Recently, Prof. ZHOU Xia from the Pulsar Research Group at the Xinjiang Astronomical Observatory, Chinese Academy of Sciences, along with her collaborators made a significant progress in understanding ultra-long-period radio transients (ULPTs). The researchers propose that these enigmatic sources could be strange dwarf pulsars. 

The results have been published in The Astrophysical Journal.

The rotation periods of ULPTs reach thousands of seconds, far exceeding the millisecond to tens-of-seconds range of conventional radio pulsars. According to standard pulsar theory, such extremely slow rotators should lie below the “death line” of radio pulsar, meaning their rotational energy insufficient to power radio emission. However, recently discovered sources like GLEAM-X J1627-52 and GPM J1839-10 exhibit periodic radio activity, posing a severe challenge to existing models.

To uncover the nature of these sources, the researchers suggest that these isolated ULPTs could be strange dwarf pulsars, rotating compact objects consisting of a strange-quark-matter core surrounded by a normal-matter crust. The theoretical framework has been successfully applied to four known isolated source: GLEAM-X J1627-52, GPM J1839-10, ASKAP J1832-0911, and ASKAP J193505.1+214841.0., The analysis shows that strange dwarf pulsars, with their significantly larger radii compared to conventional neutron stars, can naturally produce persistent, coherent radio emissions even at extremely long periods. 

This study reveals that these objects occupy a distinct niche in the magnetic-field-period diagram, with estimated surface magnetic field from 10⁶ to 10¹⁰ Gauss. The consistent lower limit of around 10⁶ G suggests that a fundamental threshold is required for the electron-positron pair production in the magnetosphere that powers the coherent radio emission. Despite their extremely slow rotation, their radio efficiencies of about 10^-4–10^-2  are comparable to those of normal pulsars.

The model naturally accommodates multi-wavelength observational features, especially the X-ray data from ASKAP J1832-0911. This source shows a two-component X-ray spectrum and its X-ray and radio emissions vary in lockstep—features strongly predicted by the strange dwarf pulsar model. The observed X-ray to radio luminosity ratio of ~10³ for this source fits neatly within the model's predicted range of 10³−10⁵.

This study not only furnishes a physical explanation for the newly recognized population of ULPTs, but also carries profound implications for understanding the equation of state of strange quark matter deep inside compact objects. The unique structure of strange dwarf pulsars allows them to remain "radio-loud" at extreme periods, offering a new window into the physics of dense matter.

This research also complements the team’s previous works of the formation mechanism of long-period (10—100 s) radio pulsars. Together, these works probe the origins of both long-period pulsars and ULPTs from complementary angles, offering fresh insights into the evolution and radiation mechanisms of compact objects.

“We plan to use next-generation facilities like the Square Kilometer Array (SKA) and the Five-hundred-meter Aperture Spherical Telescope (FAST) to further test the strange dwarf model by probing the formation, interior structure, and population of these unique objects.” said Prof. ZHOU Xia.

                                                                                          The death valley of strange dwarf pulsars in the magnetic-field — period diagram