Revealing the Unseen: The Discovery of Long-Awaited Radiation from the Intermittent Pulsar PSR B1931+24

Abstract

Pulsars are typically characterized by their stable, highly magnetized, and fast-rotating nature, which underpins their persistent radio emissions. However, the discovery of prolonged radio-quiet ("off") states in intermittent pulsars, such as PSR B1931+24, has challenged the most fundamental theory of pulsar magnetospheric emission. Despite long-term monitoring with several telescopes, continuous emission during these "off" states had not been detected in 20 years of observations. Fortunately, sensitive observations via Five-hundred-meter Aperture Spherical radio Telescope (FAST) revealed the mysterious weak emission containing occasional bursting dwarf pulses during the "off" states of PSR B1931+24. Along with a substantial decrease in flux density, a significant contraction in the integrated pulse width is measured in the "off" state compared to the radio-loud ("on") state, indicating alterations in the plasma supply and magnetospheric structure. Additionally, a previously unobserved dyssynchronous, nonuniform emission pattern is found in both states, supporting theories of a spatially inhomogeneous pair-cascade mechanism and challenging models of spatially coherent discharge. Furthermore, occasional dwarf pulses detected during the "off" state show flux and width distributions similar to those of the "on" state pulses, suggesting a potential link between the "on" and "off" state emissions of PSR B1931+24. Consequently, dwarf pulses are unlikely to represent a distinct emission mode as previously thought; instead, they appear to be part of a continuum within the pulsar's emission behaviors observed during the "on" state. These findings strongly support the basic theory of the pulsar magnetospheric emission and significantly advance our understanding of pulsar magnetospheric dynamics and their emission mechanisms.