The Internal Engine Model

The assumed association of SLSN events with the deaths of massive stars naturally begins to draw parallels to better understood observations of massive stellar collapse; Long Gamma Ray Bursts (LGRBs). The current theory of LGRB production is

that, within massive (&40 M ) stars, a core of_⇠2 M of56_{Fe will form, which will}

collapse to form a black hole during the final moments of core collapse [Stevenson, 2014]. If the progenitor star was rapidly spinning, the internal stellar envelope begins to form an accretion disc, which starting at the polar regions, feeds the new central

�� ��

��

Figure 1.7: The pulsational pair instability CSM interaction model for SLSN pro- duction. Episodic mass loss is a result of an unstable CO core compressing (A) following spontaneous pair production, briefly igniting thermonuclear fusion within the core and releasing enough energy to expel a layer of hydrogen rich material from it’s outer envelope. This shell of material travels slowly outwards from the core, which regains stability (B). This cycle may repeat several times, creating successive shells of material from the envelope at large radii from the core. When the core finally collapses, the fast SN shockwave catches up with these slowly moving shells, interacting with them and creating a more luminous event (C).

black hole. This configuration then launches ultra-relativistic bipolar jets along the rotational axis (although the precise mechanism for how this occurs is currently not fully understood), which break through the remaining stellar envelope and emerge as beams of high energy radiation. Collapses such as this which are viewed along the beaming axis are observed as LGRB events. This is more commonly known as the ‘Collapsar Model’ [Woosley, 1993; MacFadyen and Woosley, 1999].

If the stellar remnant is capable of driving large amounts of energy (which we observe as GRB jets), it is possible that this energy could be captured by the SN event, serving as an additional power source to drive its lightcurve. Energy released from the internal engine acts to re-energise the outgoing shockwave from the SN, which in turn prolongs the lifetime of the transient whilst simultaneously providing

a substantial (factor _⇠100) boost to the transient luminosity [Kasen and Bildsten,

2010; Dexter and Kasen, 2013].

This possibility of an internal engine formed by the remnant of the collapsed core has great appeal for SLSNe, given our much sounder knowledge of the formation of LGRB events and their association with massive stars. However, although the principle of the internal engine model is similar for LGRB and SLSN events (the observed transient is powered by an additional energy source to a standard core

collapse), they di↵er primarily in the timescale over which this energy is deposited.

Within a GRB event, the relatively brief observed burst of gamma rays observed is powered via rapid accretion onto the newly formed black hole, which occurs on a

very short timescale [⇠10’s of seconds, MacFadyen and Woosley, 1999]. Although

adjustments to this model can be made by introducing a faster rotating progenitor (which in turn increased the fraction of the stellar envelope which may form a torus, which extends the period of accretion, Janiuk and Proga [2008]), the energy released is observed very shortly after the core collapse, delayed only by the time taken to break out of the stellar envelope [Bromberg et al., 2012].

In order for an internal engine to modify a SN event, the energy released must firstly occur at later times (as any energy released at early times is dissipated by the adiabatic expansion of the ejecta, [Kasen and Bildsten, 2010]), and be maintained over a much longer timescale (100’s of days), in order to match the observed light curves of SLSNe. The engine succeeds by driving outflows of energy along the polar axis from the core, which accelerate through the envelope until it collides with the outward moving supernova shock wave. As it does so, the ejected material is heated up, increasing the luminosity of the event [Kasen and Bildsten, 2010; Stevenson, 2014]. This heating continues while ever the central engine remains active, thus prolonging the transient.

Such an internal engine has two proposed di↵erent forms; accretion onto a central compact object, or the spin down of a rapidly rotating neutron star.

Black Hole Accretion

This model builds upon the ground work laid down by the LGRB collapsar model. Here too a massive iron core collapses to form a black hole, and it’s subsequent accretion launches jets along the polar axis [Woosley, 1993; MacFadyen and Woosley, 1999].

In this scenario, following core collapse some of the ejected material remains gravitationally bound to the core, and thus begins to fall inwards, until it encounters the newly formed black hole. This material is accreted at much later times (either because the progenitor has a particularly extended envelope, or because a reverse shock slows down the progress of inner ejecta layers Dexter and Kasen 2013), and unlike a GRB event, the outflows which result from this never break out through the stellar envelope. Instead they become trapped behind the SN ejecta, and thus begin to heat it up. This may potentially result in particularly luminous optical emission if the black hole releases significant energy on a long time scale (weeks-

months), comparable to the photon di↵usion time through the ejecta [Dexter and

Kasen, 2013].

Magnetar Spin Down

If a rapidly rotating iron core collapses into a neutron star, angular momentum is conserved, such that the newly formed remnant is rotating with a period of mil- liseconds, inducing a strong magnetic field. Neutron stars with unusually strong

magnetic fields (>1014 _{Gauss), are more commonly known as magnetars.}

The intense magnetic field begins to interact with the surrounding stellar material. This generates a powerful electric field within the hot gas, which in turn

forms a counteracting magnetic field. These opposing fields e↵ectively act as a brake

to the magnetar spin period, and as it loses energy it begins to severely heat up the internal envelope, generating a rapidly expanding bubble of hot plasma. These plasma bubbles move out along the magnetic poles of the magnetar, moving more quickly than the expanding SN shock wave until it catches up with the outer shells of moving material. As the plasma bubbles collide with the outer moving material, they cause it to heat up, generating additional luminosity. In theory, this should produce a rebrightening ‘bump’ within the SN light curve.

Figure 1.8: The magnetar internal engine model for SLSN production. The rapidly rotating central magnetar induces strong magnetic breaking as it’s magnetic field interacts with the surrounding envelope. Lost angular momentum is converted into heat, which drives strong plumes of plasma out along the magnetic poles. When this plasma reaches the SN, it begins to heat up the ejected material, creating additional luminosity. As the magnetar continues to spin, it continues to drive these plasma bubbles, in turn generating a longer and brighter SN event.

spin down [Bucciantini et al., 2009], however the magnetars required to do so must have much stronger magnetic fields (10 - 100 times stronger than those required for SLSN events) and spin down quickly (within a few 100 seconds) so that the release of energy is much more rapid, which allows the jet of energy to break through the stellar envelope [Metzger et al., 2015].

The remnant left from such an event would be a slowly rotating magnetar with a period of only a few seconds, and although such magnetars have been observed within the Milky Way and nearby galaxies [e.g. Duncan and Thompson, 1992], the remnants left from GRB or SLSN events would be more luminous than these known Galactic magnetars, with longer spin periods [Rea et al., 2015], and as such appear

to have di↵erent origins.