Saturday, October 26, 2013

Smashing onto a Magnetar

Neutron stars are very compact objects that form from the gravitational collapse of massive stars. A typical neutron star packs as much mass as half-million Earths within a diameter of only ~20 km. Magnetars are part of a very rare group of neutron stars that have extremely powerful magnetic fields. Occasionally, magnetars exhibit ‘glitches’ that are observed as sudden spin-ups of these compact objects. Glitches are believed to be caused by the sudden transfer of angular momentum from the faster rotating superfluid interior to the slower rotating solid outer crust of a magnetar.

Figure 1: Artist’s impression of a neutron star shown to scale with Manhattan Island. Credit: NASA.

In the May 30 issue of the journal Nature, R. F. Archibald et al. (2013) report the discovery of an ‘anti-glitch’ (i.e. a sudden spin-down) of the magnetar 1E 2259+586. This unexpected sudden spin-down is contrary to the spin-ups caused by glitches. Ordinarily, the magnetar has a spin period of 7 seconds, but the anti-glitch slowed its spin by 2 millionths of a second. In another paper by Huang and Geng (2013), the authors suggest that the sudden spin-down of 1E 2259+586 is caused by the collision of a solid object with the magnetar. The solid object came in from a direction that is opposite to the spin of the magnetar, collided with the magnetar and led to the sudden spin-up.

Observations of 1E 2259+586 reveal a decaying X-ray afterglow that is associated with the anti-glitch. Based on the energy released, the mass of the colliding solid object is estimated to be ~1/5,000,000th the mass of Earth. If the solid object is a dense iron-nickel body, it would have a diameter of 64 km. The impact of the solid object onto the surface of the magnetar is an incredibly violent process. As the solid object approaches, the immense gravity of the magnetar would stretch the object into an elongated shape before breaking it up entirely. Material from the destructed object is then compressed to ultra-high densities before slamming onto the surface of the magnetar. This process releases a huge amount of energy in a very short period of time. In fact, an intense burst of hard X-rays lasting 36 milliseconds was detected by the Fermi Gamma-ray Burst Monitor on 21 Aril 2012, consistant with the timing of the anti-glitch.

Figure 2: Artist’s impression of a magnetar. Credit: ESA.

Observations have already confirmed the existence of planetary systems around neutron stars. As a result, there are a number of ways in which a solid object, like an asteroid, can be placed on a collision trajectory with a neutron star. Firstly, the presence of planets can gravitationally perturb and scatter asteroids towards the central neutron star. Secondly, like for the Solar System, an extended cloud of small objects might also exist around the neutron star and some of them could fall towards the neutron star due to disturbances from nearby stars.

Thirdly, in a system with multiple planets, planets may collide, throwing off chunks of material where some would eventually impact the central neutron star. Lastly, the neutron star could be speeding on an escape trajectory out from its own planetary system due to the large velocity ‘kick’ it received at birth from asymmetric gravitational collapse. As the neutron star speeds through its planetary system, it can capture and ram into small objects that happen to lie in its path, resulting in the anti-glitch and X-ray burst observed for the magnetar 1E 2259+586.

- R. F. Archibald et al., “An anti-glitch in a magnetar”, Nature 497, 591-593 (30 May 2013)
- Y. F. Huang and J. J. Geng (12 October 2013), “Anti-glitch induced by collision of a solid body with the magnetar 1E 2259+586”, arXiv:1310.3324 [astro-ph.HE]