Oddball Pulsars: First They Appear And Then They’re Gone
By Judith E Braffman-Miller
Pulsars are rapidly rotating baby neutron stars that are born from the funeral pyre of a massive progenitor star that has ended its stellar “life” in the violent, tragic tantrum of a supernova explosion. This final blaze of glory blasted the glowing, shimmering, varicolored outer gaseous layers of the doomed massive star into ambient space–as the core of the now “dead” star collapsed under the relentless crush of its own merciless gravity. Pulsars are dense, highly magnetized, rotating stellar remnants that emit a regular beam of electromagnetic radiation, and have brief rotational periods that hurl their brilliant light out into space with a regularity that is frequently likened to that of a lighthouse beacon on Earth. However, in January 2017, a team of astronomers announced that they have discovered strange pulsar behavior that indicates that a new class of pulsar “oddballs” seem to travel to the beat of a different drum. The study performed at the Arecibo Observatory in Puerto Rico revealed a duo of extremely bizarre pulsars that perform a weird “cosmic vanishing act”–sometimes they appear, and then for very long periods of time, they are gone.
It took a great deal of determination on the part of a team of radio astronomers at the University of Manchester’s Jodrell Bank in the UK to recognize the occurrence of the odd behavior of this duo of strange beasts inhabiting the cosmic zoo. The team, led by Dr. Andrew Lyne of the University of Manchester, showed great patience in their careful endeavor to confirm the real existence of these ghostly pulsars, that are invisible most of the time.
Pulsars are approximately 20 miles across, and possess masses equivalent to 500,000 Earths. Their rapid rotation hurls charged particles screeching out from the highly magnetized poles. This results in pulses, which can be received by radio telescopes on Earth.
The Lighthouses Of The Universe
The radiation that pulsars shoot out into the Universe can only be seen when the beam of emission is directly pointing toward Earth, which accounts for the pulsed appearance of emission.
The usually precise periods of pulsars have made them very valuable tools for astronomers to use. Indeed, astronomers have used observations of binary pulsars’ regular pulses to indirectly confirm the existence of gravitational waves–which are ripples in the fabric of Spacetime itself–that were predicted by Albert Einstein in his Theory of General Relativity (1915). In addition, the first exoplanets–alien worlds that belong to the families of stars beyond our own Sun–were discovered circling a pulsar, dubbed PSR B1257+12, by Dr. Aleksander Wolszczan. In fact, certain types of pulsars are just as reliable as atomic clocks in their accuracy in keeping time.
The first pulsar was discovered on November 28, 1967, by Dr. Jocelyn Bell Burnell and Dr. Antony Hewish. The astronomers observed pulses separated by 1.33 seconds that originated from the same location in space, and kept sidereal time. In trying to determine explanations for the mysterious pulses, the short period of the pulses eliminated most of the potential astrophysical sources that could account for the strange radiation, such as stars. Furthermore, since the pulses followed sidereal time, it could not be made by intelligent alien beings. At this frustrating point, Dr. Burnell has noted of herself and Dr. Hewish that “we did not really believe that we had picked up signals from another civilization, but obviously the idea had crossed our minds and we had no proof that it was an entirely natural radio emission. It is an interesting problem–if one thinks one may have detected life elsewhere in the Universe, how does one announce the results responsibly?” In spite of this, the two astronomers went on to nickame the mysterious signal LGM-1, for “little green men”. “Little green men” is a playful reference for intelligent alien beings from distant worlds.
It was not until a second mysterious pulsating source was discovered, in a different region of the sky, that the LGM hypothesis was discarded as merely an interesting, and somewhat humorous, diversion from scientific reality. Dr. Burnell’s and Dr. Hewish’s pulsar was later named CP 1919, and is currently known by a number of different designations including PSR 1919+21, PSR B1919+21, and PSR J1921+2153. Even though CP 1919 does emit in radio wavelengths pulsars, in general, have subsequently been determined to emit in visible light, X-ray, and/or gamma ray wavelengths.
The word pulsar first appeared in print in 1968:
An entirely novel kind of star came to light on August 6 last year and was referred to, by astronomers, as LGM (Little Green Men). Now it is thought to be a novel type between a white dwarf and a neutron [star]. The name Pulsar is likely to be given to it.
The existence of these strange and puzzling beasts inhabiting the celestial zoo was first proposed by Walter Baade and Fritz Zwicky in 1934, when the two astronomers argued that a dense, small star composed primarily of neutrons could be left in the wreckage of a massive star that had gone supernova. The core of the “dead” massive star–that tragically collapsed under the heavy, heartless, relentless weight of its own crushing gravity–is compressed to the point that its constituent protons and electrons merge together to form neutrons. In a sense, these city-sized stellar relics are really one big atomic nucleus.
Neutron stars may wander through the wilderness of our Cosmos either as lonely, isolated worlds, or as members of a binary system residing in close contact with a still “living” hydrogen-burning main-sequence star (as designated in the Hertzsprung-Russell Diagram of Stellar Evolution), or with a kindred stellar corpse similar to itself. Neutron stars have been observed embedded within the hearts of brilliantly colorful, beautiful, and glowing supernova remnants.
A dense newborn pulsar retains most of the angular momentum of its progenitor massive star, and since it has only a small percentage of its progenitor’s radius, it is born with a very high rate of rotation. A beam of radiation is sent forth along the magnetic axis of the pulsar, and it whirls along with the rotation of the newborn neutron star. The magnetic axis of the pulsar is what determines the direction of the electromagnetic beam, with the magnetic axis not necessarily being the same as the rotational axis. It is because of this misalignment that the beam is observed once for every rotation of the neutron star. This is what leads to the “pulsed” nature of its appearance.
In general, there are three distinct classes of pulsars that are currently recognized, based on the source of the power of a pulsar’s electromagnetic radiation:
–Rotation-powered pulsars, where the loss of rotational energy of the stellar relic provides its source of power.
-Accretion-powered pulsars, where the gravitational potential energy of matter that is being accreted by the pulsar provides its power. This class contains most (though not all) of X-ray pulsars.
-Magnetars, where the decay of a very powerful magnetic field accounts for the electromagnetic power of the magnetar pulsar.
Even though all three known classes of pulsars are all baby neutron stars, wildly twirling in the exuberance of their infancy, their observable behavior and the underlying physics are very different.
The theory that defines pulsars as rapidly rotating baby neutron stars is widely accepted. However, not everyone is in agreement. As Dr. Werner Becker, of the Max Planck Institute for Extraterrestrial Physics in Germany, noted in 2006, “The theory of how pulsars emit their radiation is still in its infancy, even after nearly forty years of work.”
Oddball Pulsars: First They Appear And Then They’re Gone
The oddball intermittent pulsars that perform a weird “cosmic vanishing act” represent an unusual and rarely observed population of pulsars, which show two states–one when they pulse like normal pulsars (ON state), and the other when they mysteriously have gone missing, and emit no radio waves whatsoever (OFF state). “They switch instantaneously between the states. They’re ON and then they’re gone, disappearing without any apparent warning,” Dr. Lyne commented in a January 4, 2017 Universities Space Research Association (USRA) Press Release. The USRA is in Columbia, Maryland.
A pulsar study team, consisting of 34 members, used the 7-beam receiver at Arecibo to go on a routine treasure hunt for pulsars in what they refer to as the Pulsar Arecibo L-Band Feed Array (PALFA) Survey. The study team determined that the mysterious duo of recently discovered oddball intermittent pulsars spend the greater part of their existence in the OFF state. A trio of other similar pulsars are also known but, in marked contrast, are mostly ON.
In September 2012, one of the newly detected pulsar oddballs was discovered as it was sending forth unusually brilliant pulses, and it was designated PSR J1929+1357. Of the 169 new pulsars found in this survey, follow-up observations of 50 % of those had been started at Jodrell Bank and this especially bright candidate was confirmed as a pulsar in February 2013 using Jodrell Bank’s 250-foot Lovell Telescope for the second attempt. “During the next 9 months it was observed no fewer than 650 times–100 hours. It was ON on only 5 occasions–just 0.8% of the time,” explained Dr. Benjamin Stappers in the January 4, 2017 USRA Press Release. Dr. Stappers is of the Jodrell Bank Center for Astrophysics, and a co-author of the research paper describing this discovery.
The most important implication of this study is that the small number of oddball intermittent pulsars, that have been discovered, must represent only the tip of a very large iceberg. In fact, there is the inescapable suggestion that there is an extremely large number of these “vanishing act” pulsars performing their mysterious disappearing act throughout the Universe. The PALFA survey, which is focused on a region of the Milky Way that is visible to the Arecibo radio dish, only covers each position in the survey once. It likely missed more than 130 similar “vanishing act” pulsars–and the one that was discovered just happened to be the only oddball that happened to be ON at the time of observation. In addition, if it were not for the early signals at Jodrell Bank, this oddball pulsar could very easily have been dismissed as a false detection–probably resulting from radio-frequency interference. The PALFA team calculates that there are probably about 3,000 such “vanishing act” pulsars lurking in the survey area that are still unseen–a number that is far greater than the population of “normal” pulsars.
“These disappearing pulsars may far outnumber normal pulsars. In fact, they may redefine what we think of as normal,” Dr. Victoria Kaspi noted in the January 4, 2017 USRA Press Release. Dr. Kaspi is of Mcgill University in Canada and the principal investigator on the PALFA project.
But what accounts for the unusual behavior of these oddball pulsars? Ever since the first pulsars were discovered back in 1967, astronomers have marveled at their remarkable lighthouse-like precision. Indeed, ever since their discovery fifty years ago, pulsars have served as accurate cosmic clocks that tick regularly for millions of years–and their regularity surpasses the ticking of the best laboratory clocks on Earth. However, these mysterious and rare long-term intermittent pulsars are usually invisible. This makes them about as useful as an old alarm clock, kept well-hidden in a box up in the attic, that is covered with layers of accumulated dust. “The explanation of the ON-OFF remains a puzzle. It indicates that the pulsar environment is changing, but just what those changes entail is open to debate,noted Dr. Andrew Seymour in the January 4, 2017 USRA Press Release. Dr. Seymour is a USRA postdoc at Arecibo.
Recent observations of these “vanishing act” pulsars apparently indicate that their rotational slow-down rate when OFF is only about 80 percent of the rate when ON. One of the attributes of so-called “normal” pulsars is their pulse rate only slows down very gradually with the passage of time. The PALFA team is considering the possibility that the flow of charged particles, which drive the radio beams traveling from the pulsar, also causes the pulsar to spin down much more rapidly than its more “normal” kin. When OFF, the stream of particles for some reason fails–and, as a result, the spin rate slows down. However, as Dr. Seymour notes in the January 4, 2017 USRA Press Release, there is currently no agreement among astronomers in respect to what triggers the ON-OFF mechanism.
The altering spin rate can be calculated by the number of beats missed during the pulsar’s invisible phases.
PALFA surveys are ongoing, and no one is able to predict if and when more examples of this interesting newly recognized phenomenon will be discovered. Catching another intermittent pulsar in its ON phase is purely a matter of good luck. Is there another invisible pulsar lurking out there ready to unveil its strange long-hidden face–or will it lurk forever swathed in the secretive darkness of the unexplained mysteries of the Universe?
Dr. Lyne hopes that future measurements of PSR J1929+1357 will provide a rare peek into the mysterious physics of the pulsar emission mechanism and the ever-changing spin-down phenomenon.
Dr. Victoria Kaspi presented the results of this study on January 4, 2017, at a press conference held at the American Astronomical Society’s winter meeting in Grapevine, Texas.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. Although she has written on a variety of topics she particularly loves writing about astronomy, because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.