SETI Institute observations of the Allen Telescope Array over about 10 months have shown how the twinkling of pulsars varies over time, creating tiny delays that require corrections in precise timing measurements. The subtle twinkling of a pulsar reveals how space silently warps and delays the signals we receive from the universe.
For about ten months, scientists led by the SETI Institute have been closely monitoring the pulsar PSR J0332+5434 (also called B0329+54). Their goal was to see how its radio signal "scintillates" as it travels through gas between the pulsar and Earth. Using the Allen Telescope Array (ATA), the team collected observations in the frequency range from 900 to 1956 MHz. Over time, they noticed slow but distinct changes in the pattern of the scintillation.
Why are pulsars so important to astronomers?
Pulsars are the dense, rapidly rotating remnants of massive stars that have ended their lives in an explosion. As they spin, they send out radio pulses with remarkable regularity. Because of this constant rhythm and their high density and speed, astronomers can use sensitive radio telescopes to record the exact arrival time of each pulse. These precise measurements can reveal subtle patterns related to phenomena such as low-frequency gravitational waves.
The journey through space complicates things. The gas between the stars can scatter the radio waves, slightly delaying the arrival time of each pulse on Earth. These delays can be very small, sometimes just tens of nanoseconds (a nanosecond is a billionth of a second). It is very important to learn how to measure and correct for these varying delays to keep the timing of pulsars as accurate as possible.
How the interstellar medium causes the signal to flicker
Pulsar signals behave much like starlight seen from Earth. Just as stars appear to twinkle because of Earth's atmosphere, radio waves from pulsars twinkle as they travel through space. Clouds of electrons between the pulsar and Earth cause the signal to form bright and faint patches across different radio frequencies. These patterns change over time as a result of the pulsar's motion, gas drift, and Earth's motion through space.
This changing flicker affects the arrival time of each pulse. The stronger the flicker, the longer the arrival delay. By repeatedly observing a bright, nearby pulsar, the researchers were able to track the changes in the patterns and convert them into precise corrections to the arrival times. These corrections can be applied to experiments that rely on very precise pulsar measurements.
The consequences for SETI and other fields
"Pulsars are wonderful tools that can teach us a lot about the universe and our stellar neighborhood," says SETI project manager Grace Brown. "Results like this help not only pulsar science, but also other fields of astronomy, including SETI."
Every radio signal traveling through the interstellar medium undergoes some level of flicker. A detectable flicker can be useful to SETI researchers because it helps separate signals originating from human technology from signals originating from beyond our solar system.
Long-term tracking reveals hidden cycles
The ATA study relied on a wide range of radio frequencies and many short observations. Almost every day for about 300 days, the team measured the brightness bandwidth (the size of the bright spots in the brightness pattern). They found that the brightness intensity varied on cycles ranging from days to months. The data also indicated an overall change occurring over a period of about 200 days. They also introduced a new, more reliable technique for estimating how brightness increases with radio frequency.
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