Blazar's Outburst Hid Repeating Signal

Amidst the turmoil that was 2020, you might not have noticed an obscure celestial object having its own conniption fit.

Originally thought to be a variable star, the light from the object called BL Lacertae actually comes from a huge jet of plasma that’s pointed almost straight at us. That plasma, shooting toward us at nearly the speed of light, is in turn powered by a supermassive black hole 170 million times the mass of the Sun. This blazar is always flickering hugely, but in 2020 it underwent a dramatic outburst, flaring by more than a factor of 10 beyond its usual.

Fortunately, someone was watching. Svetlana Jorstad (Boston University) and colleagues monitor more than a dozen blazars like BL Lacertae at visible and radio wavelengths with the Whole Earth Blazar Telescope; additional gamma-ray data come from the Fermi Space Telescope. Using these facilities, the team caught the whole flare-up across visible and gamma-ray wavelengths.

And within that outburst, they reported in Nature, they saw something tantalizing: a repeating signal.

Pulses from a Blazar

No, it’s not aliens. The 14 pulses, emitted roughly every 13 hours over a two-week timeframe, are quasi-periodic oscillations (QPOs), which means they’re not perfectly regular and have more prosaic explanations. But exactly where they came from wasn’t immediately clear.

For other gas-guzzling supermassive black holes, for which we aren't looking down the barrel of the jet, such QPOs could come from a hotspot spiraling around and into the black hole. But in the case of BL Lac, the light appears to come from gas spiraling not in the vast disk around the black hole but in the narrow jet pointing out of the black hole’s pole.

Jorstad and her colleagues based this conclusion on how much of the light is polarized, that is, vibrating along a certain direction. Polarization tells the story of what light has been through. Sunlight, for example, isn’t polarized, but reflected light is (that’s why sunglasses filter out polarized light, to reduce glare). In the case of blazars, radiation emitted from the disk around the black hole isn’t polarized, but that from the jet is.

That the 14 pulses from BL Lac coincide with its changes in polarization, Jorstad says, is key to identifying the source of the periodic signal: It’s coming from the jet, not the disk.

Radio imaging of the disk reveals the pulses’ likely origin: a kink in the jet. A kink can make a big change in how bright the jet is, because the material in the jet is going so fast that the laws of relativity are coming into play. Countless subatomic particles are coursing downstream in our direction at nearly the speed of light, so when they emit radiation, it appears significantly boosted, or beamed, in brightness.

But change the angle just a little and that boost can go way down — or up. “As the particles in the jet flow through the kink, the amount of beaming changes back and forth, which causes the QPOs,” Jorstad explains.

Validation Needed

As astronomers work on the signal’s origins, though, they’ll also have to validate the signal itself. The detection of repeated signals around big black holes has a contentious history, cautions Matthew Middleton (University of Southampton, UK), who wasn’t involved in the study. And Phil Uttley (University of Amsterdam), who also wasn’t involved, says understanding the signal’s background is vital.

“I think that detection against a background of long-term variability, as is the case here, is very challenging, without being certain of the nature of that variability,” says Phil Uttley (University of Amsterdam). “In this paper, methods are used to try to account for this variability, but all of them make assumptions that may not be appropriate for this AGN. So, I think it is fair to say that the detection is interesting but not yet definitive.”

Team member Alan Marscher (Boston University) counters that even if the varying brightness were due to “noise,” they would still have physical underpinnings, such as changes in magnetic field strength or particle density. “One should still try to explain the variations based on physical grounds, which is what the kink model attempts to do,” he says.

If the QPO is there, though, Middleton thinks it’s reasonable that it would come from the jet. He has previously made the case that QPOs around many supermassive black holes originate not in the disk but in the hot, X-ray-emitting gas that hovers above the inner disk, termed the corona. “The corona is widely believed to segue into the jet,” Middleton explains, “and there are a number of ways in which the QPO signal could make its way into those bands where the jet is bright.”

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Editorial note (Sept. 14, 2022): Edited to add clarifying statement from team meber Alan Marscher.

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