A 'Magnetar' is an extremely magnetic type of neutron star which radiates high energy photons in regular discrete bursts, with periods ranging from a few milliseconds to ten seconds or so. (ref.) It's estimated that they have solid cores and are probably the remnants of supernovae.

The pulsed nature of their emissions is thought to be due to their rotation, but current theory holds longest possible period of the electro-magnetic 'pulsed' emissions is just a few tens of seconds.

A newly discovered object, now called GPM J1839-10, and thought to be 15,000 light-years away from Earth, emits in the infra-red waveband every 22 minutes. There is currently no known mechanism to account for its long-period emissions.

“This enigmatic object has been hiding in the data for decades – we just did not know we had to look for it until its discovery by the MWA”, said Dr. Simona Giacintucci, NRL Research Astronomer. GPM J1839-10 emits a five-minute pulse of radio wavelength emission every 22 minutes, and it's been doing this for at least 33 years.
“Findings like this are exciting because they highlight the gaps in our understanding of the physics of these extreme stars known as magnetars,” said Dr. Tracy Clarke, NRL Research Astronomer and VLITE Project Scientist. “Current understanding says this object should not emit radio waves and yet we are detecting them across several decades and we are not sure why. That is an exciting mystery.”

Source :Press release Press release from US Naval Research Laboratory, July 2023

Other long period magnetars have recently been discovered, but GPM J1839-10 has the longest period found so far.