Probing Variable Black Holes

Artist impression of a black hole feeding off its companion star... and a rogue Higgs particle (ESO/L. Calçada)
Artist impression of a black hole feeding off its companion star... and a rogue Higgs particle (ESO/L. Calçada/Particle Zoo)

Black holes are voracious eaters. They devour pretty much anything that strays too close. They’re not fussy; dust, gas, plasma, Higgs bosons, planets, stars, even photons are on the menu. However, for astronomers, interesting things can be observed if a star starts to be cannibalized by a neighbouring black hole. Should a star be unlucky enough to have a black hole as its binary partner, the black hole will begin to strip the stars upper layers, slowly consuming it on each agonizing orbit. Much like water spiralling down a plug hole, the tortured plasma from the star is gravitationally dragged on a spiral path toward the black hole’s event horizon. As stellar matter falls down the event horizon plug hole, it reaches relativistic velocities, blasting a huge amount of radiation into space. And now, astronomers have taken different observations from two observatories to see how the visible emissions correlate with the X-ray emissions from two known black hole sources. What they discovered came as a surprise

When matter falls toward a black hole, the volume surrounding the event horizon will flare electromagnetic radiation in the form of X-rays. It is commonly believed that stellar black holes will then glow in lower energy wavelengths once the surrounding gas and dust has been irradiated with this burst of energy. So, generally, we’ll see the primary X-ray emission of relativistic matter being quickly converted into energy, then a secondary burst in visible wavelengths being emitted from the black hole accretion disk. The optical light received will therefore lag the X-ray radiation and will not vary in intensity as quickly. [Note: when talking about black hole emissions, all radiation emitted comes from outside the event horizon. Nothing, not even light, can escape the clutches of the event horizon. We don’t know what happens inside the horizon, and to be honest, I hope we never find out…]

However, new results from a comparative study of data from NASA’s orbital Rossi X-ray Timing Explorer (RXTE) and the European Southern Observatory’s (ESO) Very Large Telescope (VLT) have shown that the theory behind black hole emissions may not be as complete as believed. The study focused on two black hole candidates: GX 339-4 and SWIFT J1753.5-0127, the remnants of dead massive stars. Both have orbital companions and therefore belong in binary systems.

Visible light brightness variations from flickering black hole GX 339-4 (P. Gandhi/ESO/ULTRACAM)
Visible light brightness variations from flickering black hole GX 339-4 (P. Gandhi/ESO/ULTRACAM)

For starters, by using a visiting instrument called ULTRACAM (a high-speed camera, taking 20 images per second) attached to the VLT, observations of visible wavelengths were made. “These are among the fastest observations of a black hole ever obtained with a large optical telescope,” remarked Vik Dhillon, an ULTRACAM collaborator. The animation (right) shows the variability of black hole GX 339-4 over a period of 10 seconds. As can be seen, this black hole has a very rapid variability in optical wavelengths.

Although this is already fairly ground breaking — after all, these are the first visible observations of a rapidly flickering black hole at a sub-second temporal resolution — it’s not the only thing the ESO researchers noticed. The visible light emissions from two black holes varied more rapidly than the X-ray emissions (contrary to what is thought to be the case). What’s more, there was little evidence to suggest that the visible light emissions lagged behind the X-ray emissions. Instead, the visible light and X-rays appear to follow a definite pattern: before an X-ray flare the visible light dims, and then surges to a bright flash for a tiny fraction of a second before rapidly decreasing again.

Although the variability of the emissions appear to be chaotic, this pattern demonstrates some underlying mechanism that generates both X-rays and visible light in the same volume of space surrounding a black hole. Visible light emission is not a secondary effect after all. “The rapid visible-light flickering now discovered immediately rules out this scenario for both systems studied,” said Poshak Gandhi, international team leader. “Instead the variations in the X-ray and visible light output must have some common origin, and one very close to the black hole itself.”

So what mechanism could generate both X-rays and visible light in the same volume of space? Powerful magnetic fields surrounding the black hole may be the best candidate for this physical process. Energy released near the black hole may be stored in the magnetic flux, acting as a plasma reservoir. When a certain threshold is reached, energy may be liberated through multi-million degree plasma (akin to solar coronal loops trapping and heating X-ray emitting plasma in the solar atmosphere) or through streams of relativistic particles. By invoking strong magnetic fields as a possible energy reservoir surrounding a black hole, the pattern observed in X-ray and visible light variations may be explained.

Source: ESO

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