The Mystery of Part-Time Pulsars

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). “They switch instantaneously between the states,” notes Lyne. “They’re ON and then they’re gone, disappearing without any apparent warning.”

A 34-member pulsar study team, including Dr. Andrew Seymour, a USRA postdoc at Arecibo, used the 7-beam receiver 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 in February 2013 using the 250-ft Lovell Telescope at the second attempt. “During the next 9 months it was observed no fewer than 650 times — 100 hours,” said Professor Benjamin Stappers of the Jodrell Bank Center 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 these vanishing act 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 signals 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 3,000 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 the principal investigator on the PALFA 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.