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Type II Supernova: Accretion Fallback Onto Magnetars

19/07/2011

Type II supernovae events are amongst the most energetic events in the Universe and represent the core-collapse of massive stars. The remaining by-products of such cataclysmic events are the most compact objects possible, either neutron stars…or denser still, black holes. A slightly more exotic class of neutron stars are highly magnetic neutron stars: “magnetars“.

Magnetars are perhaps the most magnetic objects physically possible, with dipole magnetic fields as strong as B ~ 1014 – 1015G (Duncan & Thompson 1992; Thompson & Duncan 1993). Although at an age of 1000−10000 years they have spin periods of P = 5 − 12 s, as measured from soft gamma-ray repeaters and anomalous X-ray pulsars, it is an outstanding question of how rapidly they rotate when first born. Short initial spin periods, typically of the order of a few milliseconds (P0 ~ 1−10 ms), have been favoured theoretically so that the dynamo process that creates these strong magnetic fields may operate efficiently (Akiyama et al. 2003; Thompson et al. 2005).

Motivated by this, many groups have investigated the possible impact of the spin-down of this newly formed magnetar in powering an explosion (e.g. Bodenheimer & Ostriker 1974; Thompson et al. 2004; Dessart et al. 2008).

The major assumption of all the above listed studies is that the supernova progenitor to the magnetar was successful in ejecting the majority of the progenitor star’s envelope. This is clearly correct in many cases, since we know that neutron stars with more modest magnetic fields (~1012G) are created in supernovae.

But surely it is possible that some subset of supernovae which produce neutron stars have small injected explosion energies? As is expected for massive stars that give rise to black holes, these would not be successful in ejecting the majority of the envelope and a sizable amount of fallback would occur (as found for ≈ 25−40 stars by Heger et al. 2003). Asymmetries in the supernova explosion may still perhaps result in fallback, perhaps even if the outer envelope is ejected. For these reasons, it is plausible that there exists a population of massive stars that give birth to magnetars that are subsequently subject to accretion of the envelope material.

Another motivation for studying fallback accretion onto magnetars is the presence of magnetars near clusters of massive stars. SGR 1806 − 20 and CXOU J164710.2 − 455216 are associated with the clusters Cl 1806 − 20 and Westerlund 1, respectively, and are inferred to have had progenitor masses of ≈ 40 (Figer et al. 2005; Bibby et al. 2008; Muno et al. 2006). Furthermore, the expanding HI shell around the magnetar 1E 1048.1 − 5937 also argues for a ≈ 30 − 40 progenitor (Gaensler et al. 2005).

Such massive stars are typically assumed to give rise to black holes (Fryer 1999; Heger et al. 2003), although we note that this will depend sensitively on the details of mass loss during stellar evolution (Smith et al 2010; O’Connor & Ott 2011) and on whether these magnetars have binary progenitors (Belczynski & Taam 2008). It is therefore, in my opinion, worth exploring whether the presence of a highly-magnetized neutron star qualitatively changes the outcome of the collapse of massive stars.

Journal References:

  • Akiyama, S. et al. (2003) The Magneto-Rotational Instability In Core-Collapse Supernova Explosions. The Astrophysical Journal, 584 (2) pp.954-970.
  • Belczynski, K. & Taam, R.E. (2008) The Most Massive Progenitors Of Neutron Stars: CXO J164710.2-455216. The Astrophysical Journal, 685 (1) pp.400-405.
  • D’Angelo, C.R. & Spruit, H.C. (2010) Episodic Accretion Onto Strongly Magnetic Stars. Monthly Notices Royal Astronomical Society, 406 (2) pp. 1208-1219.

Suggested Reading:

  • Duncan, R.C. & Thompson, C. (1992) Formation Of Very Strongly Magnetized Neutron StarsThe Astrophysical Journal392 (1) pp. L9-L13.
  • Metzger, B. D. et al. (2010) The Proto-Magnetar Model For GRBsMonthly Notices Royal Astronomical Society413 (3) pp. 2031-2056.
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