The search for failed supernovae with the Large Binocular Telescope: confirmation of a disappearing star

Abstract: We present Hubble Space Telescope imaging confirming the optical disappearance of the failed supernova (SN) candidate identified by Gerke et al. (2015). This $\sim 25~M_{\odot}$ red supergiant experienced a weak $\sim 10^{6}~L_{\odot}$ optical outburst in 2009 and is now at least 5 magnitudes fainter than the progenitor in the optical. The mid-IR flux has slowly decreased to the lowest levels since the first measurements in 2004. There is faint ($2000-3000~L_{\odot}$) near-IR emission likely associated with the source. We find the late-time evolution of the source to be inconsistent with obscuration from an ejected, dusty shell. Models of the spectral energy distribution indicate that the remaining bolometric luminosity is $>6$ times fainter than that of the progenitor and is decreasing as $\sim t^{-4/3}$. We conclude that the transient is unlikely to be a SN impostor or stellar merger. The event is consistent with the ejection of the envelope of a red supergiant in a failed SN and the late-time emission could be powered by fallback accretion onto a newly-formed black hole. Future IR and X-ray observations are needed to confirm this interpretation of the fate for the star.

• The paper presents observational evidence from LBT, HST, and SST confirming the optical disappearance of a massive star, supporting the failed supernova hypothesis.
• The study utilized multi-wavelength observations to track the star's significant optical decline and subsequent changes in near and mid-infrared luminosity.
• Quantitative analysis shows the transient is significantly fainter than expected for standard events, providing evidence for non-explosive black hole formation instead of a typical supernova.

An Analysis of "The Search for Failed Supernovae with the Large Binocular Telescope: Confirmation of a Disappearing Star"

The research presented by Adams et al. aims to provide evidence for a failed supernova event, specifically detailing the observed optical disappearance of a massive star candidate using data collected from the Large Binocular Telescope (LBT), Hubble Space Telescope (HST), and Spitzer Space Telescope (SST). The study is deeply embedded in the framework of addressing the phenomenon whereby some massive stars, instead of undergoing typical supernova explosions, collapse into black holes without a significant explosive optical signature. This inquiry is motivated by existing theoretical expectations and some indirect observational evidence suggesting the presence of such events.

Observational Evidence and Methodology

The study focused extensively on observations collected across different wavelengths and utilizing a variety of equipment, including the LBT, HST, and SST, to continuously monitor several stars with the goal of identifying signatures of failed supernovae. A critical component of this study was the confirmation of the optical disappearance of a red supergiant progenitor, approximately 25 solar masses, identified in NGC 6946. The evidence for its disappearance included a well-documented series of observational sessions: pre-outburst visibility followed by a significant optical decline, alongside near and mid-infrared observation of luminosity changes.

Results and Numerical Claims

The research presents compelling quantitative evidence for the case of a failed supernova. It reports a transient that was approximately 5 magnitudes fainter than the progenitor's optical visibility, following a weak outburst with a primary optical luminosity around $10^6 L_{\odot}$. Following this event, infrared emissions decreased to historically low levels since initial measurements. The paper quantitatively argues against the possibility of standard obstructive transient phenomena, such as SN impostors, by demonstrating that the late-time luminosity is less than 6 times the progenitor's bolometric luminosity and is on a downswing, following a power-law decay ($\sim t^{-4/3}$).

Theoretical and Practical Implications

The findings have significant implications for both theoretical models of stellar evolution and practical astronomical surveys. They bolster the theoretical framework suggesting that some red supergiants form black holes without a typical supernova event. This has the potential to adjust our understanding of stellar death and black hole formation pathways. Furthermore, this research elucidates methodological approaches necessary for detecting failed supernovae, setting a precedent for future large-scale surveys targeting this phenomenon.

In terms of the broader landscape of stellar astrophysics, confirming failed supernovae extends implications for the understanding of the missing mass problem in massive star deaths and the apparent frequency mismatch between star formation rates and observed supernovae occurrences.

Prospects for Future Research

Continued observation of N6946-BH1 and similar stars identified in surveys can potentially yield further insights. Specifically, the pursuit of additional infrared and x-ray observations could substantiate the fallback accretion theory posited for powering the late-time emissions detected. Enhancing survey capabilities and improving techniques for distinguishing failed supernovae from other astrophysical transients remain significant areas for advancement.

Conclusion

This study enriches the evidence for failed supernovae by meticulously documenting the optical disappearance of a red supergiant and providing an observational cornerstone for confirming such theoretical models. The methodologies refined and elaborated by Adams et al. in identifying these elusive events offer a promising path for both understanding stellar endpoints and unveiling the complexities of black hole formation amidst the cosmos. Moving forward, this paper lays a substantive groundwork for refining theoretical models that account for massive star deaths beyond the standard supernova paradigm.