Neutron-star glitches, superfluidity and cooling

5 problems

• Is the Cassiopeia A neutron star cooling unusually fast?

  Contested

  The young neutron star in the Cassiopeia A remnant appeared to cool by a few percent in ten years, much faster than expected. This was explained as neutrons becoming superfluid, but the measurement depends on instrument calibration.

• Is the crust superfluid enough to power glitches with entrainment included?

  Contested

  Glitches need a reservoir of superfluid holding at least about 1.6 percent of the star's spin inertia. Calculations suggest the crust lattice drags on the superfluid neutrons (entrainment), which may leave the crust too small a reservoir.

• How large are neutron-star mountains, given no continuous gravitational waves?

  Open

  A spinning neutron star with a tiny bump on its crust would emit a steady gravitational-wave hum. None has been detected, and it is not known how large a bump the crust can support or how large real bumps are.

• What triggers pulsar glitches?

  Open

  Pulsars, spinning neutron stars, sometimes suddenly spin faster in a glitch. The superfluid inside is thought to dump its spin into the crust, but what starts the event, a cascade of released vortices (tiny whirlpools) or a crack in the crust, is unknown.

• Why do fast-spinning accreting neutron stars evade the r-mode instability?

  Open

  Theory says certain sloshing oscillations called r-modes should grow on their own in fast-spinning neutron stars, radiate gravitational waves and slow the stars down. Yet many observed stars spin fast in the region where this should happen.