Twinkle, Twinkle Mighty Quasar

What did Fellow Jeff Linsky come home with from a 2006 SINS conference?

He arrived at JILA with the realization that quasars twinkle for much the same reason stars twinkle: Light from both quasars and stars pass through turbulence that mixes up the light rays, causing the light to vary in intensity, or twinkle. However, the stars we see every night twinkle because of turbulence in the Earth’s dynamic atmosphere, which changes hundreds of times a second.

In contrast, quasars twinkle relatively slowly, changing in intensity over several hours. And — here’s what triggered Linsky’s aha moment at the conference — they twinkle more slowly or more rapidly at certain times of the year. Sitting in the audience in Socorro, New Mexico, on a warm spring day, he suddenly understood that what makes quasars twinkle are turbulent clouds only a few light years away from us in the local interstellar medium.

Linsky recognized that interstellar clouds couldn’t possibly be the whole story, however. These clouds (and any turbulence within them) are virtually stationary. Something else must be moving fast enough to cause twinkling, and that something would be us. Earth races around the Sun at a speed of ~30 km/s. Twinkling patterns result from the interaction of Earth’s speed and position with the irregular shape of the turbulence in the local interstellar clouds.

The patterns of brightness created by the irregular turbulence are also uneven. When our planet reaches a position in its orbit around the Sun that crosses the short axis of this brightness pattern, the quasar twinkles more rapidly. Three months later when the Earth’s motion around the Sun is perpendicular to its earlier motion, it crosses the brightness pattern along its long axis. This time, the quasar twinkles more slowly.

If the Earth didn’t move through space, quasars wouldn’t twinkle. The twinkling of quasars at radio wavelengths and the twinkling of stars at visible wavelengths are essentially the same process. The only difference is that for the quasars, it’s us (the observers) who move. For the stars, it’s Earth’s atmosphere that moves.

Linsky recently verified his insight regarding the origin of quasar twinkling. In a study of three quasars that twinkle at radio-wave frequencies, he and his colleagues from the University of California at San Diego and the University of Texas at Austin were able to identify the specific interstellar clouds involved. The researchers showed that the high turbulence areas of those clouds were typically near cloud edges in close proximity to a least one other cloud. They suggested that cloud-cloud interactions were likely responsible for the higher levels of interstellar turbulence involved in quasar twinkling.   - Julie Phillips