Sunday, July 10, 2016

Supernovae & Magnetars that Collapse into Black Holes

Figure 1: Artist’s impression of a neutron star.

When a massive star undergoes core collapse in a supernova explosion, the collapsed core of the star can form a rapidly rotating, strongly magnetized neutron star. Such a neutron star can be referred to as a “millisecond magnetar". The tremendous reservoir of rotational energy possessed by the millisecond magnetar can be rapidly released through electromagnetic dipole spin-down of the millisecond magnetar. This can greatly energise and boost the luminosity of the supernova explosion, potentially resulting in a superluminous supernova explosion.

Furthermore, a millisecond magnetar formed following core collapse of a massive star can have a large enough mass to be classified as a supramassive neutron star. Such an overly-massive neutron star is supported against gravitational collapse by its rapid rotation and it is only temporarily stable. As the supramassive neutron star, in the form of a millisecond magnetar, loses rotational energy though electromagnetic dipole spin-down, it can collapse and transform into a black hole. The formation of a black hole results in the sudden loss of the central engine (i.e. the millisecond magnetar) to energise the supernova explosion.

Figure 2: Light curves of supernovae explosions powered by magnetar spin-down. The black hole transformation time ranges from 0.25 to 7.5 days. This is basically the time required for a magnetar to spin down sufficiently and collapse into a black hole. The dotted line is the input magnetar spin-down energy without black hole formation. The magnetar-driven break out bump is more prominent in the light curve in cases of early black hole formation. Moriya et al. (2016)

Moriya et al. (2016) modelled the light curves of supernovae explosions powered by the spin-down of millisecond magnetars that eventually collapse to form black holes. Although the light curves can reach peak luminosities that are within the regime of superluminous supernovae explosions, they decline very rapidly after peaking because of the abrupt loss of the central engine. Basically, the light curve of a supernova explosion powered by the spin-down of a supramassive neutron star which then collapses into a black hole declines much quicker than the light curve of a supernova explosion powered by the spin-down of an indefinitely stable, lower-mass magnetar.

A notable feature in these light curves is the presence of a magnetar-driven breakout bump. This "bump" in the light curve is basically energy released from the spin-down of the millisecond magnetar. The magnetar-driven breakout bump is more noticeable when the black hole formation occurs early because the sudden cessation of energy input from the abrupt lost of the central engine allows the breakout bump to shine more prominently on its own.

Moriya et al. (2016), "Supernovae powered by magnetars that transform into black holes", arXiv:1606.09316 [astro-ph.HE]