Part-time pulsars

A new discovery has upended the widely held view of pulsars as the orderly ticking clocks of the universe. 

Artist's impression of a pulsar (Michael Kramer, Jodrell Bank Centre for Astrophysics)
Artist's impression of the radio beams from a pulsar. Credit: Michael Kramer, Jodrell Bank Centre for Astrophysics

A survey done at the Arecibo Observatory in Puerto Rico has fortuitously discovered two extremely strange pulsars that undergo a “cosmic vanishing act.”  Sometimes they are there, and then for very long periods of time, they are not. 

Recognizing the existence of this strange behavior was fortuitous in itself.  It took great patience on the part of a team of radio astronomers led by Professor Andrew Lyne using the Lovell Telescope at Jodrell Bank to confirm the existence of these mostly invisible pulsars.

Pulsars are rapidly rotating, highly magnetized neutron stars. They are about 20 kilometres across with a mass about one and a half times that of the Sun (or about 500,000 that of the planet Earth).  The rotation sends charged particles streaming out from the magnetic poles, causing beams of radio waves to sweep around the sky - like the light beams from a lighthouse. This appears as flashes, or pulses, which can be received by terrestrial radio telescopes.

Intermittent pulsars are a rarely observed population of pulsars, which have two states - one when they pulse like normal pulsars (the ON state), and another when they mysteriously fail to work, producing no radio waves at all (the OFF state).

“These pulsars switch instantaneously between the states,” notes Lyne of The University of Manchester's Jodrell Bank Centre for Astrophysics. “They’re ON and then they’re gone, disappearing without any apparent warning.”

A 34-member pulsar study team used the 7-beam receiver at Arecibo to conduct routine pulsar searches in what they call the PALFA (Pulsar Arecibo L-Band Feed Array) Survey.  The two recently discovered intermittent pulsars spend most of their time in the OFF state.  Three other similar pulsars are also known, but they are mostly ON.   

In September 2012, one of these new objects was discovered to be emitting very bright pulses, and it was labeled PSR J1929+1357.   Of 169 new pulsars, follow-up observations of half of those had been initiated at Jodrell Bank and this candidate was confirmed as a pulsar at the second attempt in February 2013 using the 76-metre Lovell Telescope. 

“During the next 9 months it was observed no fewer than 650 times for a total of 100 hours,” said Professor Ben Stappers, also of the Jodrell Bank Centre for Astrophysics, and a co-author on the publication. “It was ON on only 5 occasions - just 0.8% of the time.” 

The most important implication of this discovery is that there must exist an extremely large number of these 'part-time' pulsars.  The PALFA survey, which is aimed at a section of the Milky Way visible to the Arecibo dish, only covers each position in the survey once.  It probably passed over 130 similar pulsars, but this was the only one that was ON at the time of observation.  Furthermore, if it were not for the early detection of pulses at Jodrell Bank, this pulsar could easily have been discarded as a false detection, likely arising from radio-frequency interference.  The PALFA team estimates that there are about 3000 such intermittent pulsars in the survey area, far greater than the population of normal pulsars.

“These disappearing pulsars may far outnumber normal pulsars,” said Dr Victoria Kaspi of McGill University in Canada and a coinvestigator on the project. “In fact, they may redefine what we think of as normal.”

Why this odd behavior?  After all, since the original pulsar discovery in 1967, they have been referred to as marvelously accurate cosmic clocks that tick steadily for millions years with a cadence that surpasses the ticking of our best laboratory clocks.  But these long-term intermittent pulsars are mostly invisible, which is about as useful as having the clock on the wall that is hidden behind a curtain for most of the time.

“The explanation of the ON-OFF behavior remains a puzzle,” says Dr. Andrew Seymour, a USRA postdoc at Arecibo.  “It indicates that the pulsar environment is changing, but just what those changes entail is open to debate.” 

Recent observations of these odd pulsars is that the slow-down in their rotation rate during the OFF state is 80% lower than when they are ON.  A property of “normal” pulsars is their pulse rate slows very gradually over time.  The changing spin rate of these part-time pulsars is inferred by calculating how many beats were missed during the pulsar’s invisible OFF phases. The PALFA team suspects that the stream of charged particles, which drive the radio beams emanating from the pulsar, also causes the pulsar to spin down more rapidly.  When OFF, this particle stream fails for some reason and the spin down rate is reduced. 

As Seymour notes, there is as yet no agreement in the pulsar community as to the mechanism which operates in these unusual objects. Lyne hopes that follow-up measurements of PSR J1929+1357 will provide a rare insight into the physics of the pulsar emission mechanism and the changing spin down phenomenon.

PALFA surveys are on-going, and no one can predict if and when more examples of this fascinating new phenomenon will be found. Catching another intermittent pulsar in its ON mode is down to chance. Is there another candidate out there ready to reveal its secrets, or will it remain hidden in the depths of space?

 

Research paper

Two Long-term Intermittent Pulsars Discovered in the PALFA Survey

Lyne et al, The Astrophysical Journal, 834, 9 (2017)

Open-access: https://arxiv.org/abs/1608.09008 

 

Further information

The Arecibo Observatory is operated by SRI International under a cooperative agreement with the National Science Foundation (AST-1100968) and in alliance with Ana G. M´endez-Universidad Metropolitana and the Universities Space Research Association.

The Lovell Telescope at Jodrell Bank Observatory is owned and operated by The University of Manchester. Pulsar research at Jodrell Bank and access to the Lovell Telescope is supported by a Consolidated Grant from the UK’s Science and Technology Facilities Council.

 

Contacts

Andrew Lyne     andrew.g.lyne@manchester.ac.uk

Andrew Seymour   aseymour@naic.edu

 

 

 

 

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