Mystery Signal from a Black Hole-Powered Jet

Mystery Signal from a Black Hole-Powered Jet

Astronomers have spotted what appears to be a regular signal coming from the blazar known as PG 1553+113.

This artist's concept shows a supermassive black hole shooting out a jet of plasma headed almost straight for Earth. In the telescope, though, this object would appear as a (usually) randomly flickering point of light.
NASA / JPL-Caltech

Observing a blazar is a little like standing beneath a relativistic waterfall. Look up: that flickering point of light is a head-on view of the powerful plasma jet shooting out from a supermassive black hole.

The free-flying electrons within that mess of plasma twirl at almost light speed around magnetic fields, and they radiate across the electromagnetic spectrum, often drowning out any other forms of emission. We might even see a sudden outburst when turbulence, a sudden influx of plasma, or some other force roils the jet.

But when Markus Ackermann (DESY, Germany) and colleagues pored through almost seven years of data collected with the Fermi Gamma-Ray Space Telescope, they saw something completely unexpected: a regular signal coming from a blazar. Gamma rays from PG 1553+113 seem to brighten roughly every 2.2 years, with three complete cycles captured so far.

Moreover, other wavelengths seem to echo this cycle. Inspired by the gamma-ray find, Ackermann’s team sought out radio and optical measurements from blazar-monitoring campaigns — and both wavelengths show hints of the same periodic signal. The team also looked at X-ray data collected over the years by the Swift and Rossi X-ray Timing Explorer spacecraft, but there weren’t enough data points for a proper analysis.

The results are published in the November 10th Astrophysical Journal Letters. (Click here for full text).

A Gamma-Ray Pulse?

This light curve shows how the brightness of blazar PG 1553+113 varies for gamma rays with more than 100 million electron volts of energy. The plot, which includes data from August 4, 2008, to July 19, 2015, displays three complete cycles of an apparently regular, 2-year cycle.
M. Ackermann & others / Astrophysics Journal Letters

If this signal is real, it has to come from the black hole-powered jet, and the authors explore a number of explanations.

For example, the jet might be precessing or rotating, sweeping its beam past Earth every 2 years or so. Or perhaps a strong magnetic field chokes the flow of gas toward the black hole, creating instabilities that then regularly flood the jet with material. The most intriguing prospect is another supermassive black hole in the system, its presence affecting gas flow and jet alignment.

At this point, though, the authors admit they don’t have enough data to distinguish between these possibilities. Further monitoring might remedy that.

Keep Watching

“I am always skeptical about claims of periodicity based on only 2 to 3 cycles,” says Alan Marscher (Boston University), a blazar expert not involved in the study. Even completely random processes, he adds, can create apparently regular signals over short periods of time.

These light curves compare how the blazar varies in X-rays (top panel), optical (middle), and radio waves (bottom). Though there aren't enough X-rays to track the regular variation seen in gamma rays, the optical and radio data seem to echo the gamma-ray cycle, which is shown as a dotted line in the middle panel.
M. Ackermann & others / Astrophysics Journal Letters

And Ackermann’s team is frank about the data’s limits. After all, blazars are known to flare randomly and, due to the length of the suspected cycle, only three complete periods have been captured so far. The authors estimate a few percent probability that this signal is indeed a chance alignment of random flares.

Still, the fact that the signal is observed across radio, optical, and gamma rays strengthens the case. “Seeing such well-correlated oscillations across the different wavebands isn't as common as simple models would expect,” Marscher notes.

“It's worth keeping an eye on this object.”