Cosmic Beacons Sing in Different Octaves
This artist's illustration depicts gas flowing from the star can spin the pulsar up to hundreds of revolutions a second and allow it to resume its lighthouse-like beams of radiation.
Credit: NASA

New observations from a trio of international telescopes have caught enigmatic radio-emitting stars called pulsars beaming out signals across different octaves, revealing more clues into how these fast-spinning stars generate their cosmic lighthouse emissions.

Using observations from the new European LOFAR telescope, the Effelsberg telescope in Germany and the Lovell telescope in the United Kingdom, astronomers were able to observe six different pulsars, each simultaneously across a range of nearly eight octaves.

"Not only do such observations give us a fantastic handle on understanding the emission of pulsars, they are also a powerful probe of the interstellar gas that is between us and the pulsar," said study team member Ben Stappers of the University of Manchester.

Pulsars are rapidly rotating neutron stars about 12 miles (20 km) across that emit beams of electromagnetic radiation. They are created when massive stars die and collapse in supernova explosions into compressed objects comprised solely of neutrons.

Since roughly the equivalent of the sun's mass is being packed into a tiny space approximately the size of a city, the angular momentum causes the neutron star to spin rapidly. In the process, pulsars emit a ray of light that sweeps around in what has been called a lighthouse effect.

If a pulsar is aligned with Earth, its light beam crosses our planet once per rotation, creating a pulse of light visible at regular intervals ranging from a few milliseconds to seconds, depending on the pulsar's mass.

Even though scientists have been studying pulsars for the past 40 years, the exact mechanisms that generate these intense beams of light remain largely a mystery. Researchers who collaborated on this new study are hoping that will soon change.

The team, which included scientists from the Max Planck Institute for Radio Astronomy (MPIfR), monitored six different pulsars across multiple wavelengths. The researchers postulated that the emissions seen at the different wavelengths emerge from different heights above the highly magnetized pulsar surface.

By examining the pulsar emissions at various wavelengths, the astronomers were able to monitor the pulsar magnetosphere (or magnetic atmosphere) from different altitudes, said study team member Kosmas Lazaridis from MPIfR.

"We could see the behavior of the particles following the magnetic field lines at various heights," Lazaridis told "We observed that higher up, the magnetic field lines open, and the pulse broadens."

This led the astronomers to suggest that pulsar emissions at different radio wavelengths may be created at different heights above a star's magnetic poles. The magnetic field lines that accelerate particles spread apart as one moves further and further away from the pulsar's surface.

Experimental support for the idea came from observational data that found pulses of some pulsars stretching out at longer wavelengths. The shape of the pulsar's emission was seen to evolve quite drastically as a function of wavelength, and maps the spreading of magnetic field lines above the pulsar's magnetic poles.

Further study of pulsars will be made even more accessible once the LOFAR telescope is fully completed in the next year. This higher sensitivity radio telescope will be able to perform more pulsar experiments, while also locating previously-undetected pulsars to study, Lazaridis said.

"This telescope represents a new era, and many more observations of this kind are to come," he said.

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