GCN Circular 44301

D. O'Neill, M. Wortley, M. Pursiainen, B. P. Gompertz, J. Lyman, R. Starling, G. Ramsay, K. Ackley, M. Dyer, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, B. Godson, T. Killestein, A. Kumar, on behalf of GOTO collaboration,

A. Aryan, T.-W. Chen, Y.-H. Lee, T. K. Ho Ngo, and H.-Y. Hsiao (all NCU), report:

We report on the discovery of the rapidly-evolving transient GOTO26djh/AT2026jpw
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  with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) during survey operations. 

The object was discovered at 2026-04-13 23:17:32 UT at co-ordinates:

 RA,DEC (J2000): = 13:22:13.21, +37:02:15.16,
                   200.555049, 37.03754

Observations consisted of 4 x 45s exposures taken in the L band (400-700 nm). Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations. 

The source was initially detected with L = 19.00 ± 0.08 AB mag at t0=2026-04-13 23:17:32 UT before fading to L = 19.61 ± 0.18 AB mag at 2026-04-14 03:21:27 UT (t0 +4.06h), suggesting a decay rate of ~3.5 mag/d. 

No underlying source is detected in either the Pan-STARRS or SDSS surveys that would suggest the presence of a host galaxy or stellar counterpart. Additionally, the high Galactic latitude (b = +78.15 deg) would be unusual for a local stellar origin.

We performed follow-up observations with the 1 m Lulin One-meter Telescope (LOT) at Lulin Observatory, Taiwan, as part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86). The first epoch of LOT observations began at 2026-04-15 14:41 UT (MJD 61145.612, t0 +39.40h). The transient was detected with an r-band magnitude r=21.55 ± 0.07 AB mag.

Using this combined dataset, the estimated decay index ranges from -1.6 < alpha < -1.1, assuming explosion times of 12h to 6h pre-discovery, consistent with the decay indices seen in GRB afterglows. GOTO26djh/AT2026jpw
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  is not detected in the most recent pre-discovery GOTO observations taken at 2026-04-12 00:00 UT (t0 -47.29h) to a 3-sigma limiting magnitude of L > 20.5. This constrains its onset to a window of < 2 days before discovery.

Despite the apparent lack of an underlying star and an evolution consistent with GRB afterglows, a local stellar origin cannot be definitively excluded with the current data. We encourage further follow-up to determine the nature of this transient. EP/FXT ToO observations have been scheduled.

GOTO magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018). LOT magnitudes were calibrated using SDSS stars. Observations are not corrected for Galactic extinction.

GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester, the  University of Birmingham and the Instituto de Astrofisica de Canarias (IAC).