Saturday, October 24, 2015

Star Destroyer

It is about 290 million light-years away, so we don't get a very detailed look at the proceedings, but it looks like a black hole just destroyed a star.

A suspicious hand goes up in the back row: "How do you know that? It is so far away you can't see anything but a dot."

Good question. What do you expect to see if a star gets too close to a black hole? Tidal forces would stretch it out--and do you wonder what you get when the pressure on the core gets smaller? That burst probably pales in comparison with the energy released as the plasma gets squeezed into new shapes around the black hole--some flying off and away and some falling in and some twisting into orbit.

You get a flash of light; visible, UV, X-ray. Where there was nothing much before--just an ordinary start--now there's a burst across the spectrum. Of course this doesn't matter at all if the black hole is very active and blowing jets; you only notice if it has been lonely and quiet up till now.

But there's more. Some of the plasma winds up in rapid orbit around the black hole, and doppler shifts turn some of the light bluer and some redder depending on whether the plasma in question is moving towards us or away. From the blue-shift you can estimate speeds, and from the time it takes for changes to happen you can estimate how big the distances are. (You can estimate blue shifts because light is absorbed by gas at certain frequencies {or emitted}, and from the spectrum you can see how much these gaps or bumps have shifted.)

Unfortunately, they noticed a flash and then turned the scopes on the spot, so all they saw was the cooling down of the plasma: it would have been cool to see the whole process.

Variability within the absorption-dominated spectra indicates that the gas is relatively close to the black hole. Narrow line widths indicate that the gas does not stretch over a large range of radii, giving a low volume filling factor. Modest outflow speeds of a few hundred kilometers per second are observed, significantly below the escape speed from the radius set by variability. The gas flow is consistent with a rotating wind from the inner, super-Eddington region of a nascent accretion disk, or with a filament of disrupted stellar gas near to the apocenter of an elliptical orbit.

"Relatively close" is an interesting phrase: they estimate 17 million km at the "innermost stable circular orbit." That's closer than Mercury is to the Sun.

(One of the experiments studying the star is called ASASSN: All-Sky Automated Survey for SuperNovae.)

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