Credit: NASA’s Goddard Space Flight Center
Researchers recently discovered an object with regular radio pulses
less frequent than those of a pulsar but more frequent than those of a
long-period radio transient. What do current observations tell us about what this object might be?
Between Two Extremes
The sky at radio wavelengths is inhabited by many objects that
brighten, fade, and emit pulses. The most rapidly and regularly varying
radio emission comes from pulsars: rotating remnant cores of massive
stars that emit beams of radio waves along their magnetic poles. As the
beamed emission from a pulsar sweeps across our field of view, it
creates the characteristic pulses that give pulsars their name.
Pulsars have pulse periods from milliseconds to a few seconds. Over the past few years, astronomers have discovered a small but growing number of objects with radio pulses with periods of minutes to hours. Though these long-period radio transients (LPRTs) share some properties with pulsars, it’s not yet clear what powers them. One path to uncovering their identity is discovering objects in the period gap between them and pulsars — a region typically missed in searches for pulsars or LPRTs.
Pulsars have pulse periods from milliseconds to a few seconds. Over the past few years, astronomers have discovered a small but growing number of objects with radio pulses with periods of minutes to hours. Though these long-period radio transients (LPRTs) share some properties with pulsars, it’s not yet clear what powers them. One path to uncovering their identity is discovering objects in the period gap between them and pulsars — a region typically missed in searches for pulsars or LPRTs.
Credit: Wang et al. 2025
Serendipitous Discovery
In a recent publication, Yuanming Wang (王远明; Swinburne University of Technology and ARC Centre of Excellence for Gravitational Wave Discovery) and collaborators reported their discovery of an object that lies within this gap, PSR J0311+1402./
The discovery was serendipitous, with three of the object’s pulses appearing in two minutes of test observations made with the Australian Square Kilometre Array Pathfinder (ASKAP)’s newly installed Commensal Realtime ASKAP Fast Transient Coherent (CRACO) system. CRACO is sensitive to pulse periods within the gap between pulsars and LPRTs.
The team followed up on this detection with observations from ASKAP, Murriyang, MeerKAT, and the Green Bank Telescope. These observations paint a coherent picture of an object with 0.5-second-long pulses every 41 seconds, placing it squarely in the gap between pulsars and LPRTs.
The discovery was serendipitous, with three of the object’s pulses appearing in two minutes of test observations made with the Australian Square Kilometre Array Pathfinder (ASKAP)’s newly installed Commensal Realtime ASKAP Fast Transient Coherent (CRACO) system. CRACO is sensitive to pulse periods within the gap between pulsars and LPRTs.
The team followed up on this detection with observations from ASKAP, Murriyang, MeerKAT, and the Green Bank Telescope. These observations paint a coherent picture of an object with 0.5-second-long pulses every 41 seconds, placing it squarely in the gap between pulsars and LPRTs.
Pulsar or Long-Period Radio Transient?
Could PSR J0311+1402 be a pulsar with an unusually long spin period?
The shape of PSR J0311+1402’s pulses and the spectral index (related to
how the pulses vary as a function of frequency) resemble those of
typical pulsars. However, the spin-down rate of this object, measured
from the minuscule change in pulse period over time, potentially places
PSR J0311+1402 below the “death line.” Objects below the death line are
theorized to be unable to produce radio emission through
electron–positron pair production.
Is PSR J0311+1402 more likely to be an LPRT with a surprisingly short period? Isolated LPRTs have strongly polarized radio emission and are located at low galactic latitudes, while PSR J0311+1402 has weakly polarized radio emission and is located at a moderate galactic latitude. While LPRTs in binary systems have also been found, and have different properties, no companion has been detected for PSR J0311+1402.
Together, these factors suggest that PSR J0311+1402 is more likely to be a slowly rotating pulsar than a short-period LPRT, though further observations to refine the calculation of its spin-down rate are needed. And where there’s one object, there are usually more, suggesting that future searches attuned to periods within the gap between pulsars and LPRTs may find more of these in-between objects and help to illuminate their properties.
Is PSR J0311+1402 more likely to be an LPRT with a surprisingly short period? Isolated LPRTs have strongly polarized radio emission and are located at low galactic latitudes, while PSR J0311+1402 has weakly polarized radio emission and is located at a moderate galactic latitude. While LPRTs in binary systems have also been found, and have different properties, no companion has been detected for PSR J0311+1402.
Together, these factors suggest that PSR J0311+1402 is more likely to be a slowly rotating pulsar than a short-period LPRT, though further observations to refine the calculation of its spin-down rate are needed. And where there’s one object, there are usually more, suggesting that future searches attuned to periods within the gap between pulsars and LPRTs may find more of these in-between objects and help to illuminate their properties.
By Kerry Hensley
Citation
“The Discovery of a 41 s Radio Pulsar PSR J0311+1402 with ASKAP,” Yuanming Wang et al 2025 ApJL 982 L53. doi:10.3847/2041-8213/adbe61
