Mystery Object in the Milky Way Flared Every 62 Minutes, Then Went Silent

Astronomers have identified another of the strange radio sources known as long-period transients, an object that sent out powerful flares every 62.2 minutes and then went quiet, ScienceAlert reported. The discovery, led by Scott Hyman of Sweet Briar College, adds a rare clue about what these objects might be: its pulses come in pairs, and their pattern points to an unusually complicated magnetic field.

The new object, called LPT J1555-5631, is part of a class that has only come into focus in the last few years. Researchers still do not know whether all of these sources are the same kind of thing.

How the discovery unfolded

• September 2002: Hyman saw a source near the heart of the Milky Way fire five powerful radio flares, spaced 77 minutes apart, and then fall silent. He spent years looking for a similar object without much success.
• 2018 and 2022: A source called GLEAM-X J1627-5235 was observed in 2018 and first reported in 2022. It established long-period transients as a category. Since then, around a dozen of these sources have turned up in radio surveys of the Milky Way, and Hyman’s 2002 detection was later counted among them.
• 2022: While searching archives for millisecond pulsars, Hyman’s team spotted a candidate in MeerKAT telescope data whose pulses arrived many minutes apart at seemingly regular intervals. Data from the ASKAP telescope, which had watched the same patch of sky on the same day, covered 10 hours and confirmed a period of about 62 minutes.

What makes this object unusual

Instead of one burst per cycle, LPT J1555-5631 produced two distinct pulses, one longer and one shorter. The radio waves in the two pulses were polarized in opposite directions, with one twisting one way and the other twisting the opposite way.

Hyman said that combination suggests the radio emission comes from regions near both magnetic poles of the object, and that its rotation axis and magnetic axis are nearly perpendicular to each other.

The spacing of the pulses is also odd. A simple magnetic field with two poles would be expected to produce pulses 180 degrees apart in the cycle. Here they were separated by 165.4 degrees. Hyman said the shortfall is an important clue that the magnetic field has a more complicated structure.

Why the identity of these objects is still unsettled

Three leading explanations are on the table. Pulsars are fast-spinning neutron stars that sweep radio beams across space like lighthouses. Magnetars are neutron stars with even stronger magnetic fields that can unleash bursts as those fields twist and rearrange. White dwarfs, which are larger and can rotate more slowly, can generate radio signals through interactions with a companion star, and some of these sources have been tied to white dwarf pairs.

None fits every case. These sources pulse extremely slowly by neutron star standards, sometimes with hours between flashes, and at such slow rotation rates ordinary pulsars should have trouble producing radio emission at all. Differences in pulse patterns, polarization and periods also raise the possibility that more than one mechanism, and more than one type of object, is involved. Hyman asked whether these sources might be a new subcategory of white dwarfs or neutron stars, or something entirely unknown.

What this means for the search

For now, this is a finding about a single object, and it does not settle the question. But each new detection gives astronomers more to compare. Hyman said he expects trends to emerge across the group once many more are found, which could help point researchers toward the right targets for follow-up.

Another puzzle is why these sources flash with clockwork regularity and then switch off. LPT J1555-5631, like most of the known catalog, has not been seen again. Hyman’s 2002 source, however, has shown repeat activity, as have two others. He said the changes in activity probably have something to do with the object’s magnetosphere, the region around it dominated by its magnetic field, though a larger catalog may be needed before clear patterns show up.

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Around Prague, based on reporting by ScienceAlert