Slowly fading super-luminous supernovae that are not pair-instability explosions

Abstract: Super-luminous supernovae that radiate more than 10^44 ergs per second at their peak luminosity have recently been discovered in faint galaxies at redshifts of 0.1-4. Some evolve slowly, resembling models of 'pair-instability' supernovae. Such models involve stars with original masses 140-260 times that of the Sun that now have carbon-oxygen cores of 65-30 solar masses. In these stars, the photons that prevent gravitational collapse are converted to electron-positron pairs, causing rapid contraction and thermonuclear explosions. Many solar masses of 56Ni are synthesized; this isotope decays to 56Fe via 56Co, powering bright light curves. Such massive progenitors are expected to have formed from metal-poor gas in the early Universe. Recently, supernova 2007bi in a galaxy at redshift 0.127 (about 12 billion years after the Big Bang) with a metallicity one-third that of the Sun was observed to look like a fading pair-instability supernova. Here we report observations of two slow-to-fade super-luminous supernovae that show relatively fast rise times and blue colours, which are incompatible with pair-instability models. Their late-time light-curve and spectral similarities to supernova 2007bi call the nature of that event into question. Our early spectra closely resemble typical fast-declining super-luminous supernovae, which are not powered by radioactivity. Modelling our observations with 10-16 solar masses of magnetar-energized ejecta demonstrates the possibility of a common explosion mechanism. The lack of unambiguous nearby pair-instability events suggests that their local rate of occurrence is less than 6x10^-6 times that of the core-collapse rate.

• The paper demonstrates that the light-curve evolution of selected SLSNe deviates from PISN predictions with rise times nearly twice as fast.
• The study provides detailed spectral analysis revealing blue spectra and the absence of iron group suppression typical of pair-instability explosions.
• The findings advocate a magnetar-powered mechanism with 10–16 solar masses of ejecta, offering a viable alternative for explaining these supernovae.

Slowly Fading Super-Luminous Supernovae That Are Not Pair-Instability Explosions

The study conducted by Nicholl et al., extensively explores the characteristics and underlying mechanisms of slowly fading super-luminous supernovae (SLSNe), examining their deviation from the traditional pair-instability supernova (PISN) models. This paper investigates the nature of two such events, PTF12dam and PS1-11ap, emphasizing their distinctive characteristics and providing implications for their theoretical classification.

The research scrutinizes the slowly evolving light curves and spectra of these supernovae, noting that they display relatively fast rise times and blue spectra that contrast markedly with the predictions of PISN models. The critical observation that differentiates these supernovae from PISN is their light-curve behavior, exhibiting rise times approximately twice as quick as typical PISNs. Furthermore, the spectral analysis indicates a lack of the expected redshift due to iron group element blanketing, which is characteristic of pair-instability events.

The authors propose that these observations are inconsistent with PISN models, which traditionally involve high-mass progenitor stars undergoing thermonuclear explosions initiated by photon pair-creation. Instead, they suggest a magnetar-powered mechanism as a more viable explanation. The model suggests that 10–16 solar masses of ejecta energised by a magnetar—a young, highly magnetic, rapidly spinning neutron star—can replicate the observed luminosity and spectral features. This model provides cohesiveness in explaining the photometric and spectroscopic properties of these phenomena without requiring the large nickel masses or ejected volumes predicted by PISNs.

The research also addresses the rate of occurrence for these phenomena, indicating an upper limit significantly lower than that expected for pair-instability supernovae. This conclusion stems from the lack of unequivocally observed PISNs at redshifts below two in current data, suggesting their scarcity in the local universe. The study further compares high redshift candidates where UV-optical data leaves room for PISN classification, noting the limitations in confirming such events without rest-frame optical observations.

Implications for future observations and theoretical studies are significant. The evidence provided against PISNs invites a reconsideration of prior classifications such as SN2007bi, which were initially thought to align with pair-instability models. This work underscores the potential for magnetar models to account for a wider variety of superluminous supernovae than previously considered. Additionally, the constraints outlined could influence the understanding of cosmic chemical evolution, as the diminished role of PISNs lessens their anticipated impact on the enrichment of early universe elements.

The pursuit of deeper knowledge into SLSNe continues to be pivotal as it fosters an enhanced comprehension of stellar evolution processes, playing an essential role in the broader cosmic context. Future observational campaigns and theoretical developments should continue to refine these models, expanding our understanding of the unique and enigmatic phenomena of super-luminous supernovae.