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Researchers Discover a New Population of Dwarf Pulses

Researchers from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) under the direction of Prof. Jinlin Han have discovered distinct “dwarf pulses” from the bright pulsar PSR B2111+46 using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). This has allowed the team to study radio emissions in unprecedented detail and investigate the undiscovered physics in the magnetosphere.

Researchers Discover a New Population of Dwarf Pulses
Pulsar radio emission from thunderstorms and raindrops of particles in the magnetosphere of PSR B2111+46 detected by FAST. Image Credit: NAOC

On August 17th, 2023, the study was published in Nature Astronomy.

Pulsars often generate radio waves. Some old pulsars, however, occasionally quench for short periods of time, a process known as “pulse nulling.”

This phenomenon could arise due to particles being unable to coalesce within the magnetosphere, potentially due to unfavorable conditions or shifts in magnetic field configuration and radiation zones. Another possibility is that the particle generation zone is inundated with plasma originating from other sources.

The precise cause of pulsar radiation absence is unknown as it is impossible to study the physical condition of the pulsar’s magnetosphere while radiation is absent.

PSR B2111+46 is a rather old pulsar, and astronomers have long known that the pulsar's radiation frequently nulls for extended periods of time. However, hundreds of previously unreported exceptionally weak, very narrow pulses were identified during routine nulling phases when it was seen on August 24th, August 26th, and September 17th, 2020, as part of the Galactic Plane Pulsar Snapshot survey, a crucial effort to see pulsars.

The researchers studied this pulsar for two hours again on March 8th, 2022, to confirm this new type of emission condition.

Finally, we picked out 175 such narrow, weak pulses.

Dr. Xue Chen, Study First Author, National Astronomical Observatories, Chinese Academy of Sciences

Such pulses, according to Dr. Chen, differ from regular pulses in terms of pulse breadth and energy and have so been dubbed “dwarf pulses.”

Unlike normal individual pulses, which emit radiation through a “thunderstorm” of particles produced by copious discharges in frequently formed gaps near the pulsar’s magnetic poles, dwarf pulses emit radiation through one or a few “raindrops” of particles produced by pair production in a fragile gap of this near-death pulsar.

These sporadic, weak, and narrow pulses characterize a novel radiation state distinct from regular pulses. Interestingly, these pulses frequently display an uncommon reversed spectrum, featuring notably intensified emissions at higher radio frequencies — a phenomenon rarely detected in such a distinguished timescale from astronomical sources.

The properties of such dwarf pulses would be hard to be measured by other radio telescopes than FAST and measurements of such a new population of dwarf pulses reveal that the magnetic field structure for the pulsar radiation remains unchanged even when the radiation is almost ceased.

Jinlin Han, Professor, National Astronomical Observatories, Chinese Academy of Sciences

Yi Yan, co-first author of the study, added, “In fact, a smaller number of dwarf pulses have also been detected from a few other pulsars. Detailed studies of such a dwarf pulse population could uncover some mysteries of unknown pulsar radiation processing and reveal the extreme plasma state in the pulsar magnetosphere.

Journal Reference:

Chen, X., et al. (2023) Strong and weak pulsar radio emission due to thunderstorms and raindrops of particles in the magnetosphere. Nature Astronomy. doi:10.1038/s41550-023-02056-z

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