Epoch J2000.0 Equinox J2000.0
|Right ascension||20h 35m 27.56s|
|Declination||+60° 08′ 08.29″|
|Apparent magnitude (V)||18.17 (max)|
|Variable type||suspected failed supernova|
|Luminosity||200,000 - ~1,000,000 L☉|
N6946-BH1 is a disappearing red supergiant star in another galaxy, NGC 6946, on the northern border of the constellation of Cygnus. The star was 25 times the mass of the sun, and was 20 million light years distant from Earth. In March through to May 2009 its bolometric luminosity increased to at least a million solar luminosities, but by 2015 it had disappeared from optical view. In the mid and near infrared an object is still visible, however, it is fading away with a brightness proportional to t−4/3. The brightening was insufficient to be a supernova, and is called a failed supernova.
The star's coordinates were at RA 20h 35m 27.56s and Dec +60° 08′ 08.29″. The brightness of the star, given by its apparent magnitude in different colour bands on 2 July 2005 is given by R = 21, V = 21, B = 22, U = 23. Prior to the optical outburst the star was about 100,000 times as bright as the sun. After the outburst it was invisible in the visual band and has declined to 5000 times as bright as the sun in infrared radiation.
One hypothesis is that the core of the star collapsed to form a black hole. The collapsing matter formed a burst of neutrinos that lowered the total mass of the star by a fraction of a percent. This caused a shock wave that blasted out the star's envelope to make it brighter. After the idea that a black holes are usually formed after a supernova, N6946-BH1 has given evidence that, instead of following this process, the star may automatically collapse into a black hole.
Observed type II supernovae do not originate from stars with initial masses greater than about 18 M☉, and the rate of large star formation appears to exceed the rate of supernovae. The expectation is that something else is happening to these extra large stars. Failed supernovae and black hole formation is one proposed explanation. If this event indeed reflected the formation of a black hole, it is the first time that black hole formation has been observed.
- Adams, S. M.; Kochanek, C. S; Gerke, J. R.; Stanek, K. Z.; Dai, X. (9 September 2016). "The search for failed supernovae with the Large Binocular Telescope: confirmation of a disappearing star". arXiv: . Bibcode:2017MNRAS.468.4968A. doi:10.1093/mnras/stx816.
- Gerke, J. R.; Kochanek, C. S.; Stanek, K. Z. (2015). "The search for failed supernovae with the Large Binocular Telescope: First candidates". Monthly Notices of the Royal Astronomical Society. 450 (3): 3289. arXiv: . Bibcode:2015MNRAS.450.3289G. doi:10.1093/mnras/stv776.
- Williams, Matt (16 September 2016). "Have we really just seen the birth of a black hole?". PhysOrg. Retrieved 16 September 2016.
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- Nowogrodzki, Anna (12 September 2016). "First glimpse of a black hole being born from a star's remains". New Scientist. Retrieved 17 September 2016.
- N6946-BH1 Giant Star Becomes A Black Hole Right Before Our Eyes!