The sudden slowing of pulses coming from a spinning neutron star defy explanation — and might require a rethink of the universe’s most exotic denizens.
Pulsars are one of the most precise timepieces in the universe, rivaling the best of humankind’s inventions. As wristwatches go, they would certainly attract attention: the 12-mile-wide spinning neutron stars have masses up to twice the Sun’s. Beams of radiation shoot out along their magnetic poles, sweeping past Earth like cosmic lighthouses. The pulses are so regular that astronomers first joked they might be a sign of life — now pulsars set some of the world’s most accurate clocks, such as that in St. Catherine’s Church in Gdansk, Poland.
But every now and then, a pulsar suffers a glitch, a sudden uptick in the lighthouse’s spin. These glitches act like windows into neutron stars’ exotic structure, something astronomers thought they more-or-less understood. But Robert Archibald and Victoria Kaspi (McGill University, Canada) led a team reporting a new observation today in the journal Nature that requires a complete rethink of how these objects work.
Peeking Inside a Neutron Star
Neutron stars lie entirely outside the realm of human experience. These stellar corpses squeeze a Sun’s worth of mass into a ball the size of Manhattan. A thin, solid crust about half a mile thick covers a core of nuclear fluid, where neutrons outnumber protons 20 to 1. Theory says the outer crust is made of iron atoms arranged in a crystal lattice. At the crust’s base, densities climb to such levels that the iron lattice melts into a superfluid, a fluid with frictionless flow. Physicists on Earth have to cool liquid helium to temperatures just above absolute zero to create the superfluid state, but neutron stars’ superfluids exist among temperatures soaring to 100 million degrees Celsius.
A neutron star’s superfluid doesn’t spin in bulk, the way that the solid outer crust does. Instead, it spins around many tiny vortices like the ones that form when a bathtub drains. The vortices pin themselves to nuclei in the superfluid, trapping angular momentum in the inner crust. So the solid outer crust spins on top of the vortex-filled superfluid, and while the outer crust’s spin slows over time, the superfluid doesn’t.
But when occasional starquakes rock the crust, the vortices are unpinned, releasing angular momentum into the outer crust. The outer crust spins up, and the pulses received at Earth suddenly increase. That spin-up is what astronomers have called a “glitch.”
Glitch Theory, Revisited
At least, that’s how it’s supposed to work. But the new study reports just the opposite: a sudden slowdown.
The team was using the Swift space telescope to monitor 1E 2259+586, a neutron star 10,000 light-years away in Cassiopeia hosting an intense magnetic field 5.9 billion Tesla strong. (A manmade magnet of just 100 Tesla screams like a banshee, which makes me grateful that space has no sound.)
During nearly a year of observing the magnetar every 2 to 3 weeks, the team watched the magnetar spin once roughly every seven seconds, with a gradual, expected slowdown. Then on April 28, 2012, the pulse rate suddenly dipped by 2.2 millionths of a second, heralded by a blast of X-rays. The dip lasted for a few weeks, until the pulse rate suddenly ticked up again. After all the weirdness, the magnetar is still spinning roughly every 7 seconds, but its spindown rate is now faster than it used to be.
Theory can’t explain what the astronomers saw — glitches should always spin up a neutron star, never down, and it’s not clear why the long-term spindown rate would have changed.
“This ‘anti-glitch’ is a very interesting result . . . An anti-glitch has not been observed before,” notes Bennett Link (Montana State University), one of glitch theory’s major proponents. “My thinking on this is that this anti-glitch is caused by different physics.”
Since glitches are one of astronomers’ only ways of peeling back the surface of a neutron star, the discovery of this new kind of glitch could reveal secrets of neutron star structure.