Supernova Light Curves Powered by Fallback Accretion

Abstract: Some fraction of the material ejected in a core collapse supernova explosion may remain bound to the compact remnant, and eventually turn around and fall back. We show that the late time (> days) power associated with the accretion of this "fallback" material may significantly affect the optical light curve, in some cases producing super-luminous or otherwise peculiar supernovae. We use spherically symmetric hydrodynamical models to estimate the accretion rate at late times for a range of progenitor masses and radii and explosion energies. The accretion rate onto the proto-neutron star or black hole decreases as Mdot ~ t^-5/3 at late times, but its normalization can be significantly enhanced at low explosion energies, in very massive stars, or if a strong reverse shock wave forms at the helium/hydrogen interface in the progenitor. If the resulting super-Eddington accretion drives an outflow which thermalizes in the outgoing ejecta, the supernova debris will be re-energized at a time when photons can diffuse out efficiently. The resulting light curves are different and more diverse than previous fallback supernova models which ignored the input of accretion power and produced short-lived, dim transients. The possible outcomes when fallback accretion power is significant include super-luminous (> 10^44 ergs / s) Type II events of both short and long durations, as well as luminous Type I events from compact stars that may have experienced significant mass loss. Accretion power may unbind the remaining infalling material, causing a sudden decrease in the brightness of some long duration Type II events. This scenario may be relevant for explaining some of the recently discovered classes of peculiar and rare supernovae.

• The paper demonstrates that fallback accretion can power supernova light curves and challenges traditional radioactive decay models.
• It employs spherically symmetric hydrodynamical simulations, showing a t^(-5/3) decline in accretion rates that enhances late-time luminosity.
• The study predicts diverse outcomes, including super-luminous events and sudden brightness drops, urging revised analyses of supernova remnants.

Supernova Light Curves Powered by Fallback Accretion

The paper "Supernova Light Curves Powered by Fallback Accretion" by Jason Dexter and Daniel Kasen investigates a novel mechanism to explain optical light curves of supernovae by considering the power from fallback accretion onto compact remnants. Using spherically symmetric hydrodynamical models, the researchers elucidate how the accretion of material that remains bound following a core-collapse supernova explosion can significantly impact the resultant luminosity of the event.

Key Findings and Numerical Results

The study emphasizes that the power associated with fallback accretion arises at late times—days to weeks after the explosion. This delayed energy input is crucial as it occurs when photons can diffuse efficiently through the ejecta. Significant energy augmentation is noted in scenarios with low explosion energies, highly massive progenitors, or stars where a strong reverse shock forms at the helium/hydrogen interface.

Numerically, the accretion rate onto the compact object, either a proto-neutron star or black hole, decreases proportionally to $t^{-5/3}$ at late times. However, the normalization of the accretion rate shows a considerable enhancement in particular cases, such as those involving substantial reverse shock effects.

The simulations predict diverse outcomes for supernova light curves, including super-luminous events with luminosities exceeding $10^{44} \, \text{ergs/s}$, and varying durations. The paper further highlights the potential for luminous Type I events stemming from compact stars experiencing significant mass loss.

Bold and Contradictory Claims

The primary claim contradicts standard radioactive decay models by proposing fallback accretion as an alternative power source for certain supernovae. This model challenges the notion that post-explosion interaction with circumstellar material or magnetar spin-down are the sole mechanisms behind super-luminous supernova emissions. Intriguingly, fallback accretion could also lead to sudden decreases in brightness during long-duration Type II events—an effect not typically associated with other supernova models.

Implications and Future Directions

The theoretical implications are profound, as fallback accretion presents a viable explanation for peculiar and rare supernova types discovered in recent optical surveys. Practically, this mechanism might necessitate revisions in how supernova remnants are analyzed, impacting our understanding of remnant mass distribution and potentially influencing pulsar formation theories.

Looking forward, the study calls for more detailed modeling to disentangle fallback-powered events from other supernova types. Observational strategies should focus on identifying the characteristic light curve features predicted by this model, such as late-time luminosity tails with a distinct $t^{-5/3}$ decline and abrupt brightness cutoffs.

In summary, Dexter and Kasen provide a compelling analysis that broadens the scope of possible supernova phenomena through fallback accretion, offering novel insights into both the power sources and dynamics of stellar explosions. Their findings support ongoing discussions and investigations into the complexities of supernova light curves and their underlying mechanisms.