  In the optical sky, minutes-duration transients from cosmological distances
are rare. Known objects that give rise to such transients include gamma-ray
bursts (GRBs), the most luminous explosions in the universe that have been
detected at redshift as high as z ~ 9.4. These high-redshift GRBs and their
associated emission can be used to probe the star formation and reionization
history in the era of cosmic dawn. Here we report a near-infrared transient
with an observed duration shorter than 245 s coincident with the luminous
star-forming galaxy GN-z11 at z ~ 11. The telluric absorption shown in the
near-infrared spectrum indicates its origin from above the atmosphere. We can
rule out the possibility of known man-made objects or moving objects in the
Solar system based on the observational information and our current
understanding of the properties of these objects. Since some long-duration GRBs
are associated with a bright ultraviolet (UV) or optical flash, we investigate
the possibility that the detected signal arose from a rest-frame UV flash
associated with a long GRB from GN-z11. Despite the very low probability of
being a GRB, we find that the spectrum, brightness, and duration of the
transient are consistent with such an interpretation. Our result may suggest
that long GRBs can be produced as early as 420 million years after the Big
Bang.
