  Long-period radio transients are an emerging class of extreme astrophysical
events of which only three are known. These objects emit highly polarised,
coherent pulses of typically a few tens of seconds duration and minutes to
hour-long periods. While magnetic white dwarfs and magnetars, either isolated
or in binary systems, have been invoked to explain these objects, a consensus
has not emerged. Here we report on the discovery of ASKAP J193505.1+214841.0
(henceforth ASKAPJ1935+2148) with a period of 53.8 minutes exhibiting three
distinct emission states - a bright pulse state with highly linearly polarised
pulses with widths of 10-50 seconds; a weak pulse state which is about 26 times
fainter than the bright state with highly circularly polarised pulses of widths
of approximately 370 milliseconds; and a quiescent or quenched state with no
pulses. The first two states have been observed to progressively evolve over
the course of 8 months with the quenched state interspersed between them
suggesting physical changes in the region producing the emission. A constraint
on the radius of the source for the observed period rules out a magnetic white
dwarf origin. Unlike other long-period sources, ASKAPJ1935+2148 is the first to
exhibit drastic variations in emission modes reminiscent of neutron stars.
However, its radio properties challenge our current understanding of neutron
star emission and evolution.
