GRB 260128B

M. Brunet, O. Godet (IRAP), S. Le Stum, A. Coleiro (APC) on behalf of the ECLAIRs team

Using the event-by-event data downloaded through the X-band ground station, we report further analysis of ECLAIRs observations of GRB 260128B (GCN 43561
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 ). 

The burst that triggered ECLAIRs consists of a single peak lasting around 10 s in the 4-50 keV energy band.

The time-averaged spectrum from T0-9.24 s to T0-0.24 s (T0 = 2026-01-28T06:48:34.72 UT) in the energy range 4-50 keV is not well fitted by a simple powerlaw model. This indicates some spectral curvature. Using a blackbody with a temperature kT = (2.29 +0.22/-0.20) keV provides a good fit. With this model, the fluence in the energy range 4-120 keV is  (1.04 +0.05/-0.30) e-7 erg/cm². 

Using either a broken power law or a power law with an exponential cutoff provides acceptable fits. However, given the faintness of the event, the model parameters remain poorly constrained. 

All the quoted errors are at the 68% confidence level.

The Space-based multi-band astronomical 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 CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC.

The SVOM/ECLAIRs point of contact for this burst is: Marius Brunet (IRAP) (marius.brunet  at irap.omp.eu)

C. Wu (NAOC), J. X. Cao (GXU), A. Saccardi (CEA/Irfu), D. Turpin (CEA/Irfu), report on behalf of the SVOM mission team:

We observed the field of the SVOM GRB 260128B (Brunet et al., GCN 43561
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 ) with the LCO 1m telescope at Cerro Tololo Interamerican Observatory equipped with the Sinistro instrument.

Our observation started on 2026-01-30 at 00:48:11 UT (about 1.75 days after the trigger) and we obtained 6x300 s exposures in the SDSS r and 5x300 s exposures in the Pan-STARRS z filters. In the stacked image, we do not detect any new source within the SVOM/ECLAIRs error box, in agreement with Méndez et al. (GCN 43566
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 ).

We measure the following upper limit calibrated against the PanSTARSS DR1 catalog, and not corrected for Galactic extinction:

r > 22.4 AB (3-sigma, mid-time 42.5 hr after the trigger)
z > 21.3 AB (3-sigma, mid-time 42.2 hr after the trigger).

This project is funded by the SVOM collaboration.

Enrique Moreno Méndez (UNAM), Jean-Luc Atteia (IRAP), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP),  Leonardo García-García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Asuka Kuwata (UNAM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), N. A. Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), and Sébastien Le Stum (APC) report:

We imaged the field of the SVOM GRB 260128A (Brunet et al., GCN Circ. 43561
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 ) using the DDRAGO two-channel wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2026-01-30 02:15 to 03:38 UTC (from 43.5 to 44.8 hours after the trigger) and obtained 60 minutes of simultaneous exposure in the r and z filters.

The data were reduced, coadded, calibrated, and analyzed 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.

WIthin the ECLAIRs 12 arcmin error circle (Brunet et al., GCN Circ. 43561
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 ), and in comparison to the corresponding PS1 images, we do not detect any new sources to preliminary 5-sigma limiting magnitudes of

r > 22.3
z > 21.5

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.

M. Brunet (IRAP), S. Le Stum, A. Coleiro (APC), O. Godet (IRAP),report on behalf of the SVOM team:

The SVOM/ECLAIRs telescope detected a transient source, labelled GRB 260128B, starting at 2026-01-28T06:48:34.72 UTC (T0), through an offline search with the event-by-event data downloaded through the X-band ground station.

The burst was detected within several energy ranges and timescales. The best detection is obtained by the Image Trigger with a signal-to-noise ratio of 7.5 within 4-20 keV over a time window of 10.24 seconds starting at T0-10.24s.

The lightcurve shows a single peaked structure lasting around 4s.

The localization of the source is RA, Dec = 27.734, -25.867 degrees:

RA (J2000) = 01h50m56.16s    
Dec (J2000) = -25d52m01.20s 

with a 90% C.L. radius of 12.0 arcmin (including systematic error of 6 arcmin added in quadrature).

We note that no bright catalogued X-ray sources are found in the error box.

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 point of contact for this event  is : Sébastien Le Stum : lestum at apc.in2p3.fr

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