Astronomers have confirmed that the quasar PG 1302-102 is probably a binary supermassive black hole, its members less than a tenth of a light-year apart.
Black holes are fickle. Their emission is often unpredictable, especially for the gargantuan ones that sit munching on gas disks in galactic centers. That’s why it was so odd earlier this year when Matthew Graham (Caltech) and colleagues found 20 of these quasars glowing with a regular beat. (They later upped the tally to 111.)
The best explanation for this periodic behavior, the team argued, was that these quasars are actually doubles: two supermassive black holes inexorably spiraling in toward each other. The time between repeated signals — in this case, on the order of several years — would then be the time it took for the black holes to orbit each other once. Analyzing the cleanest-looking of the lot, PG 1302-102, Graham’s team estimated that the black holes were less than a tenth of a light-year apart.
But the scientists didn’t know why the periodicity existed. They had several ideas — such as wobbly jets or warped accretion disks — and all of them required a binary black hole. But the team didn’t know which solution was correct.
So Daniel D’Orazio (Columbia University) and colleagues decided to take a computational whack at explaining PG 1302-102’s beat. They realized that, if a smaller black hole was circling a larger one, we would see what’s called a Doppler boost in the quasar’s emission.
Doppler shifts are common phenomena in astronomy. The Doppler shift is the change in light’s (or sound’s) wavelength because its source is moving toward or away from you. (The classic example is the ambulance siren that sounds higher pitched when the ambulance is racing towards you and lower pitched when it’s zooming away.) Light shifts to bluer (shorter) wavelengths when the star, planet, or whatchamacallit is moving toward us along our line of sight, and to redder (longer) wavelengths when it moves away.
Doppler boosting is the Doppler shift on steroids. With PG 1302-102’s black holes so close together, the smaller one is whipping around its big brother at 7% the speed of light — 22,000 km/s, or 49 million mph. That dramatically shifts the wavelength, by 14%. So if we’re observing the quasar at an optical wavelength of 600 nm, that means that when the smaller black hole is moving toward us, we’re actually seeing 700-nm light that’s been blueshifted, but when it’s moving away, we’re seeing 530-nm light that’s been redshifted.
If the quasar’s emission were the same intensity at all wavelengths, then we wouldn’t notice the difference. But it’s not: it increases as we move from optical to ultraviolet. So moving 14% up and down the wavelength scale samples different intensities, meaning that as the small black hole orbits, we’ll see a periodic change. The team reports the result in the September 17th Nature.
Graham agrees that the team’s scenario is a reasonable one. The team does have to make some assumptions, such as how close the system is to being edge on to our line of sight. But the nice thing is that the researchers make a specific prediction that can provide a thumb’s up or down for the theory.
That prediction says that PG 1302-102’s brightness variations ought to be more than twice as large at ultraviolet wavelengths as at optical ones. That’s because the quasar’s emission doesn’t just get more intense as we move from optical to ultraviolet; it also increases more rapidly in ultraviolet than in optical, explains study coauthor Zoltán Haiman (also Columbia). So if an optical wavelength shifts toward a shorter wavelength, we’ll see a bit of an increase in intensity, but if an ultraviolet wavelength shifts the same amount, we’ll see a greater increase in intensity.
The team compared PG 1302-102’s multiwavelength variations using archival spectra from the Hubble (optical) and Galaxy Evolution Explorer (GALEX, ultraviolet) space telescopes. The astronomers found the expected extra oomph in ultraviolet.
All-out confirmation will require simultaneous multiwavelength observations spanning longer time scales, but at least initially, the results do seem to confirm that a binary black hole exists in this quasar.
J. D’Orazio et al. “Relativistic boost as the cause of periodicity in a massive black hole binary candidate.” Nature. September 17, 2015.
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