Constraints on an optical counterpart for the long-period radio transient GPM J1839-10

Long period radio transients (LPTs) are periodic radio sources showing pulsed emission on timescales from minutes to hours. The underlying sources behind this emission are currently unclear. There are two leading candidates: neutron stars or white dwarfs. Neutron stars could emit at LPT timescales as magnetars, binaries, or precessing sources. White dwarfs on the other hand have only been observed to emit in radio as binary systems with companions that provide charged particles through their wind. A key distinction is that an optical counterpart is much more likely in the white dwarf scenario. GPM J1839-10 is an LPT with a radio period of 21 min for which the white dwarf scenario has been favoured, but no optical counterpart is confirmed. Using HiPERCAM, a high-speed multi-colour photometer that observes simultaneously in usgsrsiszs filters, we probe the existence of a white dwarf in GPM J1839-10. We do not directly detect a white dwarf, but cannot rule out its presence given the uncertain distance and reddening of GPM J1839-10. On the other hand, we find evidence in our data for periodic behaviour in harmonics of the radio period, as expected from the white dwarf scenario.