An extremely energetic supernova from a very massive star in a dense medium

Abstract: The interaction of a supernova with a circumstellar medium (CSM) can dramatically increase the emitted luminosity by converting kinetic energy to thermal energy. In 'superluminous' supernovae (SLSNe) of Type IIn -- named for narrow hydrogen lines in their spectra -- the integrated emission can reach $\sim 10^{51}$ erg, attainable by thermalising most of the kinetic energy of a conventional SN. A few transients in the centres of active galaxies have shown similar spectra and even larger energies, but are difficult to distinguish from accretion onto the supermassive black hole. Here we present a new event, SN2016aps, offset from the centre of a low-mass galaxy, that radiated $\gtrsim 5 \times 10^{51}$ erg, necessitating a hyper-energetic supernova explosion. We find a total (SN ejecta $+$ CSM) mass likely exceeding 50-100 M$\odot$, with energy $\gtrsim 10^{52}$ erg, consistent with some models of pair-instability supernovae (PISNe) or pulsational PISNe -- theoretically-predicted thermonuclear explosions from helium cores $>50$ M$\odot$. Independent of the explosion mechanism, this event demonstrates the existence of extremely energetic stellar explosions, detectable at very high redshifts, and provides insight into dense CSM formation in the most massive stars.

• The paper demonstrates SN2016aps as an ultra-energetic supernova with radiated energy exceeding 5×10^51 erg, supporting theories like PISNe and PPISNe.
• The study employs extensive spectroscopy and photometry, showing that interaction with a dense circumstellar medium converts kinetic energy into thermal energy.
• Key analysis of a massive progenitor (70–140 M☉) and binary merger scenarios offers insights into detecting superluminous events at high redshifts.

An Analysis of an Extremely Energetic Supernova from a Very Massive Star in a Dense Medium

The paper under discussion presents an in-depth study of SN2016aps, an exceptionally powerful supernova, likely resulting from a very massive star interacting with a dense circumstellar medium (CSM). The observations led by a collaboration of researchers, including Matt Nicholl, Peter K. Blanchard, and others, focus on the spectral and photometric properties of this event, positioned outside the core of a low-mass galaxy.

Key Findings:

• Luminosity and Energy Metrics: SN2016aps exhibited radiated energy exceeding $5 \times 10^{51}$ erg, surpassing energy levels of any previously confirmed supernova (SN), with kinetic energy estimates suggesting $\gtrsim 10^{52}$ erg. This aligns with models positing pair-instability supernovae (PISNe) or pulsational pair-instability supernovae (PPISNe) as potential origins.
• Massive Progenitor and CSM Interaction: The estimated mass for the combined supernova ejecta and CSM was between $50-100 \, M_\odot$, indicating a hyper-energetic explosion involving a massive star around 70-140 $M_\odot$. A critical factor contributing to the superluminosity appears to be the conversion of kinetic energy into thermal energy due to interacting with the dense CSM.
• Astrophysical Implications: Such events challenge the conventional understanding of core-collapse mechanisms and suggest that alternative processes, like PISNe, may play a significant role. The luminous nature and interaction with a dense CSM imply that SN2016aps could be detectable at very high redshifts, offering a glimpse into the final stages of massive stellar life cycles across cosmic history.
• Rate and Formation of Progenitors: The paper explores rate calculations for such massive progenitor formations, emphasizing binary mergers as a plausible pathway for developing the necessary massive stellar cores. The interaction and pre-explosion scenarios for SN2016aps suggest a predecessor potentially experiencing rapid episodic mass ejections before the terminal explosion.
• Spectral Analysis and Observational Strategy: The detailed spectroscopic monitoring over 500 days post-discovery revealed Balmer emission lines, corroborating its classification as a superluminous supernova (SLSN) of Type IIn. Photometric datasets spanning 1000 days, including observations from Hubble Space Telescope imaging, firmly identified the host galaxy's properties—a critical factor in distinguishing SN2016aps from possible tidal disruption events in active galaxy nuclei.

Theoretical and Future Considerations:

The study of SN2016aps serves as a crucial testbed for supernova models incorporating CSM interaction, especially in contexts where extreme mass-loss rates are essential. The results of this study support theoretical models predicting such phenomena and open questions regarding the role of extreme stellar evolutions, binarity, and conditions conducive to pair-instability processes.

Future research, leveraging facilities like the Large Synoptic Survey Telescope or James Webb Space Telescope, may further unravel the prevalence and characteristics of such energetic explosions across different environments and epochs. In particular, extending spectroscopic classifications of high-redshift transients will be vital in mapping out the early universe's stellar demographics and explosion properties.

In conclusion, the analysis of SN2016aps enhances our comprehension of SLSNe and their potential progenitors, paving the way for further investigations into the complex interactions between massive stars and their CSMs in delivering such spectacular cosmic events.