Title: Properties and observability of glitches and anti-glitches in accreting pulsars
Authors: L. Ducci, P. M. Pizzochero, V. Doroshenko, A. Santangelo, S. Mereghetti, C. Ferrigno
First Author’s Institution: Instiut fur Astronomie und Astrophysik, Eberhard Karls Universitat, Tubingen, Germany
Status: Accepted for publication in Astronomy and Astrophysics
It’s nearly impossible to escape a clock. They’re on our phones, sit on our wrists, hang from our walls, glow in our cars, and tick in our computers. Less regular but nonetheless largely reliable biological clocks beat within our chests, growl in our middles, and cause our eyelids to droop at night. And predictably varying with month or season, the oceans rhythmically rise and fall, the Sun rises and sets, the Moon waxes and wanes. All mark the irreversible march of time and measure our steps into the unknown future.
To find the oldest and most reliable clocks, one must search far beyond the Earth and among the vast sprinkle of stars across the sky. Pulsars, spinning neutron stars with strong magnetic fields born in the deaths of massive stars, sweep radio (and sometimes X-ray) beams across the universe much like a lighthouse’s lamp, appearing to “blink” or “pulse” as their beam regularly sweeps in and out our view—like clockwork. The stars spin incredibly rapidly; the fastest pulsators, milisecond pulsars, can pulse almost 1000 times a second. If you were standing on the equator of the fastest known milisecond pulsar, PSR J1748-2446ad, you’d be whizzing around and around at 75,000 km/s (about 170 million miles per hour), or about a fourth of the speed of light. The slowest pulsar, PSR J2144-3933, completes a turn once every 8.5 seconds—still much faster than the Earth’s 24 hours! The timing of the pulses of some milisecond pulsars are so reliable that they best atomic clocks, are used to keep time, and have been proposed as the basis of relativistic positioning systems (the GPS analog for the Solar System).
The hermits among these dizzily-twirling stars are a fairly predictable lot, slowly spinning down as they radiate photons and possibly a gravitational wave or two or eject relativistic material. However, there are unpredictable members among them: some suddenly jump in spin speed, what astronomers call a “glitch.” The radio pulsars’ more companionable cousins, on the other hand, are a different story. Some spin up, others spin down or have constant spins, and most perplexingly of all, some randomly switch between spinning up and down. The culprits of their non-uniformity? Their companions. A normal main-sequence companion star, mutually locked into a binary orbit with a pulsar, can eventually balloon in size as it exhausts its hydrogen reserves, evolves off the main-sequence, and begins to burn heavier elements. The pulsar can siphon off part of the outer layers of its evolved companion and/or entraps its winds, then funnel the material by its magnetic fields onto its magnetic poles, where the material is accreted. As the material hits the pulsar’s surface, the material radiates its gravitational potential energy in highly energetic X-ray photons, hence the designation bequeathed to their class: X-ray pulsars. The angular momentum in the acquired material is not so easily lost, however; if assimilated, the pulsar’s spin-down can not only slow but reverse direction. Curiously, these accretion-powered pulsars, despite their unpredictable spin ups and downs, have rarely been observed to glitch.
What would it take to observe a glitching X-ray pulsar? This requires knowing what causes a pulsar to glitch, which is yet unclear, but likely lies in the bizarre physics of matter at high densities that are at play in a pulsar. A pulsar is an incredibly dense star, consisting of one to a few solar masses in a radius about a hundredth of the Sun’s at about 12 km—thus on average, roughly a million times more dense than the Sun! Such high densities cause the protons and electrons in the star to form a fluid in the densest part of the pulsar, its core. Moving outwards from the core, the pulsar becomes less and less dense, and these charged ions crystallize into a solid crust.
As the pulsar’s spin decreases (increases), it attempts to become less (more) oblate. The pulsar’s solid crust, however, resists the change. Over time, stress builds up in the crust, which is eventually released in a sudden shift—measureable in microseconds—to the preferred oblateness, a shift of micrometers (a tenth the width of a hair or less) resulting in a starquake and a jump upwards (downwards) in rotation speed. The authors predict a quake every 10^5 years, possibly longer, for four observed X-ray binaries (for crusts accreting material are thought to be more flexible and thus prone to shifts). Such long timescales means that starquake-induced glitches from accreting pulsars are rare and difficult to observe.
A second method of producing glitches lies in the behavior of the neutrons in the star. At the high densities in the pulsar, the neutrons are found in pairs and form an unusual state of matter called a superfluid. Unlike the charged ions, the superfluid resists spinning down (up), until the rotational speeds of the superfluid and the crust are significantly different, at which point the superfluid exchanges angular momentum with the ions, increasing the spin of the crust and causing a glitch. The authors calculate that such glitches occur every tens of years in accreting pulsars—much more frequently than starquake-induced glitches—and are thus more promising events to search for. In addition, the authors predicted that the glitches occur over hours rather than seconds and that the largest jumps in rotation speed were about comparable to isolated pulsars. They find that anti-glitches, in which a pulsar suddenly spins more slowly, would appear much like glitches in accreting pulsars, except that the maximum jump in spin could be smaller by a factor of 10.
Thus it appears promising that with frequent observations of X-ray pulsars, we will observe glitches in accreting pulsars soon. Such efforts are underway with existing X-ray telescopes onboard Fermi, Swift, and INTEGRAL, and are prime targets for new instruments such as LOFT.
For a detailed review of pulsar glitch models, check out this review by Haskell & Melatos 2015.