Failed supernova

A failed supernova is an astronomical event in time domain astronomy in which a star suddenly brightens as in the early stage of a supernova, but then does not increase to the massive flux of a supernova. They could be counted as a subcategory of supernova imposters. They have sometimes misleadingly been called unnovae.^{[1][failed verification]}

Overview

Failed supernovae are thought to create stellar black holes by the collapsing of a red supergiant star in the early stages of a supernova. When the star can no longer support itself, the core collapses completely, forming a stellar-mass black hole, and consuming the nascent supernova without having the massive explosion. For a distant observer, the red supergiant star will seem to wink out of existence with little or no flare-up. The observed instances of these disappearances seem to involve supergiant stars with masses above 17 solar masses.

Failed supernovae are one of several events that theoretically signal the advent of a black hole born from an extremely massive star, others including hypernovae and long-duration gamma-ray bursts.

Structure and process

Theoretically, a red supergiant star may be too massive to explode into a supernova, and collapse directly into being a black hole, without the bright flash. They would however generate a burst of gravitational waves. This process would occur in the higher mass red supergiants, explaining the absence of observed supernovae with such progenitors.^[2][3][4]

List of failed supernovae candidates

Event	Date	Location	Notes
NGC3021-CANDIDATE-1		NGC 3021 09h 50m 55.39s +33° 33′ 14.5″	Disappearance of a 25-30 MSun F8 supergiant observed in archival HST data	[2][5]
N6946-BH1	March 2009	NGC 6946 20h 35m 27.56s +60° 08′ 08.2″	Disappearance of an 18-25 MSun red supergiant	[2][6][4]