Many supernovae are the result of massive stars no longer being able to support themselves against gravity, through fusion or electron degeneracy pressure. Eventually, in stars of anywhere from eight to over one hundred solar masses, the outer layers of the star begin to collapse inwards. The very inner layers — in some cases — form a core mostly composed of neutrons; infalling material hits this core and “bounces” off the inner layers. This material is then propelled to high speeds by neutrinos produced via electron capture, and a substantial amount of energy is released in the form of neutrinos and photons. In very massive stars, the core collapses even further, to form a black hole.
One massive star, N6946-BH1, attracted attention when it flared up in 2009, increasing its luminosity by a factor of ten, to one million times that of the Sun, before dimming over several years until it was five times dimmer than in its original state. The event was too low-energy to be a supernova, yet it was clear that the star was gone. So what happened to it?
The red supergiant problem and failed supernovae
Back in 2008, months before the anticlimactic disappearance of N6946-BH1, a team of astronomers led by Stephen Smartt (Smartt et al. (2009)) drew attention to what they termed the “red supergiant problem”. Red supergiants — extremely luminous red stars, like N6964-BH1 — had been observed to have masses up to about 25 times that of the Sun, but no red supergiants of 16–30 solar masses had been determined to be supernova progenitors. The observations seemed in conflict with stellar evolutionary theory.
Smartt et al., as well as other groups, considered several possibilities, most involving high mass loss rates via stellar winds or extreme dimming (a form of astronomical extinction) by dust from those same winds. However, some scenarios not involving winds are equally plausible. One popular proposal is the idea of direct collapse to a black hole through something called a failed supernova, where a red supergiant loses mass via neutrino emission during core collapse, detaching the loose envelope and substantially reducing the luminosity of the event.
Lovegrove & Woosley (2013) performed some interesting simulations of red supergiants about to undergo core collapse. Their numerical codes confirmed prior hypotheses that when such a star begins to undergo a supernova, the collapsing core could emit a large amount of neutrinos — so many, in fact, that it loses up to half a solar mass. The outer layers — which, in red supergiants, are loosely bound and are often lost — are expelled as the force of gravity on them quickly decreases, propelled by a small shock wave.
While most of the star collapses into a black hole with comparatively little fanfare, there should still be a sign of a failed supernova. Electrons and protons in the ejected layer recombine into atoms, releasing energy and causing the ejecta to rise in luminosity to one to ten million times that of the Sun, at optical wavelengths. Over about a year, this signal should gradually fade, until the object is very dim.
Searching for failed supernovae
One million solar luminosities may seem like a lot, but it really isn’t, for this sort of event. Luminosities of one billion solar luminosities or more are common from Type II core collapse supernovae. Even some stars — supergiants and hypergiants — can exceed several million solar luminosities in brightness. Therefore, the comparatively dim death of N6946-BH1 was notable; it was much fainters than supernova.
Teams of astronomers had surveyed stellar populations for years to try and find failed supernovae. One group used the Large Binocular Telescope in Arizona (Gerke et al. (2015)), as well as archival images from the Hubble Space Telescope and the Spitzer Space Telescope. They scanned 27 galaxies relatively near the Milky Way, most over a period of over four years. They accumulated a list of targets with changes of luminosity greater than ten thousand solar luminosities, then used an algorithm to find a specific subset with luminosity changes corresponding to optical transients of three months to three years — as would be expected in a failed supernova. Manual examination removed false positives.
The authors found four interesting candidates, but discarded three. Of those three, two showed contamination from nearby stars, and appeared to brighten and dim over a period. This variability was inconsistent with models of a failed supernova. Another candidate was removed because even though it showed the expected dimming, it suddenly reappeared three years later — again, not the signature of a failed supernova.
The final candidate was the object now known as N6946-BH1, which, as I discussed before, showed the characteristic signs the teams were looking for. Models based on the photometry data showed that it had likely been a red supergiant, as one would expect for the progenitor of a failed supernova. It was in the correct mass range (18 to 25 solar masses) and would have been surrounded by dust — again, as expected. In short, the initial data seemed to make it the perfect example. All that was needed were more observations.
The team then added new data from Hubble, and looked at more archival images (Adams et al. (2017)). The data confirmed the disappearance and allowed them to place more constraints on the properties of the progenitor. It should have been roughly 200,000 times as luminous as the Sun, and 22 to 25 times as massive. With a surface temperature of about 3,500 Kelvin, it was cooler than the Sun. The post-eruption peak luminosity was indeed at least one million solar luminosities, and the eventual dimming brought it down below its pre-eruption state — indicating that the star had disappeared.
Besides the failed supernova hypothesis, the authors considered several other possibilities, chiefly the possibility that it was a long-period variable star. Mira variables — red giants undergoing pulsations and mass loss — typically have periods on the order of a year or less, and R Coronae Borealis variables — slightly hotter supergiants with dust buildups — may dim and brighten over even longer timescales. However, N6946-BH1’s outburst fit neither of these models, and was substantially more luminous than either class of star could be.
A more feasible proposal was that the event was the result of a stellar merger. Notably, there had been faint emission at near-infrared wavelengths, and it was hypothesized that pre-merger interactions could lead to such a signal. However, observations of other stellar mergers indicate that the remnant should have been more luminous than was observed, and merger models failed to replicate the speed of the ejected mass. Similar cataclysmic scenarios, such as a supernova imposter, suffered a similar problem: namely, that the star survived the event, which appears to not be the case.
At the moment, the failed supernova model appears to be the best explanation for the disappearance of N6946-BH1. Two types of monitoring should reveal more about its fate, in the X-ray band and at optical wavelengths. X-ray emission would indicate the presence of an accretion disk, a likely sign of a black hole. On the other hand, optical emission would show that the star has reappeared — not what you would expect from a failed supernova.
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