Long-period radio transient pulsed every 62 minutes before vanishing

A radio source near the center of the Milky Way pulsed every 62.2 minutes twenty years after September 2002 and then fell silent, ScienceAlert reported. A similar source put out five powerful flares separated by exactly 77 minutes in September 2002 before falling quiet; that object was later identified as GCRT J1745-3009, one of the long-period transients. Astronomer Scott Hyman of Sweet Briar College spent years searching for similar objects after 2002 without much success before moving to other work.

The newer signal became known as LPT J1555-5631 and appeared in data while Hyman was searching for something else, ScienceAlert reported. Like most of the long-period transient catalog, it has not appeared again since.

The discovery came from archival MeerKAT radio telescope observations, and a 10-hour ASKAP radio telescope observation of the same sky region on the same day confirmed it, according to ScienceAlert. The research describing LPT J1555-5631 appeared in The Astrophysical Journal.

LPT J1555-5631 puts out two different pulses during each 62-minute cycle, one longer and one shorter, with opposite circular polarization. The pulses show radio waves twisting in opposite directions, indicating emission from regions near both magnetic poles.

The two pulses sit 165.4 degrees apart rather than the 180 degrees a simple dipolar magnetic field would suggest, ScienceAlert noted. A 180-degree separation would more easily be explained by a standard dipolar magnetosphere environment, so the observed deviation points to a more complicated magnetic field structure.

Long-period transients are mystery objects that put out long bright radio pulses after regular, lengthy pauses, and the category emerged only in the last few years, ScienceAlert said. GLEAM-X J1627-5235 established the category; researchers observed it in 2018 and first reported it in 2022. Since then, radio-sky surveys across the Milky Way have turned up around a dozen of these mystery pulsators, including GCRT J1745-3009 retroactively.

GCRT J1745-3009 has flared again, as have two other long-period transients, ScienceAlert said. Most long-period transients in the catalog have not been seen again, and why they burst on a regular schedule before going quiet remains unknown, ScienceAlert said.

Three leading candidates for long-period transient identity are pulsars, magnetars, and white dwarfs, ScienceAlert explained. Pulsars are fast-spinning neutron stars that project radio beams like lighthouses based on their rotation rate. Magnetars are strongly magnetic cousins of pulsars that produce powerful bursts when their magnetic fields shift and reorganize. White dwarf stellar remnants exceed neutron stars in size and rotate far more slowly, while companion-star interactions can generate powerful radio signals.

Long-period transients pulse extraordinarily slowly by neutron star standards, sometimes taking hours between flashes, according to ScienceAlert. Conventional pulsars rotate so slowly they would have difficulty creating the conditions required to emit radio waves. No known pulsing dead-star type can easily account for everything long-period transients do.

The variety in their pulse patterns, polarization, and periods could mean multiple physical mechanisms, or multiple kinds of object, are responsible, ScienceAlert said. Hyman has asked whether they are all one object type, a new white-dwarf or neutron-star subcategory, or something unknown. A few of these objects link to white dwarf binaries, though white dwarfs cannot explain the whole class, according to ScienceAlert.

In 2022, Hyman shifted his focus to hunting for millisecond pulsars to help account for an excess of gamma rays from the center of the Milky Way, according to ScienceAlert. The strange 62-minute signal photobombed this new research focus. A candidate found in archival MeerKAT observations showed pulsar-like properties, though its pulses came many minutes apart on an apparently regular pattern.

Hyman said the team checked other archives and learned that ASKAP had viewed the same patch of sky on that same day. ASKAP confirmed the 62-minute period in a 10-hour observation, according to the report.

Along with the other details, that confirmation led the researchers to conclude they had found a long-period transient, ScienceAlert said. Hyman said it feels very rewarding to see each new long-period transient detection after years of finding nothing like GCRT J1745-3009, ScienceAlert said.

Hyman said big swings in long-period transient activity probably relate to the object’s magnetosphere, though researchers might need a larger catalog for patterns to show up. Once researchers discover many more long-period transients, they hope to uncover trends allowing targeted searches of suspected long-period transients, according to the report. He also said blind archive searches and luck remain promising ways to make discoveries.