was covered by Earth occultation and when the satellite entered regions of high particle background. For example, the transient was occulted by the Earth for ECLAIRs when it was detected by EP-WXT. In both time windows, the transient was detected with several timescales and energy ranges. The best detection was obtained by the image trigger with a signal-to-noise ratio of 6.7 in the 4-20 keV over a time window of 1311 seconds starting at 2026-08-06T21:42:40 UTC. This event was not detected onboard because it was too faint with very long timescales.  

The localization of the source is RA, Dec = 12.355, 35.380 degrees:

RA (J2000) = 00h49m25s     
Dec (J2000) = 35d22m48s

with a 90% C.L. radius of 11.7 arcmin (including systematic error of 2 arcmin added in quadrature). This position is consistent with that measured by EP/FXT, with an angular separation of 11 arcmin. 

The ECLAIRs light curve shows a featureless structure during both time windows, with most of the detected photons below 20 keV. 

We extracted the time-averaged spectra over the two time windows: T0 – 2026-08-06T20:25:20 and 2026-08-06T21:36:00 – 2026-08-06T22:04:40. The first spectrum in the 5-20 keV is well fitted (chi2/dof=3.1/4) by a power-law model with a photon index of 1.1+0.5-0.6, yielding a corresponding 4-20 keV flux of (5.1+/-1.1)e-10 erg/cm^2/s. The second spectrum in the 5-20 keV is well fitted (chi2/dof=1.1/4) by a power-law model with a photon index of 2.9+/-0.6, yielding a corresponding 4-20 keV flux of (6.5+/-0.9)e-10 erg/cm^2/s. Adopting a redshift of z  = 0.089 (de Ugarte Postigo et al., GCN 45302), the luminosities are estimated to be 1e46 erg/s and 1.3e46 erg/s in the 4-20 keV for the two time windows, respectively.

All quoted errors are at the 68% confidence level.

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. ECLAIRs was developed jointly by APC, CEA, CNES, and IRAP.

The SVOM/ECLAIRs point of contact for this event is : Hui Yang (hui.yang@cnrs.fr) 

Please contact him by email if you require additional information regarding the SVOM follow-up of this burst.

S. Ricciarini (U of Florence), A. Yoshida, T. Sakamoto, S. Sugita, 
Y. Kawakubo (AGU), K. Yamaoka (Nagoya U), S. Nakahira (JAXA), 
Y. Asaoka (ICRR), S. Torii, Y. Akaike, K. Kobayashi (Waseda U), Y. Shimizu,
T. Tamura (Kanagawa U), N. Cannady (GSFC), M. L. Cherry (LSU), 
P. S. Marrocchesi (U of Siena),
and the CALET collaboration:

The CALET Gamma-ray Burst Monitor (CGBM) detected GRB 260806A
at 07:26:59.65 UTC on 6 August 2026 (trigger #1470036404;
https://cgbm.calet.jp/cgbm_trigger/flight/1470036404/index.html).
A CGBM Notice was distributed in near real time.

This event was also reported by Glowbug (Cheung et al., GCN #45348).

The burst signal was seen by all CGBM detectors.
The burst light curve shows a multi-peaked structure starting
at T+1.7 s, peaking at T+6.9 s, and ending at T+64.6 s.
The T90 and T50 durations measured with the SGM data are 55.7 +/- 3.1 s
and 34.9 +/- 11.5 s in the 40-1000 keV band, respectively.

The ground-processed light curve is available at:
https://cgbm.calet.jp/cgbm_trigger/ground/1470036404/

The CALET data used in this analysis were provided by
the Waseda CALET Operation Center located at Waseda University.

C.C. Cheung, R. Woolf, M. Kerr, J.E. Grove (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge, D. Kocevski (MSFC), and M.S. Briggs (UAH) report:

The Glowbug gamma-ray telescope [1,2,3], operating on the International Space Station, reports the detection of GRB 260806A, also detected by CALET (trigger 1470036404).

Using an adaptive window with a resolution of 32-ms, the burst onset is determined to be 2026-08-06 07:27:00.088 with a duration of 51.2 s and a total significance of about 46 sigma.  The light curve comprises two multi-peaked emission structures from T0 to T0+26.6s and T0+40.5s to T0+51.2s.

The analysis results presented here are preliminary and use a response function that lacks a detailed characterization of the surrounding passive structure of the ISS. Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost gamma-ray transient telescopes developed, built, and operated by the U.S. Naval Research Laboratory (NRL) with support from the University of Alabama in Huntsville, USRA, and NASA MSFC.  It was launched on 2023 March 15 aboard the Department of Defense Space Test Program’s STP-H9 to the ISS and operated until 2024 April when it was put in safe storage on orbit. Glowbug was removed from storage and resumed operation on 2025 September 12. 

[1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959 
[2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O 
[3] Woolf, R.S. et al. 2024, Proc. SPIE, 13151, id. 1315108 

Distribution Statement A: Approved for public release.  Distribution is unlimited.

Y. N. Ma, H. L. Li, Y. L. Qiu, C. Wu, L. P. Xin, Z. H. Yao, J. R. Xu, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, 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 EP260806a detected by EP/WXT (Li et al., GCN 45299). SVOM/VT began observing the field at 2026-08-06T23:46:08 UTC, 43.667 minutes after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. 

With X-band data available, no uncatalogued sources are found within the EP/FXT error circle (Li et al., GCN 45299). At the optical counterpart position reported by LCO (He et al., GCN 45300; de Ugarte Postigo et al., GCN 45302; Eyles-Ferris et al., GCN 45303), a bright cataloged galaxy is detected. Preliminary photometric analysis shows a brightening of ~0.1 mag from 0.728 h to 3.855 h in both bands. 

Further detailed analysis is still ongoing.

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.